Coalescence in foams and emulsions: Similarities and differences

Coalescence in emulsions and foams is far from being understood, despite many years of investigations. The phenomenon is not easy to be characterized because it is extremely rapid and coupled to several others, gravity effects, leading to vertical motion of drops/bubbles and ripening, leading to their growth. Coalescence implies the rupture of films between drops/bubbles and involves contributions from hydrodynamics, surface rheology, surface forces, and thermal fluctuations. Different coalescence scenarios were identified and are described. There are close similarities between emulsion and foam behavior, as remarked earlier by several researchers. Ivan Ivanov, to whom this article is dedicated, was one of them. He and his group pioneered parallel studies in both emulsions and foams, aiming to clarify coalescence mechanisms. As discussed in this review, such an approach proved very successful and deserves to be continued in the future.