Energy analysis on rebound dynamics of two droplets impacting a superhydrophobic surface simultaneously

When multiple droplets impact a superhydrophobic surface, coalescence between the droplets may lead to an increased viscous dissipation rate and thus an increased contact time. In this study, the impact of double droplets on a superhydrophobic surface is studied via a lattice Boltzmann model. The morphology and contact time of the rebounding droplet are obtained for various droplet distances and Weber numbers. The simulations show that there are three kinds of rebound patterns, complete-coalescence rebound (CCR), partial-coalescence rebound (PCR), and no-coalescence rebound (NCR); and the contact time is the shortest in the PCR regime. An energy analysis is implemented to reveal the energy conversion mechanism. It is found that viscous dissipation strongly depends on the coalescence strength, and it increases monotonously from the CCR regime to the NCR regime. This result implies that the shortest contact time in the PCR regime does not arise from the reduced viscous dissipation but is attributed to the morphology of the rebounding droplet. Moreover, the simulations also show that the total kinetic energy at the rebound moment is the highest in the PCR regime; however, the restitution coefficient or the rebound velocity is lowest in this regime because a larger proportion of the total kinetic energy occurs in the transverse direction. Therefore, a shorter contact time does not imply a higher rebound velocity.