The rebound–coalescence dynamics of rebounding–coalescing droplets on an inclined superhydrophobic surface with or without a macro-ridge

The rebound–coalescence dynamics of rebounding–coalescing droplets impacting inclined superhydrophobic surfaces is investigated via lattice Boltzmann method simulations, specifically focusing on the influence of macro-ridges, normal Weber numbers (Wen), inclination angles (α), and adhesive droplet sizes. Simulation results show that at low Wen, a complete-coalescence–rebound (CCR) is observed and driven by an asymmetrical thickness of liquid films, where the film is slightly thicker on one side of the ridge, leading to the asymmetrical retraction. At moderate and high Wen, a no-coalescence–rebound (NCR) regime emerges. The regime transition from CCR to NCR is attributed to the uneven distribution of liquid film volumes between both sides of the macro-ridge, causing the asymmetrical retraction velocities and the earlier detachment of one droplet segment before the full coalescence. Under the CCR and NCR scenarios, the increasing Wen enhances the angular momentum, hence promoting the droplet rotation, coalescence, and rebound, eventually reducing the contact time. The contact time is further reduced in the presence of the macro-ridge, attributing to the accelerated retraction at different radii of adhesive droplets.