Water colliding with oil

The collision of two liquid drops is both an applied question (in rain formation or combustion, for example) and a beautiful basic situation, where impact involves liquid phases, making this problem worth studying in its own right. In a stimulating paper, Planchette, Lorenceau & Brenn (J. Fluid Mech., this issue, vol. 702, 2012, pp. 5-25) consider collisions between oil and water, which often lead to water drops protected by a shell of oil. By looking at the deformations during impact, they characterize the dynamical conditions leading to single encapsulation, and derive a criterion for avoiding fragmentation.