Transparent Photothermal Metasurfaces Amplifying Superhydrophobicity by Absorbing Sunlight

Imparting and maintaining surface superhydrophobicity is receiving significant research attention over the last several years, driven by a broad range of important applications and enabled by advancements in materials and surface nanoengineering. Researchers have investigated the effect of temperature on droplet-surface interactions, which poses additional challenges when liquid nucleation manifests itself, due to ensuing condensation into the surface texture that compromises its anti-wetting behavior. Maintaining surface transparency at the same time poses an additional and significant challenge. Often, the solutions proposed are limited by working temperatures or are detrimental to visibility through the surface. Here we introduce a scalable method employing plasmonic photothermal metasurface composites, able to harvest sunlight and naturally heat the surface, sustaining water repellency and transparency under challenging environmental conditions where condensation and fogging would otherwise be strongly promoted. We demonstrate that these surfaces, when illuminated by sunlight, can prevent impalement of impacting water droplets, even when the droplet to surface temperature difference is 50{\deg}C, by suppressing condensate formation within the texture, maintaining transparency. We also show how the same transparent metasurface coating could be combined and work collaboratively with hierarchical micro- and nanorough textures, resulting in simultaneous superior pressure-driven impalement resistance and avoidance of water nucleation and related possible frosting in supercooled conditions. Our work can find a host of applications as a sustainable solution against impacting water on surfaces such as windows, eyewear, and optical components.

Inspired by natural examples (lotus leaf, 1 butterfly wings, 2 water strider legs 3 ), superhydrophobic surfaces have received great attention in recent years for their unique self-cleaning, 4 antifouling, 5 and anti-icing 6 properties. Their extreme water repellency, especially with respect to impacting droplets, can be achieved by introducing open and closed microstructures, 7-9 hierarchical surface roughness, [10][11][12][13][14] low-surface-energy materials, [15][16][17][18] and substrate flexibility. 16,19,20 Such surface and substrate properties act to stabilize an intervening, lubricating air layer that is responsible for high droplet mobility (so-called Cassie-Baxter wetting state 21 ) and prevent a transition to the sticky Wenzel wetting state (i.e. impalement). 22 For real-world applications, the ability to repel impacting droplets is critical with numerous studies having investigated it under a range of environmental conditions and droplet temperatures ranging from supercooled droplets 20 and surfaces 7,23 to ambient 24,25 and hot droplets. 26 Typically, impalement is prevented by ensuring that the antiwetting (capillary) pressure is greater than the wetting (droplet) pressure. 27 However, a host of additional mechanisms responsible for the loss of superhydrophobicity emerge when departing from ambient conditions including condensation within the microtexture when exposed to hot vapor 26,[28][29][30] and increased viscosity and freezing in the case of supercooled droplets. 7,20,[31][32][33] Condensation resistant superhydrophobic surfaces have been demonstrated using nanotexturing through droplet coalescence, 34,35 scaling the texture to prevent filling within the droplet contact time 36,37 and minimizing the adhesion force of filled cells. 26 Yet, all of these approaches fail to prevent the initial nucleation of condensate embryos, limiting their working envelope, especially for high supersaturation conditions. This is a critical issue when imparting nanotexture to a surface, which is routinely done to enhance impalement resistance by increasing the antiwetting capillary pressure.
More specifically, when using nanotexture, due to its significantly smaller dimensions compared to microtexture, if a droplet does nucleate within the texture, it fills the roughness very quickly and displaces the lubricating air layer. Consequently, in a supersaturated environment, despite the fact that the nanotexture increases the pressure-based impalement resistance, it can actually reduce the repellency of the surface 38 due to its vulnerability to water nucleation. Moreover, providing multifunctionality by maintaining optical transparency, 39,40 an important property in many practical applications, is inherently counteractive to imparting superhydrophobicity, since surface coating roughness inherently obscures light by causing scattering. 41 To address these challenging intertwined problems of condensation-enhanced impalement and the limitations of surface energy and texturing on resisting impalement, while maintaining transparency, we propose an additive approach exploiting unexplored aspects of the involved physics. Inspired by other innovative studies using sunlight for path-breaking applications, [42][43][44][45][46] including catalysis, 47 desalination, 48,49 materials synthesis, 50 and ice/fog repellency, 51,52 we present rationally designed, transparent yet solar light absorbing, superhydrophobic plasmonic metasurface composites. 51,52 Guided by mass diffusion, heat transfer, and nucleation theories, we investigate the effects of microtexture and nanotexture, substrate subcooling, droplet impact speed, and light intensity on their photothermal operating range and performance. We explore the repellency of water droplets and the mechanisms of texture cavity filling due to condensation across a broad range of temperatures-all while maintaining optical transparency, critical to many functional water repellent surfaces exposed to natural light. We also demonstrate how our coating works in symbiosis with nanotexture to offset inherent undesirable nucleation, which causes failure under moderate supersaturation conditions.

