Coalescence stability of gas bubbles subjected to rapid pressure change at a planar air/water interface

Abstract

We report the effects of rapid and slow bubble expansion, due to a reduction in air pressure, on bubble coalescence stability at a planar air–water interface. Bubbles stabilised by pure β-lactoglobulin (BLG), a commercial whey protein isolate (WPI) consisting primarily of BLG, and the non-ionic surfactant Tween 20, were examined. A semi-automatic image analysis routine has been developed to size and count touching bubbles in close-packed arrays. It was found that the bubble size distribution does not change significantly after expansion and coalescence, except for the lowest bulk protein/surfactant concentration ( C b). Bubbles stabilized by BLG were increasingly more stable against rapid expansion with increasing C b, compared with bubbles stabilised by WPI, suggesting significant differences between the adsorbed protein films. For both proteins the fraction of coalescence, ( F c) increased with increasing expansion rate, until a plateau in F c was reached. For Tween 20, F c decreased with increasing number of injected stable bubbles per unit area of planar interface. For bubbles formed in mixtures of BLG and Tween 20 (at high expansion rate) F c increased with increasing molar ratio of Tween 20:BLG. For proteins F c is probably determined by both adsorption kinetics and the interfacial rheology of the adsorbed films, whilst other factors, such has thin film hydrodynamics, bubble size, bubble packing and the relative mobility of the bubbles at the interface, probably have additional, but more minor influence on overall bubble stability.