Splashing of a Small Droplet Impinging on a Solid Surface at High Velocity

We photographed water droplets (550 μm diameter) as they impacted on a stainless steel surface. We varied droplet impact velocity (10–40 m/s) and the average surface roughness (0.03–0.23 μm) of the steel plates used as test surfaces in our experiments. A piezoelectric droplet generator was used to produce water droplets. The stainless steel substrate was mounted on the end of a rotating arm, giving linear velocities of up to 40 m/s. A CCD video camera was used to photograph droplets impinging on the substrate. By synchronizing the ejection of a single droplet with the position of the rotating arm and triggering of the camera, different stages of droplet impact were photographed. From these photographs we measured the size of droplets as they spread. It was observed that as the impact velocity increased, finger-shape perturbations around the spreading droplet became longer and narrower. At sufficiently high velocities the tips of these fingers detached, producing satellite droplets. Increasing surface roughness was found to promote splashing and reduce the velocity at which splashing was first observed. By increasing surface roughness, both the number of fingers and the maximum extent of spreading were decreased. At high impact velocities the spreading liquid film became so thin that it ruptured in several places.