Spreading, Breakup, and Rebound Behaviors of Compound Droplets Impacting on Microstructured Substrates

In this study, we numerically investigate the dynamic behaviors of micron-scale compound droplets impacting onto superhydrophobic surfaces patterned by micropillar arrays using a three-dimensional free-energy-based lattice Boltzmann method. We address how the interplay between physical parameters (i.e., Weber number) and geometric parameters (i.e., pillar density and spacing and the droplet core-shell size ratio) affects the spreading, breakup, and rebound behaviors of compound droplets, which remains unknown and unquantified. We identify three flow regimes in which the interfacial morphology between the core and shell evolves and breaks up in distinct ways: namely, hole nucleation at the substrate, rupture of the film at the apex of the shell, and toroidal formation of the core droplet before its detachment from the pillars. We demonstrate that the transition between the three regimes and the maximum spreading factor of compound droplets can be changed by varying the core-shell size ratio, the pillar density, and the Weber number. The non-wetting behavior of the pillar structures eventually forms a new suspended pure droplet or a new suspended compound droplet, which can be characterized by the core-shell size ratio, pillar density, and Weber number.