Bouncing dynamics and directional migration of two off-center droplets impacting on a homogeneous/heterogeneous wettability surface

Horizontal transport of droplets is important for anti-icing, self-cleaning, and water collection technologies. In this paper, the bouncing dynamics of two off-center droplets impacting on a solid surface are explored via numerical simulations. Two kinds of surface wettability are considered. One is a superhydrophobic surface (homogeneous wettability), and the other is a superhydrophobic surface arranged with a hydrophilic strip (heterogeneous wettability). First, the effect of an eccentric angle on the horizontal transport of two droplets impacting on a homogeneous wettability surface is investigated. As the eccentric angle increases, the merged droplet after impact may experience horizontal transportation in either the positive or negative direction. When a heterogeneous wettability surface is adopted, the changes in the transport direction of the merged droplet are consistent with the location of the hydrophilic strip upon eccentric impact. By adjusting the position of the hydrophilic strip, the maximum horizontal transportation distance is 7.53 times the diameter of the initial droplet when the eccentric angle is 10°, while it is only 2.01 times the diameter of the initial droplet when the eccentric angle is 20° for a homogeneous wettability surface. It is indicated that the use of the heterogeneous wettability surface is significantly efficient for droplet control after impact. The obtained results contribute to the fundamental understanding of droplet directional migration and are valuable for related engineering applications.