Results and Discussion
To study the ability of metasurfaces to amplify superhydrophobicity, we fabricated transparent polyurethane acrylate (PUA) micropillared patterns using soft lithography on glass substrates. On top of these, we deposited a well-adhering, ultra-thin metasurface coating with sputter deposition. 51 This coating, relying on surface plasmon effects, significantly enhances broadband light absorption and heating, achieved by nanoengineering of gold nanoparticle inclusions in a titanium dioxide matrix (dielectric); the mean levels of transparency and absorption are controlled by coating thickness. 51,52 The broadband absorption property is achieved by tuning the size distribution and concentration of the inclusions in titanium dioxide. For concentrations close to the percolation limit (i.e. adjacent nanoparticles forming a continuous electrically conductive path), electronic conductivity increases dramatically, leading to very effective photothermal light absorption. This is important for harvesting the infrared fraction of sunlight, which makes up more than 50% of its total power, and in contrast to tinted window films and automobile glass, which work byinstead-reflecting infrared. 53 In a subsequent step, we deposited an ultra-thin conformal low surface energy fluoropolymer coating with initiated chemical vapor deposition (iCVD), resulting in a superhydrophobic plasmonic metasurface composite. Both the metasurface and fluoropolymer coatings do not substantially alter the underlying geometrical features of our micropillared surfaces. (See Methods, Supporting Information, section "Topography of the control", and Figure S1 for fabrication steps of all types of surfaces used in this study and topography of a superhydrophobic control surface.) Figure 1a shows a scanning electron micrograph of the plasmonic composite surfaces used in this study. We observe that both the metasurface and fluoropolymer coatings cover the micropillared substrate uniformly, making them almost indistinguishable from the bare pillars. Especially for the metasurface coating, we used low-pressure and high-power sputtering to promote deposition directionality (see Methods for deposition conditions), which ensures coverage of the bottom of the texture, and also to enhance conformality through metal ion resputtering on the sidewalls of the micropillars. 54 Some submicron features on the surface are due to the soft lithography replication process, but do not negatively impact its performance. (For more information on the superhydrophobic control surface, see Supporting Information, section "Topography of the control", and Figure S1.) We measured the apparent static, advancing and receding water contact angle values on the surface to be * θ = 154° ± 2°, * a θ = 160° ± 2° and * r θ = 141° ± 3°, respectively. For reference, we measured the respective contact angles of a glass substrate coated with a fluoropolymer to be * θ = 122° ± 2°, * a θ = 125° ± 1° and * r θ = 119° ± 1°. Figure 1b shows    ples that measure the sample, s T , and ambient, T ∞ , temperatures. The normalized irradiance spectra of both light sources are shown in the inset. We note that the plasmonic composites absorb at wavelengths between 400 and 2500 nm, which is practically the energy range of the entire solar spectrum. Figure 1g shows plots of s T T ∞ − vs time, t , for surfaces illuminated with a solar simulator (power densities: P = 1 kW m -2 and 3.5 kW m -2 ; used later in this work). At t = 300 s, we see that, for the plasmonic composite, s T T ∞ − = 4°C and 16°C for P = 1 kW m -2 and 3.5 kW m -2 , respectively, while, for the same P, the s T T ∞ − of control surfaces is at least 3 times lower, demonstrating the significant role of the plasmonically captured sunlight in heating through the photothermal effect. We found that our hydrophobized metasurface adhered very well to substrates after standard tape peel and abrasion tests, and that it still retained its heating property even after partial removal due to scraping (see Supporting Information, section "Mechanical durability of the metasurface coating", and Figure S3). In the following, to avoid unwanted condensation from humidity in the environment, we heat the droplets instead of cooling the substrates to achieve supersaturation. (See also Supporting Information, section "Inhibiting fog with sunlight in plasmonic composite surfaces", and Figure S4.) Figure 2a shows an image sequence of a water droplet with an initial radius, 0 1.6 R ≈ mm, density, ρ , surface tension, γ , and temperature, w°C 70 T = , impacting onto our superhydrophobic plasmonic composite with a velocity, ). We define the Weber number, which describes the relative importance of inertia and surface tension during droplet impact, as For these experiments, We was kept constant at 26. The chosen w T T ∞ − means that there is a high supersaturation above the surface and a high probability of condensation under normal circumstances. Figure 2b depicts a droplet impacting onto the same surface under identical conditions except that now the surface is illuminated (solar simulator; power density, P = 1 kW m -2 ). In Figure 2a, we see that the droplet leaves a remnant on the surface (characteristic of impalement), while in Figure 2b the entire droplet rebounds from the surface (see Supporting Information Video S1). Figure 2c plots the probability of droplet impalement on the plasmonic composite, Φ, vs w T , for 0 P = and 1 kW m -2 . For w°C 40 T = , we note the onset of impalement for P = 0 kW m -2 while for P = 1 kW m -2 we observe no impalement. Therefore, we see that sunlight alone is capable of boosting the existing repellency of a superhydrophobic surface.
