(478c) Dynamics of Wetting Transitions in Complex Systems

Studies on wetting transition, or how the meniscus of a liquid droplet suspended over the asperities on a rough solid surface (“Cassie-Baxter state”) transitions to a Wenzel state where the liquid now occupies the space between the asperities, have been extensive and provided much insight into scientific and industrial applications such as plant leaves, skin, fabrics, as well as the development and design of everyday products such as skin creams, paints, and epidermal medicines where liquids are applied on rough and often porous surfaces. While most of these studies are focused on the equilibrium states of the liquids, they seldom discuss the mechanism with and rates at which the wetting transition occurs. In addition, most of those studies on the equilibrium states of wetting transition discuss one kind of liquid droplet placed on a rough solid surface while both of them are surrounded by air (hereby called solid-liquid-vapor systems, or SLV systems). While we encounter situations where two liquids are involved, such as skin creams, as often as we do with SLV systems, they did not attain much attention. Therefore, this talk presents the rate of wetting transition for both SLV systems and SL₁L₂ systems (solid-liquid 1-liquid 2)

First, for SLV systems, the dynamics of the wetting transitions are presented both experimentally and theoretically with water-air-silicon wafer systems where the Wenzel state is always the thermodynamically favorable state, while a temporary metastable Cassie-Baxter state can also exist to determine the variables that control the rates of such transitions. Theses variables are identified to affect the rate of wetting transition: (i) the intrinsic contact angle, (ii) the concentration of dissolved air in the bulk water phase, (iii) the liquid volatility that determines the rate of capillary condensation inside the cavities, and (iv) the presence of surfactants.

Secondly, for SL₁L₂ systems, oil-water-silicon wafer systems were used to investigate the rate of wetting transition. When two liquids are involved, one encounter oil-water mixture most frequently during dairy processes and enhanced oil recovery with low-salinity water flooding. The experiments with deionized water revealed that solubility and diffusivity were the driving factors that promoted the replacement, and the 3D confocal images confirmed the decrease in volume of hydrocarbons due to dissolution and diffusion. To exclude these factors, water was saturated with each of the seven hydrocarbons prior to experiments. For these experiments, the adhesion energy of hydrocarbons to the solid surface and the density difference between hydrocarbons and water were correlated with the rate.