On the different coalescence mechanisms in foams and in emulsions

Coalescence is the less well understood destabilization process in emulsions and foams. These dispersions are stabilized by layers of surface-active agents adsorbed at the surface of drops and bubbles. The compactness of the layer is directly linked to the surface tension; the more compact the layer, the lower the tension. It is currently admitted that in dense emulsions (high drop fraction) and foams stable for more than a few minutes, coalescence is a thermally activated process involving nucleation of holes in the films separating drops and bubbles. In general, the less compact the layer, the easier the coalescence. In some emulsions however, the opposite behavior is observed: the more compact the layer, the easier the coalescence. Different models allow accounting for these two opposite situations, but the conditions in which they should be applied are not clear. After comparing the models and the experiments reported in the literature, we propose a few ideas to explain the difference in coalescence behavior.