Aqueous film drainage at the quartz/water/air interface

Abstract

The effect of salt (sodium chloride) concentration on the drainage of a thin aqueous film between a hydrophilic quartz surface and a fast approaching air bubble has been studied using scanning interferometry. An increase in salt concentration increases the rate of drainage at the boundary ring of the draining film but is accompanied by a decrease at the centre of the film. The changes are manifested at distances beyond which equilibrium surface forces are anticipated to operate. The enhanced drainage rate at the boundary ring may arise from effects similar to those which control the flow of aqueous solutions in fine capillaries. Equilibrium thin-film thicknesses can be reasonably well predicted using standard DLVO theory. Film drainage rates exhibit a linear inverse square behaviour, of the film thickness vs. time, but diverge from extant theory.