Numerical investigation of surface curvature effect on the self-propelled capability of coalesced drops

We numerically investigate the curvature effect on the self-propelled capability of coalesced drops. The numerical method is based on a well validated multiphase flow solver that solves the three-dimensional Navier-Stokes equations. The liquid-air interface is captured using the moment of fluid method, and a direction splitting method is applied to advect the interface. Afterward, an approximate projection method is used to decouple the calculation of velocity and pressure. Different cases were validated by comparing the experimental results with the simulation results. The coalescence-induced jumping behavior on a flat surface is carefully captured using this numerical method. To investigate the effect of curvature of a curvy substrate on the self-jumping behavior, a case with a single drop impinging on a convex surface and a case with two drops' coalescence on a fiber are also studied and compared with the experimental results. The asymmetric bouncing of a single drop on the convex surface leads to 40% reduction in contact time, as found in our study. Our study also reveals that due to the curvature of the wedge, the drop forms a lobe shaped region on the symmetric sides of the wedge. The lobed region forces the drop to convert more surface energy into kinetic energy in the upward direction. The jumping capability is improved by increasing the surface curvature. Our study also shows that at lower angles of contact, the drops can easily get attached to the substrate and, at the same time, have difficulty detaching from the substrate.