[
    {
        "id": 1,
        "chunk": "Historical perspective",
        "category": " Introduction"
    },
    {
        "id": 2,
        "chunk": "# Water drop-surface interactions as the basis for the design of anti-fogging surfaces: Theory, practice, and applications trends  \n\nIván Rodríguez Durán a,b, Gaétan Laroche a,b,⁎  \n\na Laboratoire d'Ingénierie de Surface, Centre de Recherche sur les Matériaux Avancés, Département de Génie des Mines, de la Métallurgie et des Matériaux, Université Laval, 1065 Avenue de la médecine, Québec G1V 0A6, Canada b Centre de Recherche du Centre Hospitalier Universitaire de Québec, Hôpital St-François d'Assise, 10 rue de l'Espinay, Québec G1L 3L5, Canada",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# a r t i c l e i n f o",
        "category": " Abstract"
    },
    {
        "id": 4,
        "chunk": "# a b s t r a c t  \n\nAvailable online 24 November 2018  \n\nKeywords:   \nAnti-fogging surface   \nWater drop   \nFilmwise condensation Cassie-Baxter equation Self-healable coating Anti-bacterial activity  \n\nGlass- and polymer-based materials have become essential in the fabrication of a multitude of elements, including eyeglasses, automobile windshields, bathroom mirrors, greenhouses, and food packages, which unfortunately mist up under typical operating conditions. Far from being an innocuous phenomenon, the formation of minute water drops on the surface is detrimental to their optical properties (e.g., light-transmitting capability) and, in many cases, results in esthetical, hygienic, and safety concerns. In this context, it is therefore not surprising that research in the field of fog-resistant surfaces is gaining in popularity, particularly in recent years, in view of the growing number of studies focusing on this topic. This review addresses the most relevant advances released thus far on anti-fogging surfaces, with a particular focus on coating deposition, surface micro/nanostructuring, and surface functionalization. A brief explanation of how surfaces fog up and the main issues of interest linked to fogging phenomenon, including common problems, anti-fogging strategies, and wetting states are first presented. Anti-fogging mechanisms are then discussed in terms of the morphology of water drops, continuing with a description of the main fabrication techniques toward anti-fogging property. This review concludes with the current and the future perspectives on the utility of anti-fogging surfaces for several applications and some remaining challenges in this field.  \n\n$\\mathfrak{C}$ 2018 Elsevier B.V. All rights reserved.",
        "category": " Abstract"
    },
    {
        "id": 5,
        "chunk": "# Contents  \n\n1. Introduction 69   \n2. Wetting states of anti-fogging surfaces 70   \n2.1. Smooth surfaces . 70   \n2.1.1. Young's equation . 70   \n2.2. Rough surfaces . 71   \n2.2.1. Wenzel equation 71   \n2.2.2. Cassie-Baxter equation 71   \n2.3. The issue of line tension in micro/nano droplets and contact angle hysteresis . 71   \n3. How to prevent surfaces from fogging up: Anti-fogging strategies, mechanisms, and materials 72   \n3.1. (Super)hydrophilic anti-fogging surfaces: Spreading mechanism 72   \n3.2. (Super)hydrophobic anti-fogging surfaces: Rolling mechanism 74   \n3.3. Hydrophilic/oleophobic anti-fogging surfaces: Percolation mechanism . 74   \n4. Fabrication techniques toward anti-fogging property . 76   \n4.1. Bottom-up processing. 76   \n4.1.1. Dip-coating deposition 76   \n4.1.2. Spin-coating deposition . 78   \n4.1.3. Layer-by-layer deposition . 81   \n4.1.4. Physical and chemical vapor deposition 83   \n4.1.5. Electrochemical deposition 84   \n4.1.6. Others 84   \n4.2. Top-down processing. 84   \n4.2.1. Dry and wet etching methods . 84   \n4.2.2. Lithography . . 85   \n4.2.3. Template-assisted fabrication 86   \n4.3. Surface functionalization and related techniques 87   \n5. Application trends of anti-fogging surfaces 87   \n5.1. Food industry 87   \n5.2. Photovoltaic industry . 87   \n5.3. Medicine 88   \n5.4. Optical applications 88   \n6. Concluding remarks and outlook . 88   \nAcknowledgements 89   \nReferences 89",
        "category": " Introduction"
    },
    {
        "id": 6,
        "chunk": "# 1. Introduction  \n\nFogging can be defined as the natural phenomenon ocurring when water vapor condenses on a solid surface whose temperature falls below the dew point of the surrounding air-water vapor mixture [1]. The dew point is the temperature at which water vapor in air must be cooled to reach saturation (relative humidity of $100\\%$ ) [2]. From a physical point of view, the conversion of water vapor into liquid water in the presence of a solid surface involves two main stages, namely, formation of minute droplets with radii exceeding a critical value or “heterogeneous nucleation” and drop growth [3]. Once water drops are formed on the surface, the extent of fogging will primarily depend on their contact angles. In general, the higher the contact angles, the more pronounced the effects of condensation [4]. The main reason for this lies in the fact that each droplet scatters the incident light in all directions because of the small radius of curvature at the water drop /air interface [5]. The interaction between light and water droplets explains to a large extent, why transparent materials become blurry when exposed to hot and humid environments. This feature of fogged surfaces is commonly referred to in the literature as “breath figures” [6–8].  \n\nThe fogging of surfaces has been shown to cause detrimental effects on sectors of activity as diverse as the medical, the automotive, or the photovoltaic. For example, the presence of condensation on optical elements such as mirrors, lenses, and prisms decreases the precision of microscopes and chromatographs [9,10]. In the automotive and aeronautic sectors (e.g., train, vehicle, and aircraft), the fogging of windshields is quite often linked to safety concerns as it severely reduces the driver's field of view [11–14]. Fogging has also been reported to impair the visual field of endoscopes during surgical procedures [15] and lower the energy-conversion efficiency of solar cells [16]. In the food industry, condensation on greenhouse claddings limits the crop yield [17,18] and reduces the visual appearance of packaged food, which is perceived by consumers as a lack of freshness and quality [19].  \n\nThus far, two anti-fogging strategies have amply demonstrated their effectiveness in preventing these situations from occurring. The first one aims at changing certain environmental parameters, such as temperature, relative humidity, and surrounding air flow to avoid or remove condensation. Rear windshields, chiller cabinets, or swimming pool windows equipped with heat elements, are some examples of how to get rid of fogging by simply changing the temperature. Typically, the heating equipment is a conductive coating that keeps surface temperature above the dew point upon application of a voltage [20]. Quite a number of papers on electrothermal coatings based on oxides such as $\\mathrm{In}_{2}{\\sf O}_{3}–\\mathrm{Sn}{\\sf O}_{2}$ [21] and graphene oxide [22,23], metals such as Ni, Ag, and Cu [24–26], and semimetals such as carbon nanotubes [27] and graphene [28], have been published in this regard. These materials make it possible to remove surface fog with minimal energy consumption [29].The improvement of air circulation is another well-known anti-fogging approach as it promotes water evaporation and diminishes the number of potential condensation points [30,31]. The way windshield defrosting/defogging systems operate is an obvious example [12,32,33]. In the same vein, both the incorporation of moist adsorbents [34] and purging with dry air or inert gas [35,36] have also proven to be successful in preventing condensation in dual-panel lens and doubleglazed windows, respectively.  \n\nThe second category of anti-fogging strategies focuses on changing the morphology of water drops by tuning the wetting characteristics of the surface. The wetting behavior of any material can be tailored by adjusting its surface features, such as the roughness or chemistry, either by direct modification or by depositing a coating of a distinct material on the surface. As detailed in the following sections, such practices have shown to be suitable to endow anti-fogging surfaces with additional features such as icing-delay, anti-reflective, anti-bacterial, or anti-fouling characteristics. Anti-fogging strategies pertaining to the direct modification of the substrate's surface features can in turn be divided into two families. The first one is based on the creation of functional groups different from those originally found on the surface (i.e., surface chemistry modification). In this case, bulk properties and surface topography remain virtually unchanged. On the contrary, the second family of anti-fogging strategies involves either enhancing surface roughness or “carving” surface nano/micro features with well-defined geometries, by means of “bottom-up” processing. Here, a rigorous control of the surface topography is crucial, as surface features exceeding $100\\mathrm{nm}$ have been shown to compromise the optical properties, mainly because of light scattering, and the resistance to scratching and wear [37,38]. On the other hand, the deposition of thin films by “top-down” processing has also proven to be as effective as “bottom-up” processing or surface treatments in endowing polymeric and ceramic substrates with the anti-fogging feature.  \n\nGiven the above-mentioned considerations, anti-fogging surfaces can be classified into four distinct groups according to their apparent contact angles (APCAs) [39]: more specifically, superhydrophilic, hydrophilic, superhydrophobic, and hydrophobic surfaces. Hydrophilic and superhydrophilic surfaces, with an APCA in the range of $10^{\\circ}<\\theta<$ $40{-}50^{\\circ}$ and $5^{\\circ}<\\theta<10^{\\circ}$ , respectively, are made of “water-loving” materials. According to Drelich and colleagues [40], complete drop spreading or “superhydrophilicity” is possible only in textured or/and structured surfaces (rough and/or porous) featuring a roughness factor, as defined by Wenzel equation, greater than one. Surfaces with water-attracting features cause water drops to spread over the surface, thus forming a thin water film that allows for incident light to pass through without being scattered. As a result, the surface remains optically clear, even under strong fogging conditions. A water contact angle of $90^{\\circ}$ has been conventionally adopted as the cut-off value to differentiate hydrophilic surfaces from those repelling water, i.e., (super)hydrophobic surfaces [41,42]. That said, it is widely accepted that surfaces with water contact angles above $40{-}50^{\\circ}$ [4,5,43] are not able to mitigate the effects of condensation despite being hydrophilic;  \n\nhence, the above-mentioned $10^{\\circ}<\\theta<40{-}50^{\\circ}$ range. Hydrophobic and superhydrophobic surfaces, with an APCA in the range of $90^{\\circ}<\\theta<$ $150^{\\circ}$ and $150^{\\circ}<\\theta<180^{\\circ}$ , respectively, are typically prepared by a two-step process consisting of surface micro-/nano-structuration, followed by the deposition of a water-repellent material. The term “superhydrophobic” refers to a nearly non-wettable state characterized by very low contact angle hysteresis and sliding angles $(<5\\AA-10\\mathrm{^\\circ})$ [44], and can formally be described by the “Cassie air-trapping” or a closely related wetting state [39,45]. Contrary to (super)hydrophilic surfaces, the application of water repellency for anti-fogging purposes appears to have attracted less interest within the scientific community. The fact that (super)hydrophobic surfaces must be tilted to remove condensation, and that the combination of water repellency with the anti-fogging performance calls for more complex and more timeconsuming manufacturing processes may account for this divergence.  \n\nA concise overview of common fogging concerns and the fundamental aspects of fogging occurrence are introduced in Section 1. On this basis, wetting states depicting the interaction of water drops with solid surfaces are presented in Section 2. Materials and anti-fogging mechanisms are described in Section 3. In Section 4, fabrication techniques toward anti-fogging property are discussed in detail and classified into “top-down” and “bottom-up” processing, and surface functionalization. Featured applications of anti-fogging surfaces in key sectors of activity such as the food and photovoltaic industries and medicine, are addressed in Section 5. Section 6 concludes with our personal standpoint based on remaining and forthcoming challenges, current trends, and potential promising breakthroughs in this field.",
        "category": " Introduction"
    },
    {
        "id": 7,
        "chunk": "# 2. Wetting states of anti-fogging surfaces  \n\nExperience shows that condensation of water vapor on a solid surface can occur according to two distinct modes, namely dropwise and filmwise condensation [46–48]. In dropwise condensation, which typically takes place on low energy surfaces, water drops are yielded with high or very high contact angles. Owing to this particular feature, the effects of fogging materialize, although the surface is not fully wetted. On the contrary, should condensation take place on a substrate with high energy surface, water drops will exhibit very low contact angles (filmwise condensation). Here, no fogging is observed, as a thin film of water, not greatly hindering light transmission, forms on the surface. As can be inferred from what Mother Nature shows us, the morphology of water drops determines whether the condensate will fog up the solid surface or not. With this in mind, surface chemistry and topography must both be properly adjusted to change water drops shape, and in this way, design surfaces simultaneously meeting suitable wetting behavior and anti-fogging requirements. As detailed in the following section, several wetting states have been proposed to explain the wettability of solid surfaces, considering surface chemistry and topography in a straightforward way, in terms of contact angles and surface roughness.",
        "category": " Results and discussion"
    },
    {
        "id": 8,
        "chunk": "# 2.1. Smooth surfaces",
