(319i) Can We Hear the Angle of Droplet Impact?

Oblique droplet impact onto dry surfaces is ubiquitous in industry and nature, yet scarce in the literature. Here we investigate the post-impact dynamics of capillary water droplets obliquely impacting homogeneous chemically-prepared hydrophilic and hydrophobic surfaces, with equilibrium contact angles ranging from 70-105°. We analyze high-speed images (>= 5000 fps) of the impacting event. Pre-impact Weber numbers range from 0-16. Impingement angles range from 0-53°. After impact, the droplet’s contact line spreads dissipating energy and after spreading pins. Post-pinning, the droplet’s perturbed liquid/gas interface slowly decays to its rest state. During this underdamped decay, the droplet’s remaining kinetic energy partitions into a linear combination of axisymmetric and non-axisymmetric vibrational modes corresponding to those predicted by theory for the pinned horizontally-aligned (non-oblique) sessile droplet. We analyze the partitioning between these distinct vibrational modes using Fast Fourier transforms and rationalize our results using the droplet’s center of mass motion. The influence of droplet volume, impingement angle, and equilibrium contact angle on these post-impact dynamics will be discussed.