A New Analysis of Foam Coalescence: From Isolated Films to Three-Dimensional Foams

Abstract

The rupture of standing three-dimensional soap foams was examined. A foam of controlled size and water content was produced in a vertical column and the evolution of its liquid fraction under gravitational drainage was followed by electrical conductivity measurements. Using simple models to describe simultaneously foam geometry and liquid drainage (syneresis), we measured coalescence events in a bulk foam. Various surfactants were used, indicating two mechanisms for foam destruction. In one case, the rupture of soap films is induced by the increase of capillary pressure resulting from liquid drainage: foam breaks at the top of the column and its level goes down with a constant velocity. In the other case, films are not influenced by drainage and their breakage is randomly distributed throughout the foam, which coalesces homogeneously in space. These two mechanisms are interpreted with microscopic arguments, based on monolayer elasticity and disjoining pressure isotherms.