Droplet rebound from rigid or strongly hydrophobic sur- faces

– A weakly deformable droplet impinging on a rigid surface rebounds if the surface is intrinsically hydrophobic or if the gas film trapped underneath the droplet is able to keep the interfaces from touching. A simple, physically motivated model inspired by analysis of droplets colliding with deformable interfaces is proposed in order to investigate the dynamics of the rebound process and the effects of gravity. The analysis yields estimates of the bounce time that are in very good quantitative agreement with recent experimental data (Okumura et. al., (2003)) and provides significant improvement over simple scaling results. Introduction. – The collision and rebound of droplets has been a subject of interest and investigation for many decades. Impacting drops undergo energetic bounces if the pressure in the gas film separating the drops deforms the drop surfaces sufficiently to transform the drop’s kinetic energy into deformation energy before contact occurs. Collisions in a incompressible, continuum gas usually result in a bounce with contact occurring due to inter-particle forces or surface imperfections. Bouncing can also result when drops impinge on rigid super hydrophobic surfaces or on rigid walls heated to well above the Leidenfrost temperature. Very recently, Okumura et. al., presented experimental data for the collision and rebound, in air, of 400− 1000 μm water drops from a super-hydrophobic surface. They found that the measured bounce time was consistently larger than that predicted by simple scaling. Furthermore, with the drop radius held fixed, the contact time was seen to increase significantly as the velocity of impact decreased. This was attributed to effects of finite drop size and gravity. In this letter, a physically motivated model is proposed to investigate this scenario, specifically the dependence of the bounce time and deformation on droplet size and velocity. Use of the methodology developed in similar problems involving droplets colliding with deformable interfaces, indicates that even when gravity is absent the bounce time for a liquid drop increases as the impact velocity decreases. Consideration of gravity induced effects suggests that the bounce time is modestly modified relative to the zero-gravity value. The predicted results are in excellent quantitative agreement with experimental data of Okumura et. al., showing that both effects are needed to account properly for the details of the bounce.