        "category": " Materials and methods"
    },
    {
        "id": 9,
        "chunk": "# 2.1.1. Young's equation  \n\nAs depicted in Fig. 1, water drops resting on a smooth surface can be characterized by the angle $\\theta_{0}$ between the surface and the tangent line drawn along the liquid/vapor interface from the point where solid, liquid, and vapor phases meet. In the early 19th century, Thomas Young [49] stated that the contact angle $\\theta_{0}$ is governed by the mechanical equilibrium resulting from surface tensions acting on the liquid drop/surface system, as follows:  \n\n$$\n\\mathbf{cos}\\pmb{\\theta_{0}}=\\frac{\\pmb{\\gamma_{S V}}-\\pmb{\\gamma_{S L}}}{\\pmb{\\gamma_{L V}}}\n$$  \n\nwhere $\\gamma_{S L},\\gamma_{S V},$ and $\\gamma_{L V}$ are the surface tensions solid/liquid, solid/vapor, and liquid/vapor, respectively, and $\\theta_{0}$ is the so-called “static contact angle”.  \n\nStrictly speaking, Eq. (1) does not appear in Young's publication “An essay on the cohesion of fluids” [49]; however, there are two statements contained in it, namely, for each combination of a solid and a fluid, there is an appropriate angle of contact between the surfaces of the fluid, exposed to the air, and the solid and We may therefore inquire into the conditions of equilibrium of the three forces acting on the angular particles, one in the direction of the surface of the fluid only, a second in that of the common surface of the solid and fluid, and the third in that of the exposed surface of the solid, which substantiate that the contact angle can be defined in terms of the surface tensions $\\gamma_{S L},\\gamma_{S V},$ and $\\gamma_{L V}.$ Although the use of surface tensions rather than forces, as stated by Young, has been a subject of debate [50], theoretical derivation of Eq. (1) has recently been proven using thermodynamic arguments [51–54]. Despite this, Young's equation (Eq. (1)) does not adequately reflect the complexity of wetting phenomena, as it applies strictly to atomically flat and chemically homogeneous surfaces that neither dissolve nor react when in contact with the liquid. With all these constraints, interpreting water contact angles measured on real surfaces, i.e., APCAs, calls for wetting states considering, not only surface chemistry (via surface tensions) but also surface roughness.  \n\n![](images/8db9ac63e7ace8844a6fa5cedd90b266d737c8e5c3bad009dbd0443d365ab5b6.jpg)  \nFig. 1. Microscopic view of a water drop showing surface tensions at liquid/vapor, liquid/solid, and solid/vapor interfaces and forces acting on water molecules (adhesive and cohesive forces). Surface tension is the energy required to increase the surface area of a given phase by a unit of area $(\\mathrm{J}\\mathrm{m}^{-2},$ ). $\\mathrm{F}_{\\mathrm{s}}= $ forces between water molecules at the drop surface, and $\\mathrm{F_{b}=}$ forces between water molecules within the bulk.",
        "category": " Results and discussion"
    },
    {
        "id": 10,
        "chunk": "# 2.2. Rough surfaces",
        "category": " Materials and methods"
    },
    {
        "id": 11,
        "chunk": "# 2.2.1. Wenzel equation  \n\nFollowing observation of contact angles on real surfaces, Wenzel [55,56] proposed an alternative to Young's equation including the effect of surface roughness on the wetting behavior, as follows:  \n\n$$\n\\mathrm{cos}\\theta_{W}=R_{f}\\ \\mathrm{cos}\\theta_{0}\n$$  \n\nwhere $\\theta_{W}$ is the “apparent contact angle” or the contact angle measured on a rough surface, $R_{f}$ is the roughness factor, and $\\theta_{0}$ is the contact angle as described by Eq. (1). The roughness factor is defined as the ratio between the real surface area $A_{R}$ (for a given surface topography, i.e., peaks and valleys) and the geometric area resulting from the projection of the rough surface onto a hypothetical planar surface AP. Given that $A_{R}>A_{P},$ hydrophilicity and hydrophobicity are both enhanced with surface roughness. Indeed, an increase in roughness factor lowers $\\theta_{W}$ when $\\theta_{0}$ $\\angle90^{\\circ}$ yet enhances $\\theta_{W}$ when $\\theta_{0}>90^{\\circ}$ . Regardless of the hydrophilicity/ hydrophobicity of the surface, the wetting of a rough and chemically homogeneous surface results in a solid-water interface with no air trapped within, namely the Wenzel wetting state. (Fig. 2a).",
        "category": " Results and discussion"
    },
    {
        "id": 12,
        "chunk": "# 2.2.2. Cassie-Baxter equation  \n\nAs reported in the preceding paragraph, Wenzel state presumes that rough surfaces are fully wettable. Nevertheless, it has been shown that liquid drops can eventually break Wenzel's assumption, and not displace the air trapped into the cavities [57,58]. Cassie and Baxter addressed this issue by assuming that such surfaces are composed of two distinct features, one with a fractional area $\\varphi_{1}$ and Young's contact angle $\\theta_{1}$ , and the other, with a fractional area $\\varphi_{2}$ and Young's contact angle $\\theta_{2},$ , with $\\varphi_{1}+\\varphi_{2}=1$ . The apparent contact angle, which can be regarded as a weighted average of static contact angles for each fraction, is given by the Cassie-Baxter equation:  \n\n$$\n\\mathrm{cos}\\theta_{C B}=\\varphi_{1}\\mathrm{cos}\\theta_{1}+\\varphi_{2}\\mathrm{cos}\\theta_{2}\n$$  \n\nIn this case, two wetting scenarios are possible: Cassie air-trapping and Cassie impregnating sates. The first wetting state considers water drops lying on the top of protrusions of the solid surface and air trapped underneath them (Fig. 2b). This distinguishing feature of the Cassie airtrapping state has been attributed either to re-entrant geometries [59] or the combination of a hierarchical topography with a low surface energy material. On this basis, the first fraction $\\varphi_{1}$ corresponds to the solid/ liquid interface with a fractional area $\\varphi_{S L}$ and $\\theta_{1}=\\theta_{0}$ (flat protrusions), and the second one $\\varphi_{2}$ , to the liquid/vapor interface with a fractional area $\\varphi_{L V}=1\\ –\\varphi_{S L}$ and $\\theta_{2}=180^{\\circ}$ (full water repellency). In light of these boundary conditions, Eq. (3) thus becomes [60,61]:  \n\n$$\n\\mathrm{cos}\\theta_{C A}=-1+\\varphi_{S L}(\\mathrm{\\bf~cos}\\theta_{0}+1)\n$$  \n\nEq. (4) represents the so-called “Cassie air-trapping wetting state” (Fig. 2b). The resulting solid-water-air interface is a sine qua non for a superhydrophobic anti-fogging surface to operate optimally. The term “optimally” refers to the situation in which water drops leave the surface upon rolling leaving no remnants behind. Conversely, if the surface is impregnated by water (water displaces the air trapped in the cavities), Cassie-Baxter equation can be rewritten as follows:  \n\n$$\n\\mathrm{\\bfcos}\\theta_{C I}=1+\\varphi_{S L}(\\mathrm{\\bf~cos}\\theta_{0}-1)\n$$  \n\nEq. (5) represents the so-called “Cassie impregnating wetting state” (Fig. 2c) [45]. In both wetting states, $\\varphi_{S L}$ ranges from 0 to 1. When $\\varphi_{S L}=$ 1, the wetting behavior is described by Young's equation (flat surface), while $\\varphi_{S L}=0$ results in a non-wettable surface $\\m\\theta=180^{\\circ}$ ).  \n\n2.3. The issue of line tension in micro/nano droplets and contact angle hysteresis  \n\nAccording to Marmur [62], Wenzel and Cassie-Baxter equations do not adequately reflect the wetting behavior of real surfaces, as they are built on the assumption that the contact angle does not depend on the size of the drop. Although appropriate for water drops sufficiently large compared with surface features (roughness), these equations do not take into account the non-negligible effects of line tension in nano- and micro-scaled sessile droplets [63–65], which also form during condensation. Bearing this in mind, Bormashenko [66] recently reported a general formula describing the wetting behavior of rough surfaces including the effect of the tension line in minute droplets.  \n\nOn the other hand, for a specific system water drop/surface, the combination of the triad $\\gamma_{S V},\\gamma_{L V},$ and $\\gamma_{S L}$ with surface roughness must result in a unique contact angle. That said, the observed contact angles usually differ from those obtained using the above-mentioned equations, primarily because of the motion of the triple phase contact line (TPCL). The TPCL is defined as the imaginary circular line where solid, liquid, and vapor phases meet (see Fig. 2) [39]. A moving TPCL leads to a minimum value of the contact angle, or “receding angle” $\\theta_{r e c}$ and a maximum one, or “advancing angle” $\\theta_{a d\\nu},$ which can be assessed using two different methods, namely dynamic sessile drop method and tilting base method [41]. In the sessile drop method, a water droplet is dropped onto a horizontal surface from a syringe without losing contact with the needle. When water is removed from the drop, the contact angle decreases to a minimum value or the receding contact angle, $\\theta_{r e c},$ before TPCL moves inward (Fig. 3a). When liquid is added, TPCL reaches a stable state characterized by the advancing contact angle, $\\theta_{a d\\nu},$ that is, the contact angle measured just before TPCL moves outward (Fig. 3b). The difference between advancing and receding contact angles ( $\\Delta\\theta=$ $\\theta_{a d\\nu}-\\theta_{r e c})$ is called “contact angle hysteresis” (CAH) [44]. Adhesion hysteresis, chemical heterogeneities, and surface roughness have been reported as the main factors behind contact angle hysteresis [53,67–70]. Nevertheless, the pinning of the TPCL is probably the most important source of CAH, as observed in silicon wafers [69] and extruded polymer [70] films, known for being atomically smooth and free of chemical heterogeneities. In the tilting base method, a water droplet is placed on a horizontal surface as in the preceding method. The main difference here is that the angle between the surface and the horizontal plane is gradually tilted from $0^{\\circ}$ to a critical value $\\alpha,$ also known as “sliding angle” (SA), triggering drop motion (Fig. 3c) [71]. Accordingly, this method allows for the assessment of contact angles and CAH when water drops meet a tilted or a moving surface (i.e., dynamic wettability). As will be seen later in Section 3.2, the water contact angle (WCA), the contact angle hysteresis, and the sliding angle are key parameters requiring controlto design antifogging surfaces featuring rolling-mechanism.  \n\n![](images/88dfb500c65168210da6b325ecf9e7ca777f7d2e23e18234af0a29e54242d160.jpg)  \nFig. 2. Wetting regimes. (a) Wenzel state, (b) Cassie air-trapping state, and (c) Cassie impregnating state. Solid, liquid, and vapor phases meet in an imaginary circular line known as th “triple phase contact line” (TPCL).",
        "category": " Results and discussion"
    },
    {
        "id": 13,
        "chunk": "# 3. How to prevent surfaces from fogging up: Anti-fogging strategies, mechanisms, and materials  \n\nAnti-fogging strategies explored thus far can be grouped into two broad categories. The first one aims at controlling the parameters external to the liquid/solid interface, that is, those involved in the nucleation of water drops , such as temperature, air flow, and relative humidity. This category of anti-fogging strategies is outside the scope of this review, as it does deserve further, more in-depth consideration. The second category of anti-fogging strategies focuses on changing the morphology of water drops either by directly tuning the substrate's surface features (chemistry and roughness) or by coating deposition. Compared to the first category, the elaboration of surfaces endowed with the anti-fogging feature has generated more interest, in light of the numerous papers published in the last ten years. According to the most recent literature, the way these surfaces combat fogging can be explained by three different mechanisms:",
        "category": " Introduction"
    },
    {
        "id": 14,
        "chunk": "# 3.1. (Super)hydrophilic anti-fogging surfaces: Spreading mechanism  \n\nAnti-fogging surfaces featuring spreading mechanism are mostly coatings made from hydrophilic or “water-loving” materials. These materials interact with water drops causing them to spread across the surface, and thus form a scattering-free water film (Fig. 4). To date, there appears to be general agreement that wetted surfaces remain optically clear under aggressive fogging conditions, if the contact angle of water drops is ${<}40{-}50^{\\circ}$ [4,5,43]. Generally speaking, these coatings can be made either from polymers and inorganic materials or from a mixture of both (composite materials). Natural and synthetic polymers with pendant hydrophilic functionalities such as hydroxyl $(\\mathrm{OH}),$ carboxyl $(\\mathsf{C O O H}),$ ester $(\\mathsf{C O O R}),$ amino $\\begin{array}{r}{(\\overline{{(\\mathsf{N H}_{2})}},}\\end{array}$ amide $(\\mathrm{NHCOR}),$ sulfonic $(\\mathsf{S O}_{3}\\mathrm{H})$ , and dihydrogen phosphate groups $(\\mathrm{PO}_{4}\\mathrm{H}_{2}^{\\cdot}$ ) are commonly used in anti-fogging coatings. The potential applicability of natural polymers in fields impaired by fogging has been receiving increasing attention in recent years due to their unique features, including the possibility of covering thermally sensitive materials, as well as their non-toxic and environmentally friendly nature (Table 1). That said, features such as low-cost, tunable interaction with water drops, easy availability, and crosslinkable nature place synthetic polymers in direct competition with natural ones as anti-fogging materials (Table 2).  \n\nThe criterion of either being naturally-sourced or not, appears not to apply when considering inorganic materials. As a matter of fact, most of the studies reported thus far on inorganic coatings with anti-fogging performance consider more suitable to classify them, according to their response to light, into two groups. The first one comprises intrinsically hydrophilic and non-photoresponsive materials, such as $\\mathrm{SiO}_{2}$ $Z\\mathrm{r}0_{2}$ , $\\mathrm{In}_{2}\\mathrm{O}_{3}–\\mathrm{SnO}_{2}$ (ITO), $\\mathrm{MgO-Al_{2}O_{3}}$ , and graphene oxide, while the second one is integrated by materials becoming superhydrophilic upon exposure to UV light, such as $\\mathrm{TiO}_{2}$ , $Z_{\\mathrm{{nO}}}$ and ${\\mathrm{Bi}}_{2}{\\mathrm{O}}_{3}$ . These materials are typically covered with abundant hydroxyl (OH) groups per area unit. On the other hand, the combination of the photo-induced superhydrophilicity with the photocatalytic property allows for the use of $\\mathrm{TiO}_{2}$ -based materials in applications where self-cleaning and anti-fogging characteristics are required. Without going into detail, photocatalysis is basically a set of reactions whereby a dirty $\\mathrm{TiO}_{2}$ surface gets cleaned at room temperature. For this to occur, $\\mathrm{TiO}_{2}$ must absorb UV light to yield “reactive oxidizing species” (ROS), such as superoxide and hydroxyl radicals, that decompose organic pollutants into $\\mathsf{C O}_{2}$ and $\\mathrm{H}_{2}0$ [72]. Unfortunately, the fact that $\\mathrm{TiO}_{2}$ necessitates UV light to perform makes it challenging to design $\\mathrm{TiO}_{2}$ -based anti-fogging coatings for indoor applications. Indeed, when stored in a dark place, an UV-irradiated $\\mathrm{TiO}_{2}$ surface (superhydrophilic) experiences a conversion toward a more hydrophobic state, which is normally less effective in combating surface fog. To remedy this situation, a growing number of studies have focused efforts not only in enhancing the anti-fogging performance of $\\mathrm{TiO}_{2}$ in the absence of UV light, but also in broadening its photocatalytic response to visible and near-IR regions. Mixing with oxides, such as ${\\sf W O}_{3}$ , $z_{\\mathrm{nO}}$ , $\\mathrm{SiO}_{2}$ , $\\mathrm{ZnFe}_{2}{\\sf O}_{4}$ , and reduced graphene oxide [73–79]; doping with metals, such as Cu and $\\mathsf{A g}$ [80,81]; incorporating porogens, including PEG and cetyltrimethylammonium bromide (CTAB), coupled with a calcination treatment [82–86]; using “building blocks” with high surface-to-volume ratios (e.g., nanofibers, nanobelts, nanospheres) [87–93]; and increasing the surface roughness [94–98], have amply demonstrated to be suitable approaches to fabricate dual anti-fogging/self-cleaning $\\mathrm{TiO}_{2}$ -based films, with no need for UV light to perform.  \n\n![](images/ef60c78c21c4906784a3a3175fce50ab7511c8c7d029f367e36a9d587469e2d9.jpg)  \nFig. 3. Dynamic sessile drop method for the assessment of (a) receding and (b) advancing contact angles. (c) Tilting base method for the measurement of advancing and receding contact angles, and sliding angles. Adapted with permission from “Definitions for hydrophilicity, hydrophobicity, and superhydrophobicity: Getting the basics right”, Law, K.-Y., J. Phys. Chem. Lett., Volume 5, Issue 4, 2014, Pages 686-688. Copyright 2018, American Chemical Society.  \n\n![](images/3019c878adfc7c29254695599986b80d2940b1c55d83199b0b662ea7ab298254.jpg)  \nFig. 4. Illustration of the spreading mechanism. As water drops spread across the surface, total internal reflection (dashed red rays) becomes less prevalent while transmitted light (dashed green rays), increasingly less scattered, travels through the system water drop/surface. These surfaces are either hydrophilic $10^{\\circ}<\\Theta<40–50^{\\circ}.$ ) or superhydrophilic $(5^{\\circ}<\\Theta<10^{\\circ}$ .  \n\n![](images/64611d7c70e9d58f3487db7dd42502cdec4ccc311375d38586f075abf735e501.jpg)  \n\nTable 2 Repeating units of the main synthetic polymers used in anti-fogging formulations.   \n\n\n<html><body><table><tr><td>Family</td><td>Repeating unit</td><td>Anti-fogging polymer</td></tr><tr><td>Polyether</td><td></td><td>Poly (ethylene glycol) (PEG) (*) R=R=H Poly (ethyleneglycol dimethacrylate) (PEGDMA) (*) R=R Poly (ethyleneglycol methacrylate) (PEGMA) (*) R=H, R as in PEGDMA Poly (ethyleneglycol diacrylate) (PEGDA) (*)</td></tr><tr><td>Polyvinyl</td><td></td><td>R=R= Poly (vinyl alcohol) (PVA) (*) R=OH Poly (vinyl acetate) (PVAc) (***) R= Poly (vinyl-N-pyrrolidone) (PVP)(*)</td></tr><tr><td>Polyacrylates & Polyacrylamides</td><td></td><td>R= Poly (acrylic acid) (PAA)(*) R=OH Poly (2-hydroxyethyl acrylate) (PHEA)(*) OH R= Poly (acrylamide) (PAM)(*) R= NH Poly (methacrylic acid) (PMAA) (*)</td></tr><tr><td>Polymethacrylates</td><td></td><td>R=OH Poly (methyl methacrylate) (PMMA) (***) R=O-CH Poly (2-hydroxyethyl methacrylate) (PHEMA) (**) OH R= Poly (dimethylaminoethyl methacrylate) (PDMAEMA) (*) R</td></tr></table></body></html>\n\n\\*Water-soluble. \\*\\*Water-swellable. \\*\\*\\*Water-insoluble.  \n\nDespite not being implemented as widely as coating deposition, the direct modification of the substrate's surface features (chemistry and roughness) has amply proven its effectiveness in fabricating fogresistant surfaces featuring spreading mechanism. In this regard, the anti-fogging performance can be met either by modifying surface topography (Section 4.2) or by creating hydrophilic functionalities by means of surface treatments (Section 4.3).",
        "category": " Results and discussion"
    },
    {
        "id": 15,
        "chunk": "# 3.2. (Super)hydrophobic anti-fogging surfaces: Rolling mechanism  \n\nContrary to those featuring spreading mechanism, surfaces with water-repellent characteristics must be tilted to a minimal angle $\\alpha$ (sliding angle) to roll off water drops, and thus avoid the effects of surface fog (Fig. 5). There is general agreement among the scientific community that surface features required for a superhydrophobic anti-fogging surface to perform optimally are high contact angles (CA) coupled with low CA hysteresis and low slides angles. Surfaces displaying these characteristics fall into one of the following wetting states: Wenzel, Cassie airtrapping, Cassie impregnating (with a single level of hierarchy of roughness), and Lotus-like (with a double level of hierarchy of roughness, that is micro and nanoscale roughness). The rose petal-like state has not been considered here as it usually displays high contact angle hysteresis. Cassie air-trapping and Lotus-like wetting states are suitable for antifogging purposes as water drops roll off the surface leaving no others behind. In contrast, Wenzel and Cassie impregnating wetting states do not meet anti-fogging requirements. Here, drops remaining entrapped into the surface features, after the tilting of the surface, can be detrimental to the anti-fogging performance, as they scatter light as larger water drops do. With this in mind, anti-fogging surfaces featuring rolling mechanism can be fabricatedby two different routes: the “two-step” and “three-step” routes. The “two-step” route (Fig. 6a), which is typically applied to ceramic substrates, is based on the deposition of “building units” followed by a treatment with a low surface energy material [99–103], while the “three-step” route (Fig. 6b) consists of surface microstructuring of a polymeric substrate by soft lithography (Section 4.2.2), followed by coating deposition and hydrophobization [104,105].",
        "category": " Results and discussion"
    },
    {
        "id": 16,
        "chunk": "# 3.3. Hydrophilic/oleophobic anti-fogging surfaces: Percolation mechanism  \n\nAnti-fogging surfaces featuring percolation mechanism are mainly coatings made of fluorosurfactants polymers, namely, perfluorinated polyethylene glycol polymers [106–113] (Fig. 7a) and perfluoropolyether polymers (Fig. 7b) [114,115], containing hydrophilic and oleophobic domains [116]. Due to this particular feature, these surfaces enable small molecules (e.g., water molecules) to penetrate the coating faster than do larger ones (e.g., hexadecane molecules). To date, two distinct mechanisms have been proposed to explain this striking behavior. The first one, at times referred to as the “flip-flop” mechanism, relies on the presence of “defects” of appropriate size in the coating [106,107,109,110,112,113]. In general, when fluorosurfactants are deposited on a substrate, perfluorinated chains are orientated outward (hydrophobic/oleophobic region) while polyethylene glycol- and hydroxyl-containing moieties are directed toward the surface (hydrophilic/oleophobic region). Considering that this configuration results in a low surface energy barrier repelling both oil and water drops, right-sized defects in the fluorinecontaining layer are thus necessary to enable water drops to permeate and reach the interface coating/substrate (Fig. 8). Another proposed mechanism accounting for the simultaneous hydrophilicity/oleophobicity relies on the rearrangement of polymer chains when in contact with water or any other polar liquid [114,115]. Contrary to the preceding mechanism, the presence of defects in the fluorine-containing region is no longer necessary. Here, when a water drop meets the surface, perfluorinated chains rapidly rearrange inducing the formation of “channels” that allow for small water molecules to permeate quickly toward the hydrophilic region, while blocking or slowing down the passage of larger oil molecules. As in the case of $\\mathrm{TiO}_{2}$ -based materials, surfaces with simultaneous hydrophilicity and oleophobicity are suitable for use in applications requiring anti-fogging performance with a certain degree of self-cleaning activity. Badyal's group [112] proposed a “switching parameter”, defined as the difference between oil (hexadecane) and water static contact angles, to quantitatively assess the percolation mechanism. The higher the “switching parameter,” the better these antifogging surfaces perform. In another study, Howarter et al. [109] demonstrated that an advancing WCA $<30^{\\circ}$ and a receding $0C A>67^{\\circ}$ , are necessary to meet simultaneous self-cleaning and anti-fogging properties. In either case, the strong affinity between water molecules and the surface enables water drops (the cleaning fluid) to wet the surface by displacing oily substances (the pollutant).  \n\n![](images/2a45f8e9b6fb984444dae03acaf28ed069df40902e761b806cb3019a5629e57c.jpg)  \nFig. 5. Illustration of the rolling mechanism. Upon elevation of one side of the surface, water drops roll off easily thus mitigating condensation effects. These surfaces are either hydrophobic $150^{\\circ}>\\Theta>90^{\\circ}$ ) or superhydrophobic $\\mathrm{(\\Theta)}=150^{\\circ},$ and exhibit very low CAH and SA.  \n\n![](images/0696dcb5fda52e9440e7783f762322a2deca47a7134ef41e31a1a6ae9d3179b0.jpg)  \nFig. 6. Routes toward water-repellency and anti-fogging performance. (a) The “two-step” route: deposition of a layer with high specific surface followed by hydrophobization. (i) Deposition of fly-eye bio-inspired ZnO nanostructures and treatment 1H, 1H, 2H, 2H-perfluor oxysilane TES) [99]; (ii) deposition of raspberry-like SiO2 nanospheres and hydrophobization with 1H,1H,2H,2H-p lan lion-like ZnO microspheres and subsequent treatment with heptadecafluorodecyltripro oxysilane (FAS-17) [101]; (iv nd hydrophobization with PFOTES [102]; (v) deposition of multiscale ommatidial arrays of a resin propyl polysilse with 1H,1H,2H,2H-heptadecafluorodecyl methacrylate (HDMA) [103]. (b) The “three-step” route: surface roughening, coatin osition terial. (i) Dome-like surfaces on PDMS covered with solid SiO2 nanoparticles and hydrophobization with fluoroalkylsilane molecules (FAS) [105]; (ii) ZnO nanohairs on poly (vinylidene difluoride) (PVDF) microratchets treated with FAS-17 [104]. WCA: water contact angle, CAH: contact angle hysteresis, SA: sliding angle. Figures reprinted with permission from references [99-105].  \n\n![](images/dc9be6c938a739845b26a530645791e06743f3df3d5c3fbd266872fbea0afec6.jpg)  \nFig. 7. Fluorocarbon surfactants used in anti-fogging surfaces featuring percolation mechanism. (a) Linear and “Y-shaped” perfluorinated polyethylene glycol oligomers. Hydrophilic and oleophobic components are separated in the polymer chain. Depending on the hydrophilic domain, these molecules can be anionic, cationic, non-ionic, or amphoteric. (b) Family of perfluoropolyether polymers (PFPE): hydrophilic and oleophobic domains cannot be distinguished in the backbone. Red: hydrophilic domain, blue: hydrophobic domain, purple: a polymerizable vinyl group.",
        "category": " Results and discussion"
    },
    {
        "id": 17,
        "chunk": "# 4. Fabrication techniques toward anti-fogging property  \n\nThe above-illustrated mechanisms highlight the fact that a judicious combination of surface topography and surface chemistry is key to developing surfaces with fogging resistance. Bearing this in mind, a plethora of fabrication techniques aimed at adjusting the wetting behavior of water drops on solid surfaces has been thus far applied. The following sections present the most widely used techniques for the preparation of anti-fogging materials. To be consistent with the notions outlined above, these techniques have been classified into three distinct categories: bottom-up and top-down processing, and surface functionalization.",
        "category": " Materials and methods"
    },
    {
        "id": 18,
        "chunk": "# 4.1. Bottom-up processing  \n\nBottom-up processing involves the assembly of small “bricks” such as nanoparticles and polymers into more complex systems.",
        "category": " Introduction"
    },
    {
        "id": 19,
