A new analysis of foam coalescence: From isolated films to three-dimensional foams

The rupture of standing three-dimensional soap foams was examined. A foam o f controlled size and water content was produced in a vertical column and t he evolution of its liquid fraction under gravitational drainage was follow ed 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, indicatin g two mechanisms for foam destruction. In one case, the rupture of soap fil ms is induced by the increase of capillary pressure resulting from liquid d rainage: 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 drainag e and their breakage is randomly distributed throughout the foam, which coa lesces homogeneously in space. These two mechanisms are interpreted with mi croscopic arguments, based on monolayer elasticity and disjoining pressure isotherms.