We attribute impalement to water condensation within the microtexture due to the evaporation of the warmer droplets whose vapor displaces the intervening air layer, as discussed in previous works. 26,29 (See also Supporting Information, section "Enhanced impalement resistance with concentrated light" and Figure S5 for the effect of concentrated illumination on impalement resistance.) To further understand and quantify this repellency boosting mechanism, we study next the wetting and impalement behavior of sessile warm droplets as a function of incident light power density, P. Figure  and an increased droplet-surface contact area; we presume that this is equivalent to impalement. A clear trend of increasing f t with increasing P exists. To better investigate this behavior, we can theoretically calculate the approximate timescale needed for condensate, which is generated by the surface being colder than the droplet, to grow and fill the texture. Assuming that this process is diffusion limited, the theoretical filling timescale for constant s T and w T can be expressed as 26 where v D is the water vapor diffusion coefficient in air and C ∆ is the water vapor concentration difference between the droplet and the bottom of the texture. C ∆ , in turn, is a function of the local water vapor saturation pressure difference across a cavity, . However, for s T T ∞ > , which occurs naturally due to the surface being illuminated, we observed that α increases significantly to Supporting Information, sections "Effect of surface temperature on condensation nucleation on superhydrophobic control surfaces" and "Experimental setup, water temperature calibration and droplet cooling rate", Figure S6 and Figure S7). Figure 3e shows schematically the mechanism of droplet impalement due to condensation from a warm droplet over time inside the surface texture. comparable, condensation occurs and the cavity begins to fill with water, while for even longer c t , water completely fills the cavity and local superhydrophobicity is compromised.
To explore a potential coupling of pressure and condensation-based impalement mechanisms, we mapped the impalement probability, Φ , of warm droplets impacting onto our plasmonic composites for a range of We . The image sequences in Figure 4a and Figure 4b depict droplets with w s T T > impacting onto a surface at 73 We = for P = 0 and 3.5 kW m -2 , respectively. We used a concentrated illumination of 3.5 suns to better demonstrate any possible effects of light on condensation-based impalement. We see that even though the droplet is warmer in the case of the illuminated sample, and therefore more likely to impale, the value of P used here is sufficient to suppress impalement and boost the naturally existing repellency of the surface. To isolate the pressure-based impalement mechanism, we performed droplet impact experiments in isothermal con- We will now explore strategies to fundamentally address the pressure-based impalement mechanism. Figure 5a and Figure 5b show image sequences of ambient droplets with w°C 21 T T ∞ = = impacting onto our control surface (no metasurface coating) for 26 We = and 73 We = , respectively; the surfaces are not illuminated ( 0 P = kW m -2 ). We observe droplet rebound in the low We case, while, in contrast, at 73 We = , a daughter droplet remains on the surface owing to the pressure driven impalement mechanism. To address this, we improved the superhydrophobicity of the single-tier plasmonic composite by spray coating a dispersion of silanized (hydrophobic) silica nanoparticles with a polymeric binder for enhanced durability. Adding a second tier of roughness does not compromise surface transparency; see inset logo picture in Figure 5c. to 32°C leads to 100% impalement probability ( 7 n = ), Figure 5d. Finally, Figure 5e shows how effectively sunlight ( 1 P = kW m -2 ), for the same We and w T as in Figure 5d, helps mitigate cavity filling and impalement by drastically reducing Φ to 28%.
