Bubble vortex at surfaces of evaporating liquids

Inhibition of bubble coalescence: Effects of salt concentration and speed of approach

“…when the bubbles have the minimum coalescence times of <1-0.1 s) is $11.5 to 60 mm/s. However, none of the bubbles in this high salt concentration coalesced instantly on contacting the meniscus, and we suggest this is explained (see Section 4.1.2) by non-radial turbulent flows induced by surface tension gradients acting in random directions in the liquid film, as observed in previous thin film experiments [46,50,90].…”

“…They observed from high-speed photography that typical contact times of bubbles in swarms are $0.1 s. If the thin film between bubbles does not rupture within this time, then bubbles separate again without coalescing. In thin film studies it has been observed that at high salt concentrations, surface tension gradients not only produce radially inward/outward flows during film drainage, but also more complicated non-radial flows typically observed as vortices [46,90,91]. These tension gradient-driven turbulent flows are somewhat persistent and maintain the integrity of the film over a certain period of time, often several seconds, before the air-water interface thins to rupture point.…”

Stability of Aqueous Films between Bubbles. Part 2. Effects of Trace Impurities and Evaporation

“…The surface tension of the film area should be slightly higher than that of the meniscus because the film, which has a low mass and inhibited heat transfer, cools faster than the meniscus in evaporation. The film-meniscus tension difference Δγ caused by cooling is estimated to be 0.1 – 0.2 mN/m, 2,13 which is about ten times larger than the maximum possible effect of disjoining pressure. The Δγ caused by cooling actively assists the inward drag of the liquid from the meniscus into the film.…”

“…Under saturated vapor conditions, water films remain for hours and days without changing their thickness, in contrast to electrolyte solutions and volatile organic liquids which do not form films in saturated vapor. 1,2 The water films remain optically uniform (Fig. 1) and the brightness of light reflected from them does not change over several days.…”

“…The observed patterns driven by surface tension gradients are similar to those observed in films of volatile pure organic liquids formed under free evaporation conditions. 2 Volatile pure organic liquids generally do not form films under vapor saturation at a temperature above the surface freezing point 3 while they do form films under free evaporation conditions, due to tension gradients caused by evaporative cooling that have a stabilising effect because they drive fluid circulation into the film. 2 …”