Spreading and breakup of a compound drop on a partially wetting substrate

Abstract

The spreading of a compound drop on a partially wetting solid substrate is numerically simulated using a diffuse-interface method. Compared with a simple drop, the spreading of a compound drop exhibits much more complex behaviour. Depending on the core–shell size ratio and the substrate wettability, various flow regimes are identified in which the interfacial morphology evolves in distinct ways. A phase diagram is constructed in the parameter space of the core–shell size ratio and the wetting angle. For relatively small inner drops, the outer interface does not rupture during the spreading and the inner drop either remains suspended and encapsulated or attaches onto the substrate. Otherwise, the compound drop spontaneously breaks up and releases the inner drop into the ambient fluid. Several breakup scenarios are observed depending on the location of the initial rupture. In some regimes, the wetting of the substrate by one fluid can entrap secondary drops of the other, which can either attach to the substrate or stay suspended. The viscosity ratio mainly affects the spreading rate and plays a minor role in the morphology evolution.