Off-center impact dynamics of a droplet against hydrophobic surfaces

When a droplet off-center impacts a macro-ridge, it may split into two asymmetric fragments, i.e., the smaller part (SP) and larger part (LP), which then exhibit asymmetric spreading and receding phenomena. Using lattice Boltzmann method simulations, the dynamics of a droplet off-center impacting a hydrophobic surface are explored for the first time. Influences of the off-center distance, the impact Weber number, and the surface wettability on the impact outcomes and the contact time are elucidated, and the impact regime is established. Six impact outcomes are discovered as off-center distance changes, which are dependent on the interaction force between the droplet and the surface. The effect of the off-center distance and the Weber numbers on the spreading and the receding dynamics of the SP is more obvious than that of the LP, due to the fact that adhesion of the ridge on the LP can be negligible compared to the inertial force and the surface tension. The contact time is closely related to the impact outcomes, that is, when the impact outcome is type 1 (SP and LP separate rebound), the contact time can be significantly reduced compared with that on the flat hydrophobic surface, due to the fact that the ridge can reduce the asymmetry of a liquid film and the receding distance in this case; while when the impact outcome is type 6 (SP and LP completely coalescence and rebound), the contact time cannot be reduced. This work extends the fundamental knowledge of off-center impact and provides inspiration for the efficient reduction in the contact time.