In order to explain the previous-unexpected-impalement events and also the effect of illumination, we investigated the nanocavity filling dynamics. From an extended view of the inset nanoroughness micrograph in Figure 5c-e, and assuming for simplicity cylindrical nanopores with an aspect ratio of unity, we measured the average nanopore diameter and subsequently depth to be , the dominant mechanism is free molecular flow. 56 The correct theory for describing such flows is based on effusion, 57  The advantages of our plasmonic composite coating extend beyond ambient conditions into supercooled environments, where freezing is also probable to occur if condensation is not controlled. For w°C 0 T = , we compute a minimum working T ∞ of -7°C without and -11°C with solar illumination. This 4°C shift is very important, considering the exponential dependence of condensation nucleation rate, J , on surface temperature. In fact, heating by just 1°C can reduce nucleation rates by up to several orders of magnitude for both condensation and freezing processes, 51,52,58 emphasizing how sunlight can be effectively and passively employed to prevent impalement on surfaces that typically forgo condensation resilience in pursuit of superior pressure-based impalement resistance.

Conclusions
We demonstrated a facile and passive method for achieving superior water repellency while maintaining surface transparency in supersaturated environments, relying on the collaborative effect of superhydrophobicity and passive heating through plasmonic exploitation of the photothermal effect stimulated by sunlight. Harvesting solar power with ultrathin plasmonic metasurface coatings without being detrimental to transparency, we demonstrated that typical microstructures can sustain superhydrophobicity and avoid droplet impalement at substrate temperatures much lower than the droplet temperature by fully preventing or retarding cavity filling in the diffusionlimited regime due to light absorption heating. We then discovered that adding a second tier of roughness to our micropillars, intended to boost the superhydrophobicity of the surface against high Weber number (impact velocity) incoming warm droplets, counterintuitively caused the surface to be less repellent by shifting cavity filling to a nucleation-limited regime. We established that by using a metasurface coating, one could inhibit condensation nucleation and significantly expand the working envelope of such hierarchical superhydrophobic surfaces, especially for supercooled environments, where it would have the added benefit of reducing the likelihood of freezing. Our approach can be used as a stand-alone or additive method towards counteracting the negative effects of warm(er) water vapor condensation on cold(er) surfaces and whenever a good degree of optical transparency is required. We believe that our work can make its advantages evident in a plethora of applications including glasses, optical components, windows, and windshields exposed to warmer humid conditions. A heated filament at 300°C enabled breakdown of the reactants. The next step consisted of transferring the wafer pattern onto a flexible PDMS mold by means of soft lithography, for the preparation of which we mixed PDMS with a curing agent at a 10:1 ratio, poured it onto the silicon wafer and degassed under vacuum to remove all air bubbles. Curing was done in a convection oven at 70°C for 2 h. The mold, consisting of microholes, was peeled off the wafer. We then transferred the pattern from the flexible mold onto our substrates. We took microscope glass slides and thoroughly cleaned them, consecutively, in sonicated acetone, isopropyl alcohol and water baths, followed by plasma ashing for 10 min. Subsequently, we placed fractions of the PDMS mold onto thin layers of PUA resin, which we previously deposited on the glass slides. Curing and thus hardening of the pattern (micropillars) took place in a vacuum UV exposure box (Gie-Tec GmbH) for 10 min, followed by peeling off the PDMS molds. In order to deposit our light-absorbing metasurface coating on top of our structures, we employed sputter deposition (CS 320 C; Von Ardenne) method. 51 We first deposited ~20 nm of silicon dioxide (SiO2) by RF-bias and setting the following

Experimental setup and protocols
We conducted surface temperature calibrations using either a high-speed infrared camera (SC7500, FLIR), for surfaces illuminated with halogen light (Flexilux 600 longlife), with 1 P = kW m -2 and 2 P = kW m -2 , or a resistance temperature detector in the case of sunlike illumination with a xenon arc source (66902, Newport), equipped with an AM1.5 air mass filter to render it a solar simulator, for 1 P = kW m -2 and 3.5 kW m -2 . The diameter of the light spot was ~8 mm for halogen light and >2 cm for the solar simulator. With the latter, we were able to homogeneously illuminate the whole sample. We assumed thermal equilibrium at 300 t = s (light was switched on at 0 t = s) and each measurement was run three times. Light from the solar simulator was stabilized by waiting for at least 30 min prior to running any experiments. (See Figure 1f for

Supporting Information
The Supporting Information is available online free of charge. The following sections are