A note on the critical gap of bubble coalescence during foaming process: A diffuse-interface modeling

Abstract

Bubble coalescence is an important stage of foaming process. A goal of foaming is to produce numerous, uniform-size bubbles. Therefore suppression of bubble coalescence is desirable during foaming process. For stationary bubbles, if their distance be less than a critical gap they will coalesce. Actually in this case, attractive forces attract the outer surfaces to touch each other and form a growing gas bridge which merge the bubbles finally. For bigger distance, the attractive forces cannot make a bridge and coalescence will not happen. In this study the dynamics of bubble coalescence is modeled using a diffuse-interface LBM. Then critical gap of bubble coalescence is defined as the maximum distance between the stationary bubbles where the coalescence will happen. Sensibility of critical gap is obtained with respect to critical properties of material, bubble size, viscosity of gas and liquid, density ratio, surface tension, temperature and interface thickness. The results show that, interface thickness is the only factor that controls the critical gap. In the other word, in the case of stationary bubbles, by a precise estimation of interface thickness, the coalescence can be predicted. Critical gap is a useful parameter in foaming where the maximum number of bubbles is desirable.