This is important because it permits sustaining the vapor phase without actually having to boil the liquid. We term the phenomenon of sustaining the metastable vapor in the pore as vapor-stabilization. Will the metastable vapor occupy the pore and keep it dry or will it condense in the pore to make it wet? This is a critical consideration, hitherto unresolved and which is essential to enabling practically dry surfaces immersed in undersaturated liquids. This vapor inside the pore could eventually condense on the pore walls, thus providing another pathway, via condensation, to wet the pore. However, a metastable vapor can evaporate from the meniscus (hanging at the top of the pore) and occupy the pore. At temperatures below the boiling point, the liquid phase is the lower energy state. Trapped air is not the only gas that can occupy the pore. The invading liquid will lead to the wetting of the immersed surface. Consequently, air pressure inside the pore will decline and water will invade if the liquid-air interface cannot remain pinned at the top of the pore 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35. However, if the liquid is undersaturated, then air within the pore will dissolve into the liquid 20. If the liquid is supersaturated with air, an air layer covering the surface may be achieved indefinitely 13. For this air to be sustained over a long period, it should be in chemical equilibrium with air dissolved in the ambient liquid. When the surface is immersed under water, there will initially be air trapped in the pore (roughness valley).
![webook nist gov heat solidication water webook nist gov heat solidication water](https://www.mdpi.com/energies/energies-13-01065/article_deploy/html/images/energies-13-01065-g005-550.jpg)
To elucidate the fundamental principles required to sustain gas pockets, we consider a typical cylindrical pore on a surface that is immersed under water. To make robust surfaces that remain dry under water, the effect of sustaining vapor pockets also needs to be accounted in the thermodynamic analysis 18, 19. The thermodynamic analysis of underwater superhydrophobicity that accounts for only the surface energy has been theoretically studied 17. Thus, in order to keep a surface practically dry under water, the gas phase in the roughness valleys must be sustained. This is challenging 4, 5, 6, 7, 9, as air in the roughness valleys can dissolve into the liquid if the liquid is undersaturated with air. Maintaining the Cassie-Baxter state will ensure a practically dry surface while immersed in a liquid. In this case, the droplets reside on top of roughness peaks, while air occupies roughness valleys. Research on the well-known wetting behavior of non-immersed rough surfaces 1, 2, manifested in the form of liquid droplets beading up and moving with very little drag, has intensified in recent years 3. It is our intention that this work will pave way to rationally design surface texture to manipulate the phase of one material adjacent to a surface – in this instance acquiring a vapor phase between a liquid and a textured solid surface, even when the liquid is not heated or boiled.Īlthough dry immersed rough surfaces may be achieved, the underlying mechanisms that drive non-wetting to wetting transitions are not fully understood. Theoretical predictions are consistent with molecular dynamics simulations, experiments and observations of air-retaining insect surfaces 15, 16. We show that surfaces of hydrophobic solids retain non-wetting properties in the presence of sub-micrometer roughness. These are passive thermodynamic mechanisms that do not involve active generation 5 or exchange of gas 13, 14. There is a critical roughness scale, below which these mechanisms are effective. We postulate that it is essential to stabilize the vapor phase of water and sustain trapped gases in roughness valleys.
![webook nist gov heat solidication water webook nist gov heat solidication water](https://www.nist.gov/sites/default/files/styles/480_x_480_limit/public/images/fire/firefighter_conduction_1.jpg)
We investigate how immersed surfaces can remain practically dry. This depletion limits the utility of these surfaces in applications like drag reduction 4, 5, 11, boiling 12, among others. Keeping these surfaces practically dry (liquid minimally touching the solid surface) under water is challenging because the trapped air is found to deplete 4, 5, 6, 7, 8, 9, 10. Maintaining superhydrophobicity of rough textured surfaces has typically relied on the presence of trapped air pockets in the roughness valleys 3. Superhydrophobicity occurs when surface roughness enhances non-wetting properties of hydrophobic solids 1, 2.