(151b) Filling of Initially Empty Cavities By Advancing Water Fronts

Phenomena involving the wetting of liquids, especially water, on rough or patterned surfaces arise in many different areas of surface science, personal care products, etc. Many fundamental issues remain unresolved, such as the energetics of complex surface structures, and the kinetics/dynamics of cavity-filling both under the wetting liquid and ahead of it, which manifest themselves in â??contact angle hysteresisâ?? and non-uniform (e.g., stick-slip) motion of advancing or receding three-phase boundaries.

 We have investigated the factors affecting the rate at which water fills micro-scale, cavities. Three-dimensional imaging with confocal microscopy coupled with fluorescence microscopy were used in these studies. We find that the concentration of dissolved air in the water significantly changes the rate at which cavities get filled: deaerated water filled the cavities within 15 minutes, while 2 weeks was required for aerated water. Therefore, the water available for air to dissolve into plays a very significant role in cavity filling. We also tested the effects of various surfactants in the water: surfactants (dissolved at their CMC) shortened the filling time, taking less than 15 minutes in all cases, due more to the surface tension lowering than the changing intrinsic contact angle. More generally, reducing γ_LV always speeded up the filling rates. We also observed capillary condensates forming and slowly growing on the bottom of the cavities while the cavities were still only in the partially-filled state, due to the diffusion of water through the vapor to the (hydrophilic) surfaces.