A rebounding droplet impacting on a static droplet

This study aims to experimentally examine rebounding characteristics of a free-fall droplet which impacts on a static droplet with a spherical-cap shape in a low Weber number range. Two well-aligned high-speed cameras were used for visualization of droplet behaviors after impact. From the image analysis, the influence of impact velocity, offset distance and static droplet volume on the rebound characteristics was examined. The results show that the dynamic contact angle and the contact diameter of a static droplet change significantly with time after impact. In particular, a substantial change in dynamic contact angles for different offset distances is also observed. Contact diameters are also varied larger with the increase in the impact velocity. It is thought that occurrence of droplet rebound may be because of air-gap formation at the interface inhibiting viscous film drainage, coming from droplet oscillation before impact.