Nanoscale Transport during Liquid Film Thinning Inhibits Bubble Coalescing Behavior in Electrolyte Solutions

Why Bubbles Coalesce Faster than Droplets: The Effects of Interface Mobility and Surface Charge

Air bubbles in pure water appear to coalesce much faster compared to oil emulsion droplets at the same water solution conditions. The main factors explaining this difference in coalescence times could be interface mobility and/or pH-dependent surface charge at the water interface. To quantify the relative importance of these effects, we use high-speed imaging to monitor the coalescence of free-rising air bubbles with the water-air interface as well as free-falling fluorocarbon-oil emulsion droplets with a water-oil interface. We measure the coalescence times of such bubbles and droplets over a range of different water pH values (3.0, 5.6, 11.0). In the case of bubbles, a very fast coalescence (milliseconds) is observed for the entire pH range in pure water, consistent with the hydrodynamics of fully mobile interfaces. However, when the water-air interface is immobilized by the deposition of a monolayer of arachidic acid, the coalescence is significantly delayed. Furthermore, the coalescence times increase with increasing pH. In the case of fluorocarbon-oil droplets, the coalescence is always much slower (seconds) and consistent with immobile interface coalescence. The fluorocarbon droplet's coalescence time is also pH-dependent, with a complete stabilization (no coalescence) observed at pH 11. In the high electrolyte concentration, a 0.6 M NaCl water solution, bubbles, and droplets have similar coalescence times, which could be related to the bubble interface immobilization at the late stage of the coalescence process. Numerical simulations are used to evaluate the time scale of mobile and immobile interface film drainage.

Ion-Specific Bubble Coalescence Dynamics in Electrolyte Solutions

Bubble coalescence time scale is important in applications such as froth flotation, food and pharmaceutical industries, and two-phase thermal management. The time scale of coalescence is sensitive to the dissolved ions. In this study, we investigate the evolution of a thin electrolyte film between a bubble and a hydrophilic substrate during coalescence. We present a thin-film equation-based numerical model that accounts for the dependence of the surface tension gradient and electric double layer (EDL) on the concentration of ions at the air-liquid interface. The influence of Marangoni stresses and the EDL on the hydrodynamics of drainage determines the coalescence time scale. We show that the electrolytes, such as NaCl, Na2SO4, and NaI retard coalescence, in contrast to HCl and HNO3 that have little effect on the coalescence time scale. We also show that the drainage of the electrolyte films with higher concentrations is retarded due to increased Marangoni stresses at the air-water interface. The slow drainage triggers an early formation of the dimple in the thin film, thus trapping more fluid within, which further decreases the drainage rate. For a hydrophilic substrate, EDL along with van der Waals for a given concentration governs the final dynamics of thin films, eventually resulting in a stable thin layer of the electrolyte between the bubble and the substrate. The stabilizing thickness reduces by an order of magnitude as the NaCl concentration increases from 0.01 to 10 mM. For Na2SO4 solution, the film is stabilized at a smaller thickness due to higher valency cations resulting in higher screening of the EDL repulsion.