Post-Impact Dynamics of Ballistic Droplets*

Understanding the impact of droplets on a solid support is important to a broad range of processes in engineering and nature. Despite a century or more of study, a priori predictions of outcomes as simple as spreading extent are lacking owing to the rapidly-moving contact line (CL) and related complications. Our focus is on ballistic dynamics, the regime where liquid inertia and capillary forces are dominant even with contact line motion.  Here we investigate the partitioning of kinetic energy into surface energy for capillary water droplets falling onto homogeneous chemically-prepared hydrophilic and hydrophobic surfaces. We analyze high-speed images of the impact/spreading event. Pre-impact Weber numbers range from 0-15.  Post-impact, the droplet spreads initially until the contact line pins. Post-pinning, there is a slow decay to the rest state.  During this underdamped decay, the droplet’s remaining kinetic energy partitions into a linear combination of vibrational modes corresponding to those predicted by pinned sessile droplet theory.  The influence of droplet volume, surface tension, contact angle and contact angle hysteresis on this partitioning will be discussed.