Experimental study of bubble-droplet interaction in water; the effect of water salinity on the drainage time

Interaction between an air bubble and an oil droplet in aqueous mediums was studied experimentally to investigate the encapsulation phenomena in ternary fluid systems. A special experimental setup was designed, and the influence of the bubble-droplet size and water salinity on the drainage times of the intervening pseudo-emulsion film were investigated. The drainage, coverage, and rupture times for the studied sizes (200–700 µm) were found to be in the range of 0.07–5.6 s, 20–70 ms, and less than 1 ms, respectively. The drainage times for de-ionized water first increase with equivalent radius, from slightly more than 1 s to around 3 s, then decrease into 1 s again for radiuses larger than 550 µm. On the other hand, a decrease in the measured drainage times was observed in the high salt concentrations (1 M NaCl), and their values remain less than 1 s for the studied bubble-droplet radius. The results also show that the drainage times in pure water are scattered, while the obtained data for the salt solution suggests a concentrative nature. Based on the interplay between capillary and disjoining pressures, it was argued that in the latter case, the outcome of the interaction is independent of the random fluctuations and instabilities and seems to be deterministic, while for the former, these microscopic events control the drainage and coalescence process.