Contact Time of Double Unequal-Sized Droplets Simultaneously Impacting Superhydrophobic Surfaces

The impact dynamics of double unequal-sized droplets impacting superhydrophobic surfaces are investigated via lattice Boltzmann method (LBM) simulations. The simulations capture the spreading and receding dynamics of unequal-sized droplets on superhydrophobic surfaces, and reveal the influences of the radius ratio of the droplets (K), the droplet spacing (L*), and the Weber number. The radius ratio and the droplet spacing significantly affect the contact time. The contact time increases with the radius ratio when 1.2 < K < 2.1, as the increasing radius ratio leads to the intensification of the asymmetry in the impact processes that causes more intense internal collisions and energy dissipation; while it remains unchanged when 2.1 ≤ K ≤ 3.0, since the larger droplet dominates the morphological evolution and the contact time is equivalent to that of a single droplet. Besides, the contact time decreases with the droplet spacing when 1.2 ≤ L* ≤ 1.6, as the coalescence strength decreases with the increase of droplet spacing, leading to less viscous dissipation; while the contact time increases when 1.6 < L* ≤ 2.0, since the increasing droplet spacing causes the increase of the degree of separation of droplets during rebound, which reduces the kinetic energy for rebound provided by the smaller droplet. Our work demonstrates the key physics governing the impact dynamics of unequal-sized droplets impacting superhydrophobic surfaces, key for enhanced antifrosting/icing, self-cleaning, and water/energy harvesting.