Counterintuitive Wetting Transitions in Doubly Reentrant Cavities as a Function of Surface Make‐Up, Hydrostatic Pressure, and Cavity Aspect Ratio

Abstract

applications. Thus, new approaches for the long-term entrapment of air on submersion in water that does not entirely depend on coatings are needed. In this context, a new class of microtextures has been developed, comprising microcavities that broaden below the inlets such that the cross-section of the space between adjacent cavities spanning cavity inlets and the intervening wall resembles the serif-T shape [7,16-22] (Figure 1A-C). These bio-inspired [18,23] microtextures are known as doubly reentrant cavities (DRCs), and they can entrap air inside them on immersion in liquids due to their topography, regardless of their surface makeup. [21] In fact, the transition of these air-filled cavities-Cassie-states [24-26] to the fully-filled or the Wenzel-state [27] depends on a number of factors, such as the compressibility of the trapped air, liquid vapor pressure, capillary condensation, the solubility of the trapped air in the liquid, and the cavity geometry. [20] For instance, when hexadecane (vapor pressure at NTP, P V = 0.01 kPa) was used to immerse silica surfaces adorned with circular DRCs (apparent advancing contact angle on flat silica, θ A ≈ 20°), they robustly entrapped air, which remained intact even after 27 days. [20] In stark contrast, circular SCs of similar chemical make-up, diameter, and pitch as the DRCs got fully-filled by hexadecane within t ≈ 0.2 s, that is, a factor of ≈10 7 faster (or 10 9 % faster). Indeed, these observations are quite exciting because the function, that is, the entrapment of air underwater, can now be realized by the microtexture alone. A variety of materials and techniques have thus been explored toward proof-of-concept demonstrations of this approach, for example, with silica-on-silicon wafers (SiO 2 /Si) and photolithography and dry etching, [7,19,22,28-31] perfluoropolyether dimethacrylate and reverse imprint litho graphy, [32] and polypropylene and crack formation/propagation [33] among others. Thus, there is a growing expectation that this approach would yield greener and low-cost technologies. Even if a Cassie state is achieved on an intrinsically wetting material, this scenario does not pertain to the global minimum on the thermodynamic energy landscape; in fact, such a system will eventually transition to the Wenzel-state to reach thermodynamic equilibrium. [25,34] However, these wetting transitions could be impeded by kinetic barriers by controlling the micro/ nano texture-cavities/pillars, reentrant/simple profile, and Surfaces that entrap air underwater serve numerous practical applications, such as mitigating cavitation erosion and reducing frictional drag. These surfaces typically rely on perfluorinated coatings. However, the non-biodegradability and fragility of the coatings limit practical applications. Thus, coating-free, sustainable, and robust approaches are desirable. Recently, a microtexture comprising doubly reentrant cavities (DRCs) has been demonstrated to entrap air on immersion in wetting liquids. While this is a promising approach, insights into th...

A first‐principles approach for treating wastewaters

“…This approach can thus be extended to anionic/zwitterionic/uncharged dyes, other wastewater contaminants, and also for finding greener and fouling-resistant materials/approaches. [80][81][82][83][84][85] Taken together, our findings suggest that this first-principles approach, supported by laboratory experiments, for the rational design of functional materials in the treatment of wastewaters can contribute to the sustainable use of water.…”

Nature-Inspired Superhydrophobic Sand Mulches Increase Agricultural Productivity and Water-Use Efficiency in Arid Regions

“…The factors and mechanisms underlying the superhydrophobicity of SHS are presented in the Discussion section. Next, we characterized the breakthrough pressure of water on SHS, defined as the pressure at which water penetrates into the microtexture . While water spontaneously imbibes into common sand, due to capillarity, a 5 mm-thick SHS layer could prevent imbibition of the water column up to a height of h ≤ 12 cm, thereby presenting a breakthrough pressure of P h = ρ gh ≈ 1.2 kPa (as explained in the Discussion section).…”

“…Next, we characterized the breakthrough pressure of water on SHS, defined as the pressure at which water penetrates into the microtexture. 51 While water spontaneously imbibes into common sand, due to capillarity, 52 a 5 mm-thick SHS layer could prevent imbibition of the water column up to a height of h ≤ 12 cm, thereby presenting a breakthrough pressure of P h = ρgh ≈ 1.2 kPa (as explained in the Discussion section). Based on these results, we envisioned that when placed on moist topsoil, SHS would create a capillary and diffusion barrier for soil moisture and insulate it from direct exposure to solar radiation, wind, and dry air (Figure 2).…”

“…as the pressure at which water penetrates into the microtexture 32 . While water spontaneously imbibes into common sand, due to capillarity 33 , a 5 mm-thick SHS layer could prevent imbibition of the water column up to a height of h ≤ 12 cm, thereby presenting a breakthrough pressure of 𝑃 $ = 𝜌𝑔ℎ ≈ 1.2 kPa (as explained in the Discussion section).…”

Suppression of Leidenfrost effect on superhydrophobic surfaces

“…The DRP architecture helped us disentangle the effects of the penetration depth and the droplet-substrate contact area on the onset of the Leidenfrost phenomenon. However, at higher Weber numbers, the liquid meniscus might de-pin from doubly reentrant edges and land onto the DRPs' stems 50 . Subsequently, akin to the case of cylindrical pillars, the curvature of the liquid-vapor interface will reverse, the liquid-solid contact area will increase, and the conductive heat transfer will yield the Leidenfrost phenomenon.…”