Rebound of oscillating droplets on non-superhydrophobic surfaces

Abstract

Oscillating droplets rebounding completely from non-superhydrophobic surfaces (polydimethylsiloxane, PDMS) were experimentally studied and theoretically interpreted. The new experimental finding is that, with increasing the droplet impact Weber numbers ($We$), the recovery coefficient of droplet velocity, which is defined as the ratio of the rebounding velocity over the impact velocity, has an overall trend of decrease but in a fluctuating manner. Physically, a sufficiently large droplet freely falling under gravity has an inevitable oscillation, which makes the impacting droplet shape slightly deviate from being spherical and in turn affects the interaction between the droplet and the surface. The fluctuating recovery coefficient is the result of the periodically varying phase of droplet oscillation with increasing $We$, and increasing the droplet viscosity can suppress the droplet oscillation and then the fluctuation amplitude of the recovery coefficient. A theoretical model of oscillating droplet rebound is proposed and well fits the present experiments over a wide range of $We$.