Design of a robust superhydrophobic surface: thermodynamic and kinetic analysis

Abstract

The design of a robust superhydrophobic surface is a widely pursued topic. While many investigations are limited to applications with high impact velocities (for raindrops of the order of a few m/s), the essence of robustness is yet to be analyzed for applications involving quasi-static liquid transfer. To achieve robustness with high impact velocities, the surface parameters (geometrical details, chemistry) have to be selected from a narrow range of permissible values, which often entail additional manufacturing costs. From the dual perspectives of thermodynamics and mechanics, we analyze the significance of robustness for quasi-static drop impact, and present the range of permissible surface characteristics. For surfaces with a Young's contact angle greater than 90° and square micropillar geometry, we show that robustness can be enforced when an intermediate wetting state (sagged state) impedes transition to a wetted state (Wenzel state). From the standpoint of mechanics, we use available scientific data to prove that a surface with any topology must withstand a pressure of 117 Pa to be robust. Finally, permissible values of surface characteristics are determined, which ensure robustness with thermodynamics (formation of sagged state) and mechanics (withstanding 117 Pa).

Direct Electrodeposition of Superhydrophobic and Highly Oleophobic Poly(3,4‐ethylenedioxypyrrole) (PEDOP) and Poly(3,4‐propylenedioxypyrrole) (PProDOP) Nanofibers

“…For example, some species are able to resist droplet impacts during rainfalls, to slide easily on aquatic surfaces or to see clearly in foggy environments . It was shown that the presence of both micro and nanostructures and low surface energy materials can lead to “robust” superhydrophobic properties . However, it is also extremely important to repel low surface tension liquids such as oils, which is much more difficult due to their high tendency to spread across any substrate .…”

Drop impact on inclined superhydrophobic surfaces

“…The first type of wetting transition corresponds to the crossing of a critical pressure difference, driven by the dynamic pressure inside water drops. This type of failure occurs by destabilizing the triple-phase contact line (contact line where air, liquid, and solid converge) and subsequent penetration of the liquid-air interface into surface pores [23,24]. The second type of wetting transition can occur even when the contact line remains pinned to the top of the surface texture: any pressure differential across the liquid-air interface leads to the sagging of the interface, and if the bulging interface can touch the bottom of the texture, such a local wetting can quickly spread across the entire surface [20,23,25].…”

“…This type of failure occurs by destabilizing the triple-phase contact line (contact line where air, liquid, and solid converge) and subsequent penetration of the liquid-air interface into surface pores [23,24]. The second type of wetting transition can occur even when the contact line remains pinned to the top of the surface texture: any pressure differential across the liquid-air interface leads to the sagging of the interface, and if the bulging interface can touch the bottom of the texture, such a local wetting can quickly spread across the entire surface [20,23,25]. In both cases, the driving force for the transition is pressure, which, for the case of dynamic impact, originates from the transfer of momentum of the drop [18,26,27].…”

“…Many attempts have also been made to develop, both by empirical and analytical means, a model that can predict the robustness of superhydrophobic surfaces to water penetration [18][19][20][21][22]. So far, it is well established that two distinct penetration mechanisms cause most of the wetting transitions [23]. The first type of wetting transition corresponds to the crossing of a critical pressure difference, driven by the dynamic pressure inside water drops.…”

Combining Staudinger Reductive Amination and Amidification for the Control of Surface Hydrophobicity and Water Adhesion by Introducing Heterobifunctional Groups: Post‐ and Ante‐Approach

“…The main parameters influencing both the surface hydrophobicity and water adhesion are the surface chemistry and the topography of the surface roughness. Usually, low surface energy materials and dual‐scale or hierarchical surface roughness are used to obtain stable superhydrophobic properties with low adhesion but the adhesion can be increased by using materials of higher surface energy and/or by creating one‐scale surface roughness (e.g., nanofibers) …”