Early-time azimuthal undulation of liquid metal drop impacts on solid surface

Splashing of an impact droplet on dry solid surfaces has attracted numerous studies. For Galinstan drops, an oxidized skin formed at drop surface "freezes" and visualizes early-time undulation and radial ligament formation at microseconds after impact, usually requiring ultra-high-speed cameras to capture. Via optical microscopy the earliest undulation onset is found to initiate at the cusp region between ejected lamella and bulk and the location of cusp formation Rc is observed at impact Weber numbers from 150 to 600. Cusp formation velocity is predicted based on scaling analyses involving lamella ejection and drop impact velocities; our model agrees reasonably well with previous theoretical scaling and direct numerical simulations. Rather than past works assuming effective deceleration takes place instantly after drop impacts, we correctly determine deceleration at Rc for cusp formation and justify the relevance of RT instability mechanisms in