The effect of electrolytes on the critical velocity for bubble coalescence

An experimental study concerning the influence of electrolytes on the critical velocity for bubble coalescence is presented. Bubble collisions in water and two NaCl solutions (0.100 and 0.300 wt%) were recorded with a high-speed video camera at four different operating temperatures (10 ≤ TL ≤ 40 °C), using air as the dispersed phase in all cases. For all operating conditions analysed, the critical velocity was observed to be initially a decreasing function of the equivalent diameter of the colliding bubbles up to a given value, at which it became constant. This behaviour was physically reasoned in terms of the importance of bubble deformation during the drainage of the liquid film between the bubbles. Regardless of the composition of the liquid phase, an increase in the liquid temperature enhanced bubble coalescence. An empirical equation including three parameters was proposed to describe the experimentally observed trends. Two parameters were only temperature-dependent and all electrolytes effects were hence lumped together in the third parameter.