On the effect of liquid temperature upon bubble coalescence

An experimental study concerning the influence of liquid temperature on bubble coalescence is presented. Bubble collisions in two different liquids (water and ethanol) were analysed at four distinct temperatures (10⩽T⩽40∘C), using air as the dispersed phase in all cases. A quantitative criterion was developed to compute the critical velocity for bubble coalescence based on the relative velocities and the outcome (coalescence or bouncing) of the recorded collisions. The critical velocity, and consequently bubble coalescence, was shown to increase as the liquid temperature was raised, an effect whose intensity depended on the kind of liquid employed. Using some simplifying assumptions, a comparison between the experimentally observed trends and the predictions of literature models for the film drainage was drawn. The experimental data were used together with available literature data in the development of a dimensionless correlation for predicting the critical velocity as a function of the liquid properties.