        "chunk": "# 4.1.1. Dip-coating deposition  \n\nWhen vinyl/acrylic polymers such as PVA, PVP, and PAA, are used as starting materials, dip-coating deposition allows for the fabrication of anti-fogging coatings endowed with frost-resisting and even selfhealing features [117–120]. In PET and PC substrates (eyeglasses) covered with PVA-Nafion complexes prepared by Sun et al. [121] a minimum thickness of $61~\\mathrm{{nm}}$ was required to prevent fogging effects at room temperature, while a thickness of 247 nm was necessary to ensure transparency over boiling water (Fig. 9a). In addition to providing PC lenses free of frozen fog (Fig. 9b,c), PVA/Nafion films were also found to be self-healable (Fig. 9d,e). After five cycles of damage and healing tests, the transmittance of the coated PET fully recovered (Taverage $\\approx99\\%$ at $500\\mathrm{nm}$ ), as supported by the complete closing of the scratches (Fig. 9f,g). Although PEG-based coatings do not feature self-healability [122], hydrogels prepared by Molina et al. [123] using a PEG functionalized with 3-isocyanatopropyltriethoxysilane hold great promise for the manufacture of anti-fogging films with drug delivery capability, due to its water absorbing characteristics. Anti-fogging coatings made from an isosorbide-based epoxy resin, also exhibited potential applicability as drug delivery system due to the swelling capacity conferred by the epoxide groups [124]. Different research groups have recently developed fog-resistant films containing sulfonic and phosphonic groups, well known for their high water-absorbing characteristics and underwater oleophobicity [125–128]. Ezzat and colleagues [129] fabricated anti-fogging glasses with extreme wettability $(\\mathsf{W C A}<5^{\\circ}),$ by anchoring zwitterionic poly(sulfobetaine methacrylate) (pSBMA) and poly(sulfobetaine vinylimidazole) (pSBVI) polymer brushes, while Huang et al. [130] used silanized zwitterionic sulfobetaine silane (SBSi) for the same purpose (Fig. 10a). In the latter study, the coated glasses recovered up to $99\\%$ of the initial light transmission after being exposed to hot water, as well as cooled at $-20^{\\circ}C$ (Fig. ${10}\\mathsf{b},{\\mathsf{c}}^{\\cdot}$ ). This behavior was in agreement with the observed “see-through” property under different fogging scenarios (Fig. 10d,e). Furthermore, a SBSicoated stainless steel mesh selectively separated water from various oil/water mixtures and oil/water emulsions with high efficiency $(>$ $99.5\\%$ and $>98.2\\%$ , respectively) (Fig. 10f,g). Oil/water separation efficiencies $>99.5\\%$ were also observed in a stainless steel wire mesh coated with anti-fogging formulations based on phytic acid and ferric ions $(\\mathsf{F e}^{\\mathrm{III}})$ [131]; while fog-resistant coatings reported by Wu's group [132] were shown to not only repel oil underwater $(0C A>150^{\\circ}.$ ), but also to prevent bacterial adhesion (E. coli and S. aureus).  \n\n![](images/215a786c100b0dbd374f38858e083528cd3fa14c50def2d3a9bd87e3533500a9.jpg)  \nFig. 8. Illustration of the “flip-flop” mechanism. These surfaces eliminate the effects of condensation by allowing for water drops to permeate the coating while blocking or slowing down the passage of oily substances. Accordingly, oil contact angles (OCA) are greater than water contact angles (WCA). Adapted from “Bioinspired, roughness-induced, water and oil super philic and super-phobic coatings prepared by adaptable layer-by-layer technique”, Brown, P. S.; Bhushan, B.; Young, T.; et al., Sci. Rep., Volume 5, 2015, Page 14030. (Open access).  \n\n![](images/72bd13f6cf93968d3a1d135a63112fcc27884995a103e1dd123eb1a200dd8d81.jpg)  \nFig. 9. (a) Anti-fogging properties of a PVA-Nafion film with thickness of ${\\sim}247\\mathrm{nm}$ . This film was first conditioned in a - ${}^{-20^{\\circ}\\mathsf{C}}$ refrigerator for 1 h and then placed over boiling water $(\\sim50^{\\circ}\\mathsf C$ and \\~100% RH). (b) Pair of polycarbonate eyeglasses, with the left-hand lens coated with PVA-Nafion films and the right-hand one uncoated. (c) Eyeglasses after being conditioned at - ${\\cdot20^{\\circ}C}$ for 1 h and then exposed to an ambient environment of ${\\sim}20^{\\circ}\\mathsf C$ and $\\sim40\\%$ RH. (d) Digital images of the PVA-Nafion film on a glass substrate that heals scratches. (i) Film scratched with sandpaper and (ii) scratched film from panel i after healing in water for $5\\mathrm{{min}}$ . The scale bar is $1\\mathrm{cm}$ . (e) AFM images of the scratched PVA-Nafion film before (i) and after (ii) healing in water. (f,g) Changes in transmittance at $500\\ \\mathrm{nm}$ and Rrms roughness, respectively, of the PVA-Nafion film during five cycles of the scratching-healing process. Reprinted with permission from “Highly transparent and water-enabled healable antifogging and frost-resisting films based on poly(vinyl alcohol)-nafion complexes”, Li, Y.; Fang, X.; Wang, Y.; Ma, B.; and Sun, J., Chem. Mater., Volume 28, Issue 19, 2016, Pages 6975-6984. Copyright 2018, American Chemical Society.  \n\nBy combining the dip-coating deposition with the in situ nanopressing technique (Fig. 11a), Zhang and collaborators built on glass and PET samples bilayer configurations integrated either by solid silica nanoparticles $(\\mathsf{W C A}=33.1^{\\circ}$ ) [133] (Fig. 11b) or by hollow silica nanoparticles $(\\mathsf{W C A}=37.5^{\\circ})$ ) [134] (Fig. 11c) partly embedded in a thin film of thermally crosslinked PVA-PAA blends. The anti-reflective property observed in optimal SNSs-HSNs/(PVA-PAA) configurations resulted in better light transmittance $(\\mathrm{T_{average}}>93\\%$ in the visible range when compared with that of uncoated substrates $(\\mathrm{T_{average}}\\approx85{-}90\\%)$ . Coatings with anti-fogging activity can also be prepared by immobilizing inorganic nanoparticles (typically $\\mathrm{SiO}_{2}$ and $\\mathrm{TiO}_{2}$ ) either in a network of hydrophilic polymers on the substrate (e,g., PVA, PVP, and PEGMA [135], glycidoxypropyltrimethoxysilane [136] or a catechol-conjugated polymer [137]) or on the substrate by direct deposition using sol-gel [138] or aqueous solutions [139]. The use of mesoporous silica nanoparticles (MPSNPs) [140,141], hollow silica nanospheres (HSNs) [142], double-shell hollow nanospheres of $\\mathrm{SiO}_{2}/\\mathrm{TiO}_{2}$ (DSHNs) [143], and solid silica nanoparticles (SSNPs) [144], among otherbuilding blocks allows for the fabrication of anti-fogging films with hierarchical roughness featuring spreading mechanism (high specific surface). Coatings made up of SSNPs deposited on $\\mathsf{L a}(\\mathsf{O H})_{3}$ nanorods prepared by You and colleagues [145] have proven remarkable capacity to alleviate surface fog $(\\mathsf{W C A}\\approx0^{\\circ}$ ) and minimize light reflection under sun exposure. Cao et al. [146] employed very recently faujasitic nanozeolites (DZ) with an average size of $25-30~\\mathrm{nm}$ , as assembly units to prepare on glass samples coatings with anti-reflective/anti-fogging features, while Liu and colleagues [147] proposed a soft templating route combining in situ growth with a modified Stöber method to synthesize $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ nanospheres directly on glass surfaces (Fig. 12a). Upon calcination ( $500^{\\circ}\\mathrm{C}$ for $^{2\\mathrm{h}}$ ), the resulting 500-nm-sized $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ nanospheres conferred superhydrophilic property $(\\mathsf{W C A}=2^{\\circ}.$ ) suitable for anti-fogging purposes to glass samples (Fig. 12b,c).  \n\n![](images/3ba75bdc595a2f4e33f1237a5322e90206aeaf3c1cb0263e7e853ec0f0771fc3.jpg)  \nFig. 10. (a) Chemical structure of SBSi and the formation of SBSi coatings on the oxidized substrate. Light transmission through the samples of PMMA, bare glass, and SBSi-glass after the treatments of (b) hot or (c) freezing at $-20^{\\circ}\\mathsf C.$ (d) Water spray on samples of PMMA, bare glass, and SBSi-glass. (e) Anti-fogging test by treating the water steam to samples of PMMA, bare glass, and SBSi-glass. (f) Oil-water separation apparatus and images of oil-water mixtures, residues, and filtrates in vials before and after separation. The colors of organic fluids are original, without pigment added. (g) Optical images of the underwater-oil CA measurements for SBSi-glass performed with air bubbles, ether, toluene, hexane, gasoline, diesel, and soybean oil; and quantitative results of OCAs for bare and SBSi-glass samples. Reprinted with permission from “Surface modification for superhydrophilicity and underwater superoleophobicity: Applications in antifog”, Huang, K.-T.; Yeh, S.-B.; and Huang, C.-J., ACS Appl. Mater. Interfaces., Volume 7, Issue 38, 2015, Pages 21021-21029. Copyright 2018, American Chemical Society.  \n\nSeveral studies have proven, on the other hand, the feasibility of conferring anti-fogging performance to glass $(\\mathsf{W C A}<10^{\\circ}$ ) by depositing silica or titania sols by dip-coating, followed by calcination [148,149] in the absence or presence of porogens (e.g., PEG [150], CTAB [151]). The removal of residual carbon-containing groups/porogens upon calcination leads to an increase in surface roughness, which drive the surface toward a superhydrophilic state.",
        "category": " Results and discussion"
    },
    {
        "id": 20,
        "chunk": "# 4.1.2. Spin-coating deposition  \n\nSpin-coating deposition makes it possible to prepare anti-fogging coatings based on semi-interpenetrating polymer networks (SIPNs) with frost-resistance [152] and even anti-bacterial/anti-viral activity [153]. For example, Zhao and colleagues [154] prepared SIPNs based on random copolymers of poly(DMAEMA-co-MMA) within a network of UV-cured PEGDMA with anti-fogging/anti-bacterial features. Partial quaternization of DMAEMA via SN2 (substitution nucleophilic bimolecular) (Fig. 13a) using 1-bromoundecane ( $5\\mathrm{mol}\\%$ in the copolymer, “SIPN-Q-5”) yielded coatings providing glass samples, not only with remarkable optical properties $(\\mathrm{T_{average}}>90\\%$ and capacity to prevent surface fog (Fig. 13b-e), but also with very high killing efficiency against E. coli and S. epidermidis (5-log reduction) (Fig. 13f). Nam et al. [155] developed a two-step process consisting of the deposition of functionalized PEG-containing polymers with pendent polymerizable norbornene (NB) groups by spin-coating, followed by immersion in a solution of Grubb's catalyst to fabricate optimal fog-resistant glasses $\\mathrm{(NB}\\leq30\\mathrm{mol}\\%$ ) with water-attracting features. Unlike SIPNs, no UV light nor heat was required to induce crosslinking. Anti-fogging composite coatings based on a bilayer configuration with enhanced mechanical properties can be fabricated by spin-coating deposition [156,157]. Films consisting of a bottom layer (“primer”) of colloidal $\\mathrm{SiO}_{2}$ ( $30\\mathrm{wt\\%})$ embedded in cross-linked network of dipentaethritol hexaacrylate, and a top layer containing HEMA and “Tween- ${\\cdot20}^{\\prime\\prime}$ have shown encouraging results (Fig. 14a) [158]. The addition of $10\\mathrm{wt\\%}$ of “Tween- ${\\cdot20\"}$ to the coating formulation, resulted in superhydrophilic coatings with fully adherence to  \n\n![](images/023d149dee67071e444dee3cdc6cfdf612e2e7eea3490eb1bb02b1db4f5bbc06.jpg)  \nFig. 11. (a) Schematic of the in situ nanopressing process. (b) SEM images of (i) SNs/polymer/PET, and (ii) ISNW20-SNs/polymer/PET, ISNW20: 20 washing cycles. (iii) Digital images exhibiting the antifogging property of blank (lower part) and ISNW20-SNs/polymer coated (upper part) PET, respectively; (iv) Transm ion spectra of blank PET, polymer/PET, SNs/ polymer/PET, ISN-SNs/polymer/PET, ISNW20-SNs/polymer/PET, and ISNW120-SNs/polymer/PET, respectively. (c) (i) SEM image of the 2HSNs/polymer thin film coated glass, (ii) TEM image of the HSNs. (iii) Digital images exhibiting the antifogging properties of 2HSNs/polymer coated glasses (up part) and blank glasses (lower part). (iv) Transmission spectra of blank glass and glasses coated, respectively, by polymer, 1HSNs/polymer, 2HSNs/polymer, and 3HSNs/polymer. The best anti-fogging configuration is shown in a red rectangle. Figures and graphics reprinted with permission from references [146-147].  \n\nPMMA substrates and long-lasting fog-free effect $(>~1$ year) (Fig. 14b).  \n\nAs in dip-coating deposition, the spin-coating process provides a facile way to build up inorganic coating on flat substrates, employing “building bricks” such as solid and hollow nanoparticles, microspheres, and nanorods, among others [159]. Here, the idea is to procure antifogging activity by designing surfaces endowed with hierarchical topography. With this goal in mind, Shan and colleagues [160] developed 400-nm-thick coatings made up of a thin film of $\\mathrm{Cu-Bi}_{2}0_{3}$ covered with MPSNPs, by combining the spin-coating technique with the sol-gel method. Anti-fogging $\\mathsf{M P S N P S}/\\mathsf{C u{-}B i_{2}O_{3}}$ films $\\mathrm{\\Cu}{:}\\mathrm{Bi}_{2}\\mathrm{O}_{3}$ molar ratio $\\c=$ 5) performed adequately when exposed to humid air after being cooled in a freezer $(-18^{\\circ}\\mathsf C)$ and were able to degrade methyl orange and stearic acid upon exposure to UV light ( $1\\mathrm{\\mw\\cm^{-2}}$ ). Mesoporous $\\mathrm{SiO_{2}/B i_{2}O_{3}/T i O_{2}}$ triple-layered thin films prepared on glass slides using a simple sol-gel/spin-coating approach showed similar results in terms of photocatalytic response and anti-fogging performance [161]. Silica- and titania-based coatings with enhanced wetting behavior can be prepared in ways other than those involving the deposition of building blocks. Similar to dip-coating deposition, particular emphasis has been placed on the in situ generation of nanopores upon calcination, for example, by using porogens [162] or surfactants [163]. Regardless of the adopted strategy, the principle is simple: the infiltration water drops into the nanoporous network drives the anti-fogging phenomenon (spreading mechanism). Alternatively, Budunoglu and collaborators [164] fabricated $135\\mathrm{-nm}$ -thick $\\mathrm{SiO}_{2}$ films with tunable porosity on glass samples using “ormosil” (organically modified silica) gels, which were prepared via hydrolysis and condensation of TEOS and methyltrimethoxysilane (MTMS). The pore size was tuned by changing the TEOS/MTMS volume ratio in the sol-gel mixture. A rational commitment between the “seethrough” property, mechanical durability, and optical clarity (Taverage $>95\\%$ , in the visible range) was met for a TEOS/MTMS volume ratio of 3:2. Despite the conceptual simplicity behind the spin-coating technique, the feasibility of fabricating anti-fogging coatings with hierarchical surface features similar to those observed in the compound eyes of insects have been recently demonstrated. For example, Sun and colleagues [99] designed fog-free surfaces by depositing on glass samples fly-eye bioinspired ZnO microspheres (Fig. 6ai). Following hydrophobization with 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane, coated glasses prevented water drops from accumulating on the surface when placed in an artificial fogging chamber for 2 min at a tilting angle of $10^{\\circ}$ ( $\\mathsf{W C A}=162.2^{\\circ}$ and $S A\\approx3^{\\circ}$ ). Zhang and collaborators [165] reported a straightforward method involving sol-gel process and spincoating deposition to produce films with moth compound eye-like features using a mixture of MPSNPs containing surfactants and $\\mathrm{SiO}_{2}$ sol. Finally, Li’s group [166] very recently reported on the fabrication of coatings with water- and oil-attracting features made of $\\mathrm{Cu}_{3}\\mathrm{SnS}_{4}$ , a ternary semiconductor. Following annealing in $\\mathsf{N}_{2}$ at $500^{\\circ}\\mathrm{C},$ , spin-coated glasses $\\mathrm{\\DeltaR_{rms}}=0.432\\mathrm{\\nm}\\mathrm{\\Omega}$ ) displayed superamphiphilicity, as revealed by a WCA and OCA below $1^{\\circ}$ , and a band-gap $(1.74\\ \\mathrm{eV})$ compatible with applications in the field of photovoltaic cells (see Section 5.2).  \n\n![](images/c52bef8c5d54cdd0a5e7c9314aa5fb2b4eb2d0cebcfc966ccfe700956c3dbc69.jpg)  \nFig. 12. (a) The in situ synthesis mechanism of $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ nanospheres. (b) Contact angle of the blank substrate, substrate with $\\mathrm{SiO}_{2}$ particles, and substrate with $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ nanospheres. (c) The anti-fogging property of the samples. “In situ growth of $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ nanospheres on glass substrates via solution impregnation for antifogging”, Liu, F.; Shen, J.; Zhou, W.; Zhang, S.; and Wan, L., RSC Adv., Volume 7, Issue 26, 2017, Pages 15992-15996. Published by The Royal Society of Chemistry.  \n\n![](images/a815d24ca159b381a79807146077572424540a48f0fbec1de8d6e8216cdc98f5.jpg)  \nFig. 13. (a) Schematic illustration of partial quaternization of poly(DMAEMA-co-MMA). Photos of different samples: (b) control glass and (c) SIPN-Q-5, which were first stored at ${}-20^{\\circ}{\\mathsf{C}}$ for $30\\mathrm{min}$ and then exposed for 5 s to ambient lab conditions $(\\sim20^{\\circ}C$ , $50\\%$ RH). Light transmittance at normal incident angle for various samples: (d) as prepared and (e) 5 s under ambient condition $(\\sim20^{\\circ}C$ , $50\\%$ RH) after being stored at ${}^{-20^{\\circ}\\mathsf{C}}$ for $30\\mathrm{min.}$ . (f) Zone-of-inhibition test result of (a) SIPN-Q-5 and (b) SIPN-Q-10 in a cultured lawn of E. coli. “SIPN-Q-X”, X: x mol% in the copolymer of quaternized DMAEMA. Reprinted with permission from “Dual-functional antifogging/antimicrobial polymer coating”, Zhao, J.; Ma, L.; Millians, W.; Wu, T.; and Ming, W., ACS Appl. Mater. Interfaces., Volume 8, Issue 13, 2016, Pages 8737-8742. Copyright 2018, American Chemical Society.  \n\n![](images/e1b5e192824dc8a5ca6f134016b94fae3654607d7a469dcd7831f508cb0c0e99.jpg)  \nFig. 14. (a) Bilayered anti-fogging coating. (b) Steam anti-fogging tests of coatings in AF10 after 1 year in service. Reprinted with permission from “Preparation of water-resistant antifog hard coatings on plastic substrate”, Chang, C.-C.; Huang, F.-H.; Chang, H.-H.; Don, T.-M.; Chen, C.-C.; and Cheng, L.-P., Langmuir, Volume 28, Issue 49, 2012, Pages 17193-17201. Copyright 2018, American Chemical Society.",
        "category": " Results and discussion"
    },
    {
        "id": 21,
        "chunk": "# 4.1.3. Layer-by-layer deposition  \n\nThe layer-by-layer (LbL) deposition is a straightforward coating technique suitable for the fabrication of anti-fogging coating based on multi-layer structures. This bottom-up approach involves sequential assembling of thin layers by dipping the sample into different solutions, followed by rinsing cycles. In general, the coating's robustness is ensured either by electrostatic interactions or by covalent and non-covalent interactions between adjacent layers, namely, hydrogen, hemiacetal, and ester bonds. In the last five years, various research groups have demonstrated that the incorporation of natural polymers, such as carboxymethyl cellulose (CMC), chitosan (CHI), and other polysaccharides [167–169]; and synthetic polymers, such as polyvinyl and polyacrylic compounds [170–174], into anti-fogging formulations can be successfully attained via LbL. For example, Spiroiu's group [175] fabricated anti-fogging layers with WCA exceeding $90^{\\circ}$ , based on selfassembled structures of CHI and sodium lauryl ether sulfate micelles, while Lee's group [176] developed zwitter-wettable coatings comprising a hydrophilic bottom layer of $(\\mathrm{CHI}/\\mathrm{CMC})_{30}$ capped with three hydrophobic (CHI/Nafion) bilayers $(\\mathsf{W C A}\\approx110^{\\circ}.$ ). As did Shibraen's and Cohen's groups [169,170], these research groups considered the water-absorbing characteristics of these coatings to account for the observed anti-fogging performance and, in some cases, the frosting delay capacity (percolation mechanism) [170,171]. Sun and colleagues [177] designed anti-fogging films with oil-repellent features via the assembly of hyaluronic acid (HA) and branched poly(ethylenimine) (bPEI) and subsequent hydrophobization with perfluorooctanesulfonic acid potassium salt (PFOS). It was found that glass and plastic lenses coated with PFOS- $({\\mathrm{HA}}/{\\mathrm{bPEI}})_{50}$ films were able to heal cuts of $80\\upmu\\mathrm{m}$ in width after $5\\mathrm{min}$ in water. Very recently, Shiratori et al. [178] demonstrated that films composed of multistacked layers of negatively charged PVA-PAA blends and positively charged PAH-PVA-PAA blends featured not only capacity to minimize fogging effects but also anti-reflective and antithrombogenic properties. The hierarchical topography observed in ((PAH-PVA-PAA)/(PVA-PAA)) $_{10}$ -coated glasses coupled with abundant OH groups per area unit translated to extreme wetting behavior (WCA $<5^{\\circ}.$ ) (Fig. 15a). Qualitative assessment of the anti-fogging performance revealed that ((PAH-PVA-PAA)/(PVA-PAA))10 coatings conferred noticeable visual characteristics to glasses when in contact with a moist environment at $35^{\\circ}\\mathsf{C}$ (Fig. 15b). Furthermore, the light transmission values $(\\mathrm{T}_{\\mathrm{average}}\\approx95\\%$ ) were greater than those of a bare glass (Taverage $\\approx91\\%$ , in $450\\mathrm{-}850~\\mathrm{nm}$ range) (Fig. 15c). In view of the FITR results, these anti-fogging coatings prevented the adhesion of fibrinogen, thus revealing a potential application as “anti-anticlotting” material (Fig. 15d,e).  \n\n![](images/4915658c313f57410863dfeb10d8e65095f99c9731d500d74c621e34c0ef5ec1.jpg)  \nFig. 15. (a) Scanning electron microscopy image of ((PAH-PVA-PAA)/(PVA-PAA))10 films. (b) Photography of a cooled glass slide with (left) and without (right) the coating in a highhumidity environment $(90\\%\\mathrm{RH})$ at $35^{\\circ}\\mathrm{C}$ after being cooled in a refrigerator to $<5^{\\circ}C.$ (c) Transmittance of films with different numbers of bilayers on glass substrates. Fourier transform infrared spectra of (d) bare silicon wafer substrate and fibrinogen and (e) ((PAH-PVA-PAA)/(PVA-PAA))10 films before and after contact with a fibrinogen solution. Reprinted with permission from “Antifibrinogen, antireflective, antifogging surfaces with biocompatible nano-ordered hierarchical texture fabricated by layer-by-layer selfassembly”, Manabe, K.; Matsuda, M.; Nakamura, C.; Takahashi, K.; Kyung, K. H.; and Shiratori, S., Chem. Mater.,Volume 29, Issue 11, 2017, Pages 4745-4753. Copyright 2018, American Chemical Society.  \n\n![](images/5cc6bbfd87b991d9f02a8f1477a9c07b4954a31330a9c5f1dd187f617c89cd93.jpg)  \nFig. 16. LbL strategies for the deposition of inorganic materials used in anti-fogging coatings. C: Carbon (template), MPSNPs: Mesoporous silica nanoparticles, NS: Nanosheets, PC: Polycarbonate (template), PDDA: Poly(diallyldimethylammonium chloride), PSS: sodium Poly(4-styrenesulfonate), SSNPs: Solid silica nanoparticles. Figures reprinted with permission from references [183,185,188,193].  \n\nRegarding inorganic anti-fogging layers, studies carried out by various research groups in the last seven years show that, solid and mesoporous $\\mathrm{SiO}_{2}$ nanoparticles, i.e., SSNPs and MPSNPs, can be assembled in three different ways [179–181]. The first one involves combining SSNPs with nanosheets (Fig. 16a). In this context, worthy of mention are the studies conducted by Byeon and colleagues [182,183], who designed coatings with luminescent/anti-fogging features by assembling nanosheets of RE-doped gadolinium hydroxides ( $\\boldsymbol{\\mathrm{RE}}=\\boldsymbol{\\mathrm{Eu}}$ , Tb, and Dy) with SSNPs. Following annealing at $500{-}600\\ ^{\\circ}\\mathrm{C},$ the resulting $(\\mathrm{Gd}_{2}\\mathrm{O}_{3}{\\mathrm{:RE/SSNPS}})_{\\mathrm{n}}$ coatings $\\mathbf{\\dot{\\zeta}}n=7\\mathbf{-}9$ , 30) prevented fogging via spreading mechanism $(\\mathsf{W C A}<5^{\\circ}).$ ). Depending on the dopant, the coated glasses featured efficient red (Eu), green (Tb), and blue (Dy) light emissions when illuminated with light of $254~\\mathrm{nm}$ . In a similar manner, stacking of reduced graphene oxide (RGO) nanosheets with nanoparticles of $\\mathrm{SiO}_{2}$ [184] or $\\mathrm{TiO}_{2}$ [77] has also been carried out to produce fog-resistant coatings with high specific surface area. The second way to prepare anti-fogging coatings with hierarchical porosity involves using “building blocks”, such as raspberry-like [100,185–187] and mulberry-like [89,188,189] nanospheres, which are synthesized prior to the deposition process by a judicious assembly of nanospheres (Fig. 16c,d,f,g). Many other research groups have followed a protocol similar to that depicted in Fig. 16c,d,f, to elaborate super wettable surfaces with hierarchical roughness, using only nanoparticles of $\\mathrm{SiO}_{2}$ [106,107,179,190], $\\mathrm{TiO}_{2}$ [74,88], $Z\\mathrm{r}0_{2}$ [191], $\\mathsf{A l}(0\\mathsf{H})_{3}\\mathsf{-M g}(0\\mathsf{H})_{2}$ as building units or PDDA‑sodium silicate complexes [192]. The third way to produce hierarchically rough anti-fogging surfaces is based on the assembly of mesoporous silica nanoparticles (MPSNPs) according to the protocol depicted in Fig. 16b [193–196]. On the other hand, several studies have shown that anti-fogging activity comparable to the one obtained these ways can be attained, without the need for calcination or annealing post-treatments [197,198] coupled, in some cases, with a reduction in the number of deposition cycles. For example, Sun and collaborators [199] evidenced that only three deposition cycles of MPSNPs $(\\sim50\\ \\mathrm{nm})$ ) alternating with PDDA sufficed to retain transparency when coated polycarbonate was exposed under very humid conditions. Analogously, Guo et al. [200] used the LbL assembly technique to produce fog-free films consisting of discrete layers of poly(ethylenimine) (PEI) and PSS containing clusters of calcium silicate hydrates (CSH). Interestingly, coatings integrated by multistacked layers of $z_{\\mathrm{{nO}}}$ nanoparticles (NP)/nano-flowers (NF) and PAA proved to not only be effective in eliminating the effects of condensation but also in blocking UV light [201] and killing bacteria [180]. Notable capacity to block UV light was also noticed in a multistack configuration consisting of discrete layers of PEI and CMC-modified $\\mathrm{TiO}_{2}$ nanoparticles recently prepared by Li and collaborators [202]. Further to this, $(\\mathrm{PEI}/\\mathrm{CMC@TiO_{2}})_{15}$ coatings were found to delay aging of PET substrates while conferring them anti-fogging performance, because of their superhydrophilic nature.",
        "category": " Results and discussion"
    },
    {
        "id": 22,
        "chunk": "# 4.1.4. Physical and chemical vapor deposition  \n\nSputtering methods such as RF magnetron sputtering, and evaporation methods such as electron beam deposition has proven to be suitable to design nanostructured anti-fogging inorganic coatings, with high deposition rates, excellent adhesion, and uniformity [203,204]. For example, Kwak and colleagues [205] reported a two-step process to fabricate ZnO-based anti-fogging coatings consisting of the deposition of a $z_{\\mathrm{{nO}}}$ seed layer on glass samples by RF sputtering, and subsequent growth of ZnO nanorods using ammoniacal solutions of zinc nitrate hexahydrate. Because of a light transmission as high as bare glass $\\approx90\\%$ in the $400{-}700~\\mathrm{nm}$ range) and the ability to block light below $370\\mathrm{nm}$ as in [180,201,202], these surfaces hold promise for fenestration purposes. ITO nanorods prepared by RF magnetron sputtering, followed by in-air annealing at $250^{\\circ}\\mathrm{C}$ have shown to endow glass samples with satisfactory anti-fogging and self-cleaning properties [206] (Fig. 17a,b). Coatings met extreme wettability $(\\mathsf{W C A}<1^{\\circ}),$ with sputtering times $>40$ min, because of the increase in size of nanorods (Fig. 17c). No fogging was observed in the samples treated for $60\\mathrm{min}$ under an aggressive cold fog test a − $20~^{\\circ}\\mathrm{C}$ (Fig. 17d). Following functionalization with 2H-perfluorodecyltrichlorosilane, the asprepared surfaces were easy to clean, as a drop of green powder phosphor lying on the surface was easily removed when water was added, leaving no remnant. (Fig. 17b).  \n\nRF magnetron sputtering makes it possible to build multifunctional $\\mathrm{TiO}_{2}$ -based configurations showing tremendous potential in smart window applications, as observed in glasses covered with a $\\mathrm{TiO}_{2}$ (anatase)/ $\\mathsf{V O}_{2}$ (monoclinic) $\\slash\\mathrm{{IiO}}_{2}$ (rutile) tri-layered film [207] or with a multistacked $\\mathrm{TiO}_{2}$ (anatase) $/\\mathrm{Si}/\\mathrm{Ag(Cr)/TiN_{x}}$ structure [208]. Using electron beam evaporation, Eshaghi and collaborators [209] developed a multistack configuration consisting of discrete layers of $\\mathrm{SiO}_{2}$ and $\\mathrm{TiO}_{2}$ that proved to be effective in preventing condensation effects on glass. In the same vein, Palmisano and colleagues [210] demonstrated the feasibility of depositing smooth $\\mathrm{TiO}_{2}$ coatings with better anti-fogging and self-cleaning performances than the ones observed in a commercial anti-fogging glass (Pilkington Activ™ glass). Using the CVD technique on glass samples, Chen and collaborators [211] deposited $\\mathrm{SiO}_{2}$ coatings with a regular convex nipple structure employing ammonia-catalyzed sol-gel solutions of TEOS. Even though all of the treated samples remained fog-free when placed over hot water or cooled at $-18^{\\circ}C$ the best optical properties (Taverage $\\approx95\\%$ in the $400{-}800~\\mathrm{nm}$ range)  \n\n![](images/5eb92920379673a3ee9ecf2fe86c0bde48f04529b69219be5b206559d5a26f86.jpg)  \nFig. 17. Schematic illustration of the fabrication procedures for preparing a multifunctional ITO nanorod film: (a) superhydrophilic ITO nanorods $\\angle W C A<1^{\\circ}$ ) displaying anti-fogging behavior when exposed to a humid environment $(\\mathrm{RH}>80\\%)$ after storage at $-20\\ ^{\\circ}\\mathsf C,$ and (b) superhydrophobic ITO nanorods $(\\mathsf{W C A}=172.1^{\\circ}$ , $S\\mathbb{A}\\mathfrak{n}\\mathbb{0}^{\\circ}$ ) featuring self-cleaning activity. (c) WCA of the post-annealed ITO nanorod films on glass substrates as a function of the growth time. The insets show the water CAs of a bare glass substrate and of an ITO nanorod film grown on a glass substrate for $60~\\mathrm{{min}}$ . (d) Top- and side-view SEM images of the ITO nanorod film grown on a glass substrate for $60~\\mathrm{{min}}$ . Reproduced from “Fabrication and characterization of large-scale multifunctional transparent ITO nanorod films”, Park, H. K.; Yoon, S. W.; Chung, W. W.; Min, B. K.; and Do, Y. R., J. Mater. Chem. A, Volume 1, Issue 19, 2013, Pages 5860-5867. Copyright 2018, with permission of The Royal Society of Chemistry.  \n\n![](images/e099837ce7aa5f4217ab0eddea4e576098d67862b98e8286e65b849063ebcf8c.jpg)  \nFig. 18. (a) Schematic drawing of the synthesis and hydrogen-bond-driven stabilization of titanate nanobelts. (b) Schematic illustration of the electrophoretic deposition process to prepare a TNB/FAS film. (c) SEM image of the as-prepared superhydrophobic TNB/FAS film (2 min). The inset image shows water droplets on the transparent TNB/FAS film on ITO glass. (d) Time sequence of the self-cleaning process on the superhydrophobic coating with low water adhesion. (e) Water droplet on the superhydrophilic TiO2 film. (f) Photograph of an ITO substrate deposited with superhydrophilic coatings (bottom) and a control ITO substrate without any coating deposition (upper) taken from a refrigerator $(-4^{\\circ}C)$ to the humid laboratory air (ca. $50\\%$ RH). Reproduced from “Transparent superhydrophobic/superhydrophilic TiO2-based coatings for self-cleaning and anti-fogging”, Lai, Y.; Tang, Y.; Gong, J.; Gong, D.; Chi, L.; Lin, C.; Chen, Z.; Liu, M. J.; Zheng, Y. M.; Zhai, J.; et al., J. Mater. Chem., Volume 22, Issue 15, 2012, Pages 7420-7426. Copyright 2018, with permission of The Royal Society of Chemistry.  \n\nwere noticed in glasses treated for $^{10\\mathrm{h}}$ . Shoji et al. [212] prepared silicon resin thin films on PC substrates with tunable hydrophobic/hydrophilic features using a low-pressure RF plasma. When plasma polymerization was performed in $0_{2}/\\mathrm{HCOOH}$ atmosphere under a power input between 50 and $150\\mathrm{W}$ , the coated PC displayed extreme wettability $(\\mathsf{W C A}<5^{\\circ})$ ) and remained fog-free when exposed to steam, breath, and room conditions after cooling at lower temperature.",
        "category": " Results and discussion"
    },
    {
        "id": 23,
        "chunk": "# 4.1.5. Electrochemical deposition  \n\nMeroni and colleagues [213] reported on the feasibility of an electrochemical method (potentiostatic deposition), similar to that employed by Patel et al. [214], to deposit several layers of $\\mathrm{TiO}_{2}$ on glass. Following application of $3.6~\\mathrm{V}$ for $60\\ s$ to a glass sample immersed in a $\\mathrm{TiO}_{2}$ sol, the resulting crack-free $\\mathrm{TiO}_{2}$ coatings were fully wettable but degraded transparency of glass substrates, as supported by the observed decrease in the average transmittance from $92\\%$ (uncoated glass) to approximately $75\\%$ in the visible region. In addition, the anti-fogging property was consistent with a decrease in WCA from 40 to $0^{\\circ}$ following exposure to UV light $(30\\mathrm{\\mW\\cm}^{-2}$ ). In contrast, $\\mathrm{TiO}_{2}$ coatings with nanofiber morphology fabricated by Tricoli's group [92] by electrospinning, displayed non-UV-activated anti-fogging features. Following thermal treatment at $500~^{\\circ}\\mathrm{C}$ , the resulting $\\mathrm{TiO}_{2}$ nanofibers of $200~\\mathrm{{nm}}$ in thickness provided glasses with excellent capacity to avoid blurry view when exposed to vapor, because of the great amount of hydroxyl on the surface (specific area $=106\\ \\mathrm{\\bar{m}}^{2}\\ \\mathrm{g}^{-1}$ , WCA $\\mathit{\\Theta}<10^{\\circ}$ ), as well as acceptable light transmission $(\\mathrm{T}_{\\mathrm{max}}~\\approx~93\\%)$ for incident light of 400 and $600\\ \\mathrm{nm}$ . Similarly, films composed of $\\mathrm{TiO}_{2}$ nanobelts were shown to be superhydrophilic with no previous UV exposure [93]. In this instance, titanate nanobelts (TNB), which were synthesized via a hydrothermal method, were deposited on ITO glass via electrophoretic deposition, and then functionalized with 1H,1H,2H, 2H-perfluorooctyltriethoxysilane (FAS) (Fig. 18a,b). When the functionalization time was 2 min, the resulting wetting behavior of the FAS-treated surfaces $(\\mathsf{W C A}\\approx156.2^{\\circ}$ and ${\\bf S}{\\bf A}\\approx8.6^{\\circ}$ ) led to the easy cleaning against yellow nitrogen-doped titanate powder (Fig. 18c,d). Upon calcination at $500~^{\\circ}\\mathrm{C},$ , a drastic shifting toward a super wettable state $(\\mathsf{W C A}\\approx0^{\\circ}$ ) was noticed, because of the removal of the hydrophobizing agent and the conversion of TNB into porous $\\mathrm{TiO}_{2}$ (anatase) (Fig. 18e). Under fogging conditions, the $\\mathrm{TiO}_{2}$ -coated glasses exhibited higher transmissivity than did uncoated ones (Fig. 18f).",
        "category": " Results and discussion"
    },
    {
        "id": 24,
        "chunk": "# 4.1.6. Others  \n\nThe techniques mentioned above cover the most common bottomup approaches for producing anti-fogging surfaces; however, other not less important ones have not been addressed here. These include: solvent casting methods [215–217]; bar coating methods [218–223]; spray coating techniques [91,224–226]; and multi-step approaches [227,228].",
        "category": " Results and discussion"
    },
    {
        "id": 25,
        "chunk": "# 4.2. Top-down processing  \n\nTop-down processing is based on the removal of material from a starting sample either to increase surface roughness or to create fine patterns, and thus drive the surface toward anti-fogging property.",
        "category": " Results and discussion"
    },
    {
        "id": 26,
        "chunk": "# 4.2.1. Dry and wet etching methods  \n\nIn dry etching, the sample is subjected to either high energy particles (e.g., electrons, X-rays), ions, or both; while in wet etching, the sample is dipped in an acid or in an alkaline solution for a certain period of time to “carve” the surface. In wet etching, the surface morphology, the rate at which the material is removed, and the resulting optical properties can be tailored, by varying certain experimental parameters such as the temperature, the concentration of reactive species, and the etching time [229–231]. Bearing this in mind, He et al. [229] elaborated dual anti-fogging/anti-reflective glasses $(\\mathsf{W C A}=4.3^{\\circ}$ ), with nanoflake-like surface features using a liquid alkali etching ( $5\\mathrm{g}\\mathrm{L}^{-1}$ of NaOH, $85~^{\\circ}C.$ ). A similar approach was reported by Myoung and colleagues [230]. In this instance, glass samples with variable $w t\\%$ of $\\mathsf{N a}_{2}0$ were dipped in KOH solutions at $95^{\\circ}C$ for 4, 12, and $24\\mathrm{h}$ (Fig. 19a–c).  \n\nEven though hydrophilicity was shown to increase with the etching time, at least $^{4\\mathrm{~h~}}$ of etching treatment were required to obtain “A” glasses with resistance to fogging (Fig. 19d). Furthermore, this treatment increased the maximum transmittance of “A” glasses from $\\approx90$ to $97.7\\%$ (at $630\\ \\mathrm{nm}$ ) due to a concomitant variation in size of nanoflake-like structures (Fig. 19e). Aqueous ${\\mathsf{N a H C O}}_{3}$ solutions have also shown an ability to “chisel” glass surfaces to produce sponge-like structures with a notable capacity to alleviate fogging effects [231].  \n\n![](images/58c7a8cae1e52211a3b196609d76d35c5384473c1d5d05d33f34f9ff1445845f.jpg)  \nFig. 19. SEM images of glasses etched for different periods of time (4, 12, and 24 h): (a) glass “A” (27.42 wt% of $\\mathrm{\\tilde{Na}_{2}O}$ ), (b) glass “B” (24.08 wt% of $\\mathbf{\\dot{Na}}_{2}0$ ), and (c) glass $\"C\"$ (0.35 wt% of $\\mathrm{\\DeltaNa_{2}O}^{\\prime}$ . (d) Transmittance spectra of glass A before and after etching (KOH 1M) at different etching times, and (e) anti-fogging performance of etched $\"A\"$ glasses (4 h) when cooled at - ${}-10^{\\circ}{\\mathsf{C}}$ and exposed thereafter to steam (right: before etching and left: after etching). Reproduced from “A multifunctional nanoporous layer created on glass through a simple alkali corrosion process”, Xiong, J.; Das, S. N.; Kar, J. P.; Choi, J.-H.; and Myoung, J.-M., J. Mater. Chem. Volume 20, Issue 45, 2010, Pages 10246-10252. Copyright 2018, with permission of The Royal Society of Chemistry.  \n\nYao et al. [232] fabricated fog-free glasses in a sequential approach consisting in chemical dry etching using a $\\mathrm{H}_{2}\\mathrm{SiF}_{6}$ -containing vapor $(\\leq20~^{\\circ}\\mathrm{C})$ , annealing at $720^{\\circ}\\mathrm{C}$ for $135\\ s,$ and low-pressure $0_{2}$ plasma treatment for $25{\\mathrm{~min}}$ . On the other hand, etching can be used to “activate” surfaces prior to coating deposition. Here, the goal is to ensure the adherence of anti-fogging coatings on the substrate to prevent them from detaching when exposed to a humid environment or under normal cleaning practices. For example, Lam et al. [85] deposited $\\mathrm{TiO}_{2}/\\mathrm{SiO}_{2}$ bilayers on NaOH-etched and UVirradiated PC, while Yao and collaborators [233] dipped glasses, which were previously treated following the above-mentioned protocol [232], in a SSNPs solution $(20~\\mathrm{\\nm})$ to ensure the “seethrough” property. In another study, Di Mundo's group [234] conferred anti-fogging capability to PC films through a self-masked plasma etching and subsequent deposition of a superhydrophilic silica-like coating, using a low-pressure $\\ensuremath{\\mathrm{~\\textrm~{~~}~}}0_{2}/\\ensuremath{\\mathrm{Ar}}$ plasma fed with hexamethyldisiloxane (HMDSO).  \n\nEvidence shows that the above-illustrated etching methods do not allow for the design of surface structures with desired geometric order and well-defined shapes (e.g., subwavelength structures, SWSs). To overcome this drawback, reactive ion etching (RIE) has emerged as a promising tool due to its unique ability to etch with finer resolution, and higher aspect ratio than isotropic etching does [235]. For example, Lee and colleagues [236] tailored the wettability of borosilicate glass substrates by means of a self-masked RIE operating under controlled conditions, namely, $50\\mathrm{W}$ and $\\mathrm{CF}_{4}{:}0_{2}$ ratio of 4:1. When the etching time was $7\\mathrm{min}$ , the glasses became hydrophilic $(\\mathsf{W C A}=12.5^{\\circ}.$ ) in response to a concurrent formation of tapered SWSs with aspect ratios in the 1.5–2 range. Both the low WCA and the high surface energy $\\left(87.8~\\mathrm{mN}~\\mathrm{m}^{-1}\\right.$ ) substantiated the observed fog-free effect when the etched glasses were exposed to steam. Alternatively, Xu et al. [237] built up tapered conical structures (aspect ratio of 2.8) by reactive ion etching ( $100\\mathrm{W}$ and $\\mathrm{CHF}_{3}$ :Ar ratio $=2$ ), using a thin film of Ag nanoparticles as etching mask. As in the previous study, the judicious combination of the inherent hydrophilicity of $\\mathrm{SiO}_{2}$ with the nanohole egg-crate-like structure was behind the observed broadband optical transmissivity $(400-1400\\mathrm{nm}$ ) as well as the anti-fogging performance $(\\mathsf{W C A}\\approx0^{\\circ}$ ) of quartz slides. RIE in combination with bottom-up processing makes it possible to obtain nanostructured polymer-based anti-fogging coatings with outstanding optical performance. Of particular interest is the elegant strategy reported by Suh and collaborators [238] to fabricate super wettable glasses $(\\mathsf{W C A}<5^{\\circ})$ ). Their approach involved the deposition of an UV-curable polyurethane acrylate by rollpressing and subsequent self-masked RIE. Following the same idea, Sim et al. [239] elaborated anti-fogging layers with graded roughness (gradient-index anti-reflection coating, GIARC) using block copolymers of polystyrene (PS) and polydimethylsiloxane (PDMS) (i.e., PS-b-PDMS) as starting materials. Briefly, glass substrates were first coated with PSb-PDMS films and subjected thereafter to RIE to convert the copolymer into a nano-structured $\\mathrm{SiO}_{2}$ (Fig. 20a). Surface features such as roughness and porosity, as well as the optical properties of the resulting coating were found to depend on both the molecular weight of PDMS and its fraction in the copolymer (Fig. 20b,c). Optimized nanoporous silica films “SD55k” $\\mathrm{'}\\mathrm{f}_{\\mathrm{PDMS}}=9.1\\%$ , enabled an easy legibility of the letters behind the coated glasses when exposed to super-saturated water vapor at 90 $^{\\circ}{\\mathsf C}$ (Fig. 20d). This behavior was consistent with an average transmittance remaining almost unchanged at approximately $97\\%$ under the same conditions (Fig. 20e).",
        "category": " Results and discussion"
    },
    {
        "id": 27,
        "chunk": "# 4.2.2. Lithography  \n\nThis top-down approach makes it possible to design subwavelength structures, SWSs ( $\\leq100~\\mathrm{{nm}}$ ) with excellent precision and accuracy using photons, electrons, or ions. Park et al. [37] applied the so-called “orthogonal interference lithography” to fabricate periodic square arrays of tapered SWSs on silica samples with an aspect ratio of 5.5 and packing densities $>10^{6}\\mathrm{mm}^{-2}$ . The resulting surfaces simultaneously met anti-fogging performance, with a WCA ${\\approx}0^{\\circ}$ , and minimal reflection over a wide range of incident angles (0 to $80^{\\circ}$ ) in the visible and near-IR wavelengths. Mao and collaborators [240] very recently reported on the potential applicability of “direct laser interference lithography” (DLIL) for the manufacture of anti-fogging eyeglasses. A square periodic array of inverted nanocones made of polyurethane acrylate, which was fabricated in a sequential approach consisting of DLIL, dry etching, and UV replication process, has also generated worthwhile results [241]. Due to the superhydrophilicity $(\\mathsf{W C A}\\approx0^{\\circ}$ ) conferred by the SWSs with an aspect ratio $\\approx4$ , no fogging was observed when the nanotextured glasses were placed over saturated steam. Moreover, such glasses displayed remarkable light transmission $\\mathrm{'T_{average}}>95\\%$ , incidence angle of $0^{\\circ}$ ) over the $350\\mathrm{-}1400\\mathrm{nm}$ range. In another study, Duan and colleagues [242] combined a sol-gel/dip-coating method with DLIL to design non-UV-activated anti-fogging $Z\\mathrm{r}0_{2}$ coatings with a grooved or a mastoideus surface. Soft lithography, on the other hand, allows for the preparation of polymer-based anti-fogging coatings with micro/ nanostructured surface features using mechanical procedures, such as stamping and molding. Zheng's group [104] fabricated anti-fogging surfaces with water-repellent and icing-delay characteristics by planting onto poly(vinylidene difluoride) microratchets, which were obtained by the heat-pressing pattern-transfer technique, nanohairs of ZnO (Fig. 6bii). Following hydrophobization with heptadecafluorodecyltripropoxysilane (FAS-17), water drops remained in a non-freezable state at $-5^{\\circ}\\mathsf{C}$ and rolled off the surface when exposed to breeze, because of the water-repellent properties of the surface $(\\mathsf{W C A}\\approx150^{\\circ}$ ). Epoxy micropillars arrays covered with $z_{\\mathrm{{nO}}}$ nanohairs fabricated by soft replication methods (“Bosch process”) and crystal-growth techniques have shown better results in terms of ice formation delay [102]. In this case, an icing delay time as high as 9839 s was noticed in the FAS-treated surfaces (an icing delay time of 7360 s at $-10^{\\circ}C$ was reported in the previous study), even though the contact angle was virtually the same $\\mathrm{^{\\prime}W C A}\\approx152^{\\circ}.$ ) (Fig. 6aiv).  \n\n![](images/80e592b40c5a6f5292ebf44a7dfa3d30afcd46dfed00873680ea8fa671540e43.jpg)  \nFig. 20. (a) Facile solution-based procedure for the preparation of the gradient-index anti-reflection coating (GIARC) based on Si-containing block copolymers. SEM images of a doublelayered GIARC consisting of (b) SD55k $\\mathrm{\\Delta}\\mathrm{\\cdot}\\mathrm{f_{PDMS}}=0.091$ ) and (c) SD43k $\\mathrm{^{\\prime}f_{P D M S}}=0.488\\mathrm{^{\\cdot}}$ . (d) Comparison of the anti-fogging properties of GIARC and a bare glass substrate. e) Changes in transmittance with the exposure time to water vapor. Reprinted from “Ultra-high optical transparency of robust, graded-index, and anti-fogging silica coating derived from Si-containing block copolymers”, Sim, D.; Choi, M.-J.; Hur, Y.; Nam, B.; Chae, G.; Park, J.; and Jung, Y., Adv. Opt. Mater. Volume 1, Issue 6, 2013, Pages 428-433. Copyright 2018, with permission from John Wiley and Sons.  \n\nAlthough lithography has great potential for the fabrication of intricate structures, surprisingly only a few research groups have focused their expertise toward developing anti-fogging films with topographical features similar to the ones found in insect's eyes [243–245]. For instance, moth eye-like nanostructures integrated by polydimethylsiloxane domes have been elaborated employing the lift-up softlithography technique (Fig. 6bi) [105]. Following deposition of SSNPs and subsequent treatment with monolayers of self-assembled fluoroalkylsilane (FAS), the resulting hydrophobicity $(\\mathsf{W C A}=155^{\\circ}$ and $S\\mathsf{A}=15^{\\circ}$ ) supported the rolling mechanism behind the observed anti-fogging activity. By means of sacrificial layer-mediated nanoimprinting (SLAN), Raut and collaborators [103] deposited on glass samples a moth eye-like structure made from a resin containing methacryloyloxypropyl polysilsesquioxane (Fig. 6av). Following treatment with 1H,1H,2H,2H-heptadecafluorodecyl methacrylate, optimal surfaces with ommatidial features of $20\\upmu\\mathrm{m}$ in diameter $(\\mathsf{W C A}\\approx151^{\\circ}$ and ${\\mathsf{C A H}}\\approx2^{\\circ}$ ) displayed very low average reflectance (ca. $4.8\\%$ ) and very fast transmittance recovery $(\\mathrm{T_{average}}=100\\%\\mathrm{in}\\approx10\\mathrm{s})$ ) after exposure to saturated steam. Without the need for hydrophobization posttreatments, moth eye-like nanostructures consisting in PMMA nanonipples covered with solid silica nanoparticles were also found to retain transparency under fogging conditions [246]. Despite an aspect ratio as low as 1, nanostructured surfaces reduced drastically glare and remained optically clear when exposed to moisture for $15~\\mathrm{min}$ due to their superhydrophilicity $(\\mathsf{W C A}=2^{\\circ}$ ).",
        "category": " Results and discussion"
    },
    {
        "id": 28,
        "chunk": "# 4.2.3. Template-assisted fabrication  \n\nGenerally speaking, template-assisted fabrication involves two basic steps. An anti-fogging solution is deposited into a micro/nanoporous material (template), allowing the solvent to evaporate. Afterwards, the template is selectively removed, yielding micro/nanostructured arrays or freestanding 3D structures. Following this protocol, Han and collaborators [247] recently developed a relatively complex strategy to elaborate biologically-inspired anti-fogging films. Here, butterfly's wing scales were used as a template to produce a $\\mathrm{SiO}_{2}$ film with multiscale hierarchical pagoda structures. The hierarchical surface roughness resulting in the significantly high surface density of the hydrophilic OH groups, translated to extreme wetting behavior. Coated glass samples featured excellent anti-fogging activity, as supported by the observed optical transparency $(\\mathrm{T}_{\\mathrm{average}}\\approx95\\%)$ under aggressive fogging conditions. Using the colloidal templating method, Vogel et al. [248] demonstrated the feasibility of preparing a $\\mathrm{SiO}_{2}$ -based periodic array of nanopores with tunable re-entrant geometry. Regardless of the pore size and the opening angle, which were changed by adjusting the TEOS/EtOH ratio in the starting sol-gel solution, all of the coated glasses exhibited extreme wetting behavior $(\\mathsf{W C A}\\approx0^{\\circ}$ ) following calcination at $500~^{\\circ}\\mathrm{C}.$ $\\mathrm{SiO}_{2}$ layers prepared from colloidal particles of $200\\ \\mathrm{nm}$ in diameter imparted superior anti-fogging capacity to glass slides.",
        "category": " Results and discussion"
    },
    {
        "id": 29,
        "chunk": "# 4.3. Surface functionalization and related techniques  \n\nIn addition to “top-down” and “bottom-up” processing, another way to confer anti-fogging performance to a given material consist in modifying its surface chemistry. Surface treatments such as plasma treatment [249] and ionic implantation [43] have amply demonstrated their effectiveness in conferring resistance to fogging to poorly wettable polymers such as polyethylene, polypropylene, and polyethylene terephthalate. The main reason for this relies on the formation of hydrophilic groups on the surface, such as OH, COOH, COH, CN, $\\mathsf{N H}_{2}$ , etc., wellknown for their favorable interaction with water drops (spreading mechanism) [250–252]. Worthy of mention are the studies conducted by Patel and collaborators [214,253], who prepared anti-fogging polyethylene terephthalate (PET) using low-pressure plasmas operating under a controlled $0_{2}$ gas atmosphere $(20\\ s c c m)$ ) (sccm: “standard cubic centimeters per minute). Following plasma treatment for $5\\mathrm{min}$ , PET films did become superhydrophilic (WCA went from 95 to $\\approx0^{\\circ}$ ) in response to a concurrent rise in the number of carbonyl-containing functionalities on the surface. Even though the plasma-treated PET retained transparency when placed over a cup of hot water, the hydrophilicity was found to degrade upon exposure to both dry and humid environments for 7 days. These authors also [214] conferred antifogging property to ITO glass, following application of $50\\mathrm{V}$ for $20\\mathrm{min}$ to an aqueous solution of ${\\mathrm{H}}_{2}{\\mathrm{SO}}_{4}$ were a ITO sample was immersed. These authors argued that the electrochemical oxidation of water yielded hydroxyl groups on the ITO surface, which explains why, WCA abruptly decreased from 80 to $0^{\\circ}$ . Although not prevented, hydrophilicity loss due to surface aging was slower than that observed with the plasma-treated PET films under the same fogging conditions. Alternatively, extremely wettable $(\\mathsf{W C A}<5^{\\circ})$ ) films of polydiethylene glycol bis(allylcarbonate) with resistance to fogging were prepared by implantation of $\\mathsf{A r}^{+}$ ions under very low $0_{2}$ pressure [43]. A pre-implantation treatment with $\\mathrm{He^{+}}$ ions was found to delay significantly the hydrophilicity loss, hence the occurrence of fogging. While the above-mentioned surface treatments hold great promise for the manufacture of agricultural and food packaging films with anti-fogging characteristics, the problem of surface aging remains unresolved. This fact may explain why the incorporation of surfactants appears to be gaining in popularity in this regard [254–256]. Surfactants are molecules consisting of two well-differentiated parts, namely, a hydrophobic tail and a hydrophilic head. In general the hydrophilic domain contains hydroxyl [257–263] or amine groups [264]. When incorporated to polymer formulations, these molecules migrate from the bulk to the film surface, where they dissolve in the condensed water, decreasing its surface energy. As a result, water drops wet evenly the surface and scattering events are mitigated [260]. According to Irustra [264] and Salmeron [265] the use of additives comes with two major problems. First, as long as a sufficient amount of surfactant dissolves in the condensate, the anti-fogging/ anti-dripping film will perform adequately; however, given that it takes a while for these molecules to migrate and dissolve in water, these films usually fog up when exposed to sudden temperature or humidity changes. Second, considering that surfactants are gradually washed away by the dripping water, the anti-fogging/anti-dripping performance deteriorates over time. Thus, controlling the migration rate of these molecules is crucial to retaining the anti-fogging performance long term. In general, the migration of surfactants can be slowed down if bonded to inorganic nanoparticles such as SSNPs [266–268] or if added to blends of hydrophilic grafted co-polymers with un-grafted ones [269]. To retain wetting features for longer periods of time, covalent grafting of “bulky” surfactants, also known as “graft copolymerization”, represents a feasible alternative to plasma and ionic implantation treatments, as well as the addition of surfactants per se[270,271]. The applicability of this surface treatment on low surface energy polymers is motivated by the fact that the steric hindrance prevents these molecules from hiding in the bulk, thus hampering surface aging. Voluminous surfactants such as monostearic acid monomaleic acid glycerol (MMGD), [272] glycerol monolauric acid monoitaconic acid diester (GLID) [273], trifluoroacetic acid allyl ester (TFAA) [274], maleic anhydride (MA) [275], or polyether pentaerythritol monomaleate (PPMM) [276] have been successfully grafted to the backbone of linear low-density polyethylene (LLDPE) without compromising its optical and mechanical properties.",
        "category": " Results and discussion"
    },
    {
        "id": 30,
        "chunk": "# 5. Application trends of anti-fogging surfaces  \n\nIn sectors of activity such as the medical, the photovoltaic, or the horticultural, the use of surfaces endowed with anti-fogging performance is on the rise and under perpetual development. In this section, some of the most relevant applications of these surfaces are briefly presented.",
        "category": " Results and discussion"
    },
    {
        "id": 31,
        "chunk": "# 5.1. Food industry  \n\nIn the horticultural sector, the presence of condensation inside greenhouses causes injury to produce (dripping water) [18,269] and favors the development of fungal diseases [254]. Further to this, the decline in sunlight passing through the greenhouse claddings due to the total internal reflection occurring at the water drop/air interface has also been reported to affect crop yield [4,5,277]. Far from being an irrelevant issue, the effects of condensation on light transmission have been studied extensively for ${>}20$ years. For instance, using different agricultural films, including polyethylene (PE), PE with IR-absorbing features, UV-stabilized PE, and double-layered PE films, Cemek and Demir [18] estimated an average loss in light transmittance between 5 and $17\\%$ for a 2-month testing period. Similar results were reported by Pearson and colleagues [278] (transmission $10s s\\approx13\\%$ and Geoola's group [279] (transmission $\\begin{array}{r}{\\mathrm{loss}=9\\mathrm{-}10\\%}\\end{array}$ ) with modified and unmodified PE films. In this context, the use of plastics containing anti-fogging/antidripping additives (Section 4.3) is more than welcome, as better light transmission translates to enhanced plant growth rates and more abundant crops. Commercial additives such as Atmer $\\cdot\\mathtt{m}_{A00}$ and Atmer $\\cdot\\mathrm{\\Delta}\\mathrm{m}_{103}$ (Uniquema Polymer Additives, Switzerland), Loxiol A4 Spezial (Emery Oleochemicals, Malaysia), Dyneon™ MM5935 EF (Dyneon LLC, USA), and AF0406PE (Tosaf, Israel) deliver proven anti-fogging/anti-dripping performance to the most commonly used cladding materials (e.g., PE, PP, PTFE, PVC, PS, and PC).  \n\nRegarding food packaging, plastic films used to pack freshly chopped meats or vegetables play two crucial roles: they help limit waste by displaying the content more attractively and provide protection, so that food remains safe to eat for a reasonable period of time. However, unless the package contains moisture absorbers (e.g., sorbitol, xylitol) or enables moisture to permeate, sudden changes in temperature results in a packed produce surrounded of condensation. Experience shows that consumers are less likely to purchase when the “seethrough” property is severely compromised. As in the case of greenhouse cladding materials, the incorporation of anti-fogging additives into polymeric films (e.g., PP [280–283], PTFE [284], LLDPE [254,285,286], and PLA [287]) represents the most cost-effective solution adopted thus far by the manufacturing sector to minimize the effects of condensation.",
        "category": " Results and discussion"
    },
    {
        "id": 32,
        "chunk": "# 5.2. Photovoltaic industry  \n\nSolar cells are electrical devices made of semiconductors that generate voltage when exposed to light [288]. It is widely known that silicon is the leading material in solar cell production; however, its use comes with a major problem: ${>}30\\%$ of the incident light is reflected because of its high refractive index. In addition to this, dust accumulation has been reported to contribute up to another $10\\%$ to overall nonabsorbed light [289]. Surprisingly, compared to existing literature on anti-reflective coatings for solar cells, few studies have addressed the issue of condensation, even though the formation of surface fog adversely affects the energy conversion efficiency of these devices. Indeed, according to Lu et al. [138] the scattering phenomenon provoked by water drops decreases the amount of photons reaching the cell surface, hence the ratio between the number of collected carriers and the number of all the incident photons, namely, the quantum efficiency. A reasonable approach to address this problem involves the use of coatings made of highly porous $\\mathrm{SiO}_{2}$ . These surfaces reduce contaminant adsorption and enable water drops to wet the surface [16,138,142]. For example, after covering the photoanodes of a high-performance solidstate dye-sensitized solar cell with SSNPs, Park and collaborators [16] observed an improvement in the photovoltaic efficiency of $5.9\\%$ in the presence of condensation. These anti-fogging coatings endowed with anti-reflective characteristics would not only improve the optical properties of future transparent solar cells but also their photovoltaic conversion efficiency, by enhancing light harvesting. Dual anti-fogging/ anti-reflective coatings with self-cleaning property, have also shown to further improve the performance of solar cells. In general, these coatings are made of $\\mathrm{TiO}_{2}$ or $\\mathrm{SiO}_{2}/\\mathrm{TiO}_{2}$ mixtures [290–292]. In addition to featuring resistance to fogging, the cell surface is cleaned at room temperature as a result of the photocatalytic activity (ROS species) and the “sweeping” effect of water (photoinduced superhydrophilicity).",
        "category": " Results and discussion"
    },
    {
        "id": 33,
        "chunk": "# 5.3. Medicine  \n\nIn light of the growing number of endoscopic procedures reported annually in developed countries (e.g., 15–20 millions in the US), it is an incontestable fact that camera-guided instruments have become indispensable surgeon's colleagues [293]. In these situations, where a sharply defined field of view is required for obvious reasons, surgeons must paradoxically struggle with the low-quality images provided by the endoscope camera. The root cause of the impaired surgeon's vision reflects the result of two factors acting together. namely, the soiling of endoscope lens caused by the physiological fluids, and the formation of surface fog induced by temperature and relative humidity differences between operating rooms and human body [294]. To recreate the view attained in an open surgery, surgeons are forced to periodically clean the lens in water or saline Unfortunately, constant intraoperative interruptions put the patient's health at risk, slow down the surgery's progress, and contribute to surgeon frustration. Indeed, several studies [295,296] have evidenced that an increase in the number of times that the endoscope is withdrawn led to the increases in both the estimated blood loss and the operative time. Longer operative times make financial costs for both hospital and patient skyrocket.  \n\nWithin this framework, the implementation of anti-fogging technology is key to ensuring a safe and a successful surgical procedure. Available strategies aimed at maintaining a clear operating field can be divided into four broad categories: endoscope lens warming, use of temporary anti-fogging coatings and modified endoscopes, and other defogging approaches [294]. Regarding those changing the morphology of water drops, anti-fogging strategies based on temporary coatings involve applying commercial solutions such as Covidien FRED [297–299], Betadine [300], Hibiscrub [297], and baby shampoo [297] on the endoscope lenses. FRED™ (Fog Reduction and Elimination Device) and Betadine™ are aqueous solutions: the first one contains isopropyl alcohol $(<15\\mathrm{wt\\%})$ and surfactants $(2\\mathrm{wt\\%})$ ; and the second one, well known for its antiseptic activity, contains povidone‑iodine $(10\\mathrm{wt\\%})$ . Cheaper alternatives such as the use of patients' saliva [301] or saline solutions [302], as well as rubbing the endoscope lens on viscera [303] have also proven to be suitable to mitigate fogging effects. Also, worthy of mention are the endoscopes incorporating lenses covered with permanent anti-fogging coatings. For instance, using the layer-by-layer assembly, Aizenberg and colleagues [293] coated a bronchoscope lens with solid silica nanoparticles embedded in a thermally cured polydimethylsiloxane resin. $100\\mathrm{-nm}$ -thick SSNPs/PDMS films endowed the lens with anti-fogging and blood-repelling characteristics. Ohdaira et al. [304,305] prepared $\\mathrm{TiO}_{2}$ -coated lenses using the spin-coating technique followed by silicone-sealing and post-treatment at $200^{\\circ}\\mathsf C$ for $10\\mathrm{min}$ . After $12{-}15\\mathrm{~h~}$ of exposure to UV light, the as-fabricated coatings displayed better anti-fogging performance than did heated or washed lenses [305], making it possible to perform surgery with no retraction of the laparoscope [304].",
        "category": " Results and discussion"
    },
    {
        "id": 34,
        "chunk": "# 5.4. Optical applications  \n\nFrom swimmers to surgical technicians to mining workers, dealing with fogged eyeglasses can be a challenging task. Indeed, this frustrating phenomenon usually forces the person to focus on wiping eyeglasses dry or wait for them to defog, putting under certain circumstances her/his safety at risk [306–311]. Protective eyewear fogging experienced by construction workers when laboring outside illustrates one among many obvious paradigmatic examples of surface fog formation, as it encompasses all of the favorable conditions to induce water condensation, namely, transitions between warm and cool environments, worker exertion, and tight eyewear. Even tough Mother Nature dictates that the fogging of eyewear must occur, human intervention can efficiently prevent it. For example, the fogging of surgical goggles can be reduced by applying a temporary anti-fogging solution called “Body Glove Fog Away” [312]. Permanent coatings of $\\mathrm{TiO}_{2}$ have been very successful in preventing condensation on mirrors [313–316]. In the same vein, coatings based on cellulosic ethers, have also shown to confer notable anti-fogging capability to a plethora of elements, including visors and transparent shields, sports goggles, safety glasses, face shields, and surgical masks, among others [317,318].",
        "category": " Results and discussion"
    },
    {
        "id": 35,
        "chunk": "# 6. Concluding remarks and outlook  \n\nAnti-fogging mechanisms and their link with recent progress in fabrication techniques toward anti-fogging property are discussed in length in this review. Anti-fogging surfaces with additional features such as self-healing, self-cleaning, and anti-bacterial properties as well as the main sectors of human activity making use of them, including food and photovoltaic industries and medical practice, are also addressed. Nevertheless, despite years of tremendous efforts and achievements made in the field of anti-fogging surfaces, some relevant challenges remain.  \n\nStandards applied in North America (e.g., CSA Z611-M86 [319] and ASTM F659–10/−06 [320,321]) and Europe (e.g., CEN EN 168 [322]) for guaranteeing (protective) eyeglasses to reliably perform under fogging conditions are quite limited and not necessarily adapted to everyday activities. In F659–10 and EN 168 standards, the sample is immersed in distilled water at room temperature $(23\\pm5^{\\circ}\\mathsf C)$ for $^{1\\mathrm{h}}$ and then placed over a water bath $(50.0\\pm0.5^{\\circ}\\mathrm{C})$ after being dried at room temperature ( $50\\%$ RH) for $\\geq12\\mathrm{~h~}$ . For a sample to be considered anti-fogging, the time required for the light transmittance to decrease to $80\\%$ of its initial value (non-fogged sample) must be lower than or equal to $30~\\mathsf{s}.$ In Z611-M86 standard, the sample is cooled at $-25~^{\\circ}C$ and exposed thereafter to ambient conditions ( $23^{\\circ}\\mathrm{C},$ $50\\%$ RH). Here, rather than measuring light transmission, the time it takes for a transparent substrate to defog is reported. The application of these standards is highly questionable when assessing fogging resistance of eyeglasses during day-to-day activities, for example, when taking a walk, when moving from a warm to a cold environment, when cooking in a steaming environment or even when breathing. In our opinion, developing a certification adapted to day-to-day activities would be certainly applauded.  \n\nAccording to Briscoe [5], Grosu [43], and Pieters [4] a WCA angle of ${<}40{-}50^{\\circ}$ is required for a surface to be anti-fogging; that said, several studies [111,114,119,121,152,172,176] disagree with this rule, as it is possible to prevent fogging effects despite WCA exceeding this cut-off value. The main reason for this lies in the fact that this rule only holds true for nonporous anti-fogging coatings whose surface features do not display time-dependent behavior. In addition to the contact angle, several factors related to fogging effects, such as the number and the size of water drops [99,109], surface rearrangement phenomena [170], as well as the capability of the coating to transport water molecules [175,176], must also be considered to establish a more robust antifogging criterion.  \n\nOn the other hand, designing of a “well-rounded” anti-fogging material is more than a simple adjustment in the morphology of water drops, as many other features, such as mechanical durability and optical properties, must also be considered. For example, the use of inorganic materials to elaborate anti-fogging coatings faces two major challenges, namely, the deposition on thermally sensitive materials and the problem of light reflection. Following coating deposition, it is standard practice to implement thermal treatments (e.g., calcination, annealing); however, high temperatures make it challenging to coat polymeric substrates because of thermal degradation concerns. In this regard, developing coating techniques adapted to thermally sensitive substrates would undoubtedly be welcome. Optical transparency is another critical parameter to consider when designing anti-fogging layers. The adjustment of the refractive indices of the coating and the substrate is of considerable relevance to minimize the reflection of light. This implies that the thickness of the coating must be equal to $\\lambda/4n_{c},$ where $\\uplambda$ is the wavelength of the incident light and $n_{c}$ is the refractive index of the coating [323]; and that the refractive index of the coating must be $n_{c}$ $={\\sqrt{n_{s}n_{a i r}}},$ where $n_{s}$ is the refractive index of the substrate and $n_{a i r}$ is the refractive index of the air $(n_{s}>n_{c})$ [324]. Simultaneously fulfilling these two design criteria is quite often more difficult than imagined.  \n\nDespite the plethora of materials and fabrications techniques employed thus far to design anti-fogging surfaces, bridging the gap between fundamental research and industry is a pending issue. Even though their large-scale fabrication is not particularly challenging, addressing the problem of mechanical durability is crucial to make it a reality. Experience shows that any surface is exposed to mechanical wear caused by rubbing during day-to-day use or by solvents under normal cleaning practices. In coated surfaces, temperature variations can lead to coating deformation or detachment because of the differences in thermal expansion coefficient between the coating and the substrate. Mechanical wear, temperature variations, and exposure to cleaning products may result in a deterioration of the anti-fogging performance over time. Thus, designing anti-fogging surfaces with abrasion resistance (e.g., durable self-healing properties) with optimal adherence to the substrate is key to ensuring a long service life once integrated in items, such as mirrors, eyeglasses, and home windows, that make our day-to-day living more comfortable.  \n\nAccording to recent studies, the future trend in this promising field points to unique anti-fogging surfaces exhibiting an optimal combination of features to cover a wide range of applications. For example, dual anti-fogging/anti-bacterial surfaces will likely be most welcome in endoscopic surgery, while anti-fogging surfaces endowed with selfhealing properties would find a niche of opportunity in swimming goggles, solar panels, or automobile windshields. Undoubtedly, anti-fogging surfaces would be welcomed in applications where a clear visualization of the liquid medium plays a crucial role. Such is the case, for example, with micro/nanofluidic devices and microreactors for chemical synthesis and cell culture. Another opportunity niche for anti-fogging surfaces can be found in fiber optics [325] as well as among amateur and professional photographers. We firmly believe that future development of anti-fogging technology will be based on two fundamental pillars, namely industrial research and the use of eco-friendly materials. Indeed, the development of less time-consuming and cost-effective fabrication techniques compatible with industrial manufacturing using anti-fogging materials coming from renewable sources is undoubtedly a pending issue. In conclusion, research focusing on fundamental aspects of anti-fogging surfaces is still necessary to make industrial and professional applications of anti-fogging technology a reality.",
        "category": " Conclusions"
    },
    {
        "id": 36,
        "chunk": "# Acknowledgements  \n\nThe authors thank Pascale Chevalier for her helpful advice concerning the redaction of this review. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada (G.L), PRIMA-Québec (G.L) and the Centre Québécois sur les Matériaux Fonctionnels (CQMF) (G.L.).",
        "category": " References"
    },
    {
        "id": 37,
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        "category": " References"
    }
]