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The effect of a range of electrolytes on the mobility of the gas-aqueous solution interface has been investigated. By measuring the terminal rise velocity of small bubbles (diameter < 100 µm, Re < 1) in electrolyte solutions devoid of surface active contaminants, we determined that they obey the Hadamard-Rybczynski model, corresponding to a mobile interface (slip boundary condition), in agreement with observations in pure water. In addition, there is no difference in the interfacial mobility between electrolytes that inhibit bubble coalescence relative to pure water and electrolytes that have no effect. This is strong evidence against the hypothesis that electrolytes inhibit bubble coalescence via immobilization of the interface in the absence of surface-active contaminants.

Very small bubbles are ideal for making measurements of both the film drainage process and the disjoining force between a bubble and a hydrophilic titania surface, immersed in water. Their small buoyancy, combined with high Laplace pressure, minimizes bubble deformation, in a flow regime where the Reynolds number approaches zero. We have used high-speed, dynamic, thin-film interferometry to measure film drainage rate and film thickness as a function of buoyancy force. Single gas bubbles, in the diameter range of 15-120 µm, were allowed to rise freely, before collision with a planar, hydrophilic titania surface. Experiments were conducted in 0 to 10 -1 M aqueous KCl or (CH 3 ) 4 NBr at pH 3.5 or pH 6.3, both below and above the titania isoelectric point. We have observed a transition in bubble boundary condition from full-slip to no-slip. Importantly, this effect is reversible and dependent on electrolyte pH, ionic strength, and film thickness. The true origin of these effects remains obscure; however, they seem to reflect the influence of surface conductance on flow through a charged capillary, particularly when the bulk conductance is small and κa is small.