Ring-bouncing induced by the head-on impact of two nanodroplets on superhydrophobic surfaces

Efficient droplet shedding from surfaces is fundamentally interesting and important due to its promising potential in numerous applications, such as anti-erosion, anti-icing, and self-cleaning. In this work, the bouncing dynamics of the head-on impact of two nanodroplets on superhydrophobic surfaces are investigated through molecular dynamics simulations. Three bouncing patterns, including regular-coalescence-bouncing, coalescence-hole-bouncing, and ring-bouncing, are identified at a wide range of impacting Weber numbers. For three bouncing patterns, the time evolutions of the spreading factors and the vertical velocity components are employed to analyze the particular dynamic behaviors and elucidate the underlying physics. As a counter-intuitive bouncing pattern, the ring-bouncing that two impact nanodroplets coalesce, spread, and then leave the surface in a ring shape without retracting exhibits a remarkable reduction in contact time by up to 60%. Considering four typical states for the ring-bouncing pattern, the comparison of the velocity distribution within the droplet clearly reveals that the ring-shaped droplet reshapes interfaces, which leads to a special hydrodynamics distribution. As a result, the internal flows at the inner and outer edges along the opposite direction collide with each other, leading to a sudden increase in the upward velocity. Combining the largely decreased contact area between solid and liquid with the small surface adhesion, the ring-shaped droplet rapidly bounces off the surface at the maximum spreading state. Finally, it is significantly highlighted that the ring-bouncing pattern offers a new avenue to break the contact time limit for efficient droplet shedding.