The contact time of rebounding-coalescing droplets on rectangular-ridged superhydrophobic surfaces

On a rectangular-ridged superhydrophobic surface, the contact time of the rebounding-coalescing droplet is for the first time investigated via lattice Boltzmann method simulations, where the rebounding-coalescing droplet is caused by an impinging droplet coalescing with an adhesive droplet. The simulation results show that at constant initial radii of impinging droplets, R0, and various initial radii of adhesive droplets, R1, the contact time of rebounding–coalescing droplets depends not only on the impact condition but also on the surface condition. Under various impact conditions, that is, with increased Weber numbers of We = 1–30, the contact time is gradually reduced, and then nearly constant, and eventually constant after slightly reduced at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is gradually reduced, then increased, and eventually constant. It indicates that the contact time of rebounding-coalescing droplets is affected by the initial radii of adhesive droplets. Under different surface conditions, that is, with increased spacing distances between adhesive droplets and ridges of L = 3–17, the contact time is reduced at the low Weber number of We = 3, constant at the moderate Weber number of We = 12, and increased at the high Weber number of We = 28 at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is reduced at both low and moderate Weber numbers of We = 3 and 12, and constant at the high Weber number of We = 28. It indicates that under different surface conditions, the contact time of rebounding-coalescing droplets is also affected by the initial radii of adhesive droplets.