Steven Hardy Page

Rates of cavity filling by liquids

Significance In engineering and natural phenomena, various fluids contact rough/textured surfaces, e.g., wicking, facial creams, corrosion-preventive paints, and rain on plant leaves. Liquids on rough surfaces, especially those with cavities, pits, or pores, may or may not transit from the unfilled or partially filled (wetted) state to the fully filled (fully wetted) state. Either one of these states may be desired for a given application (compare superhydrophobicity) or even survival (compare oil-soaked feathers). In this article, we present five variables that control the wetting behavior (cavity filling) of water on intrinsically hydrophilic surfaces with micrometer-sized cavities. Our experimental results and theoretical analysis provide criteria for maintaining either the partially filled state, or quickly transiting to the fully filled state, and insights into other related wetting phenomena. Understanding the fundamental wetting behavior of liquids on surfaces with pores or cavities provides insights into the wetting phenomena associated with rough or patterned surfaces, such as skin and fabrics, as well as the development of everyday products such as ointments and paints, and industrial applications such as enhanced oil recovery and pitting during chemical mechanical polishing. We have studied, both experimentally and theoretically, the dynamics of the transitions from the unfilled/partially filled (Cassie–Baxter) wetting state to the fully filled (Wenzel) wetting state on intrinsically hydrophilic surfaces (intrinsic water contact angle <90°, 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. We prepared silicon wafers with cylindrical cavities of different geometries and immersed them in bulk water. With bright-field and confocal fluorescence microscopy, we observed the details of, and the rates associated with, water penetration into the cavities from the bulk. We find that unconnected, reentrant cavities (i.e., cavities that open up below the surface) have the slowest cavity-filling rates, while connected or non-reentrant cavities undergo very rapid transitions. Using these unconnected, reentrant cavities, we identified the variables that affect cavity-filling rates: (i) the intrinsic contact angle, (ii) the concentration of dissolved air in the bulk water phase (i.e., aeration), (iii) the liquid volatility that determines the rate of capillary condensation inside the cavities, and (iv) the presence of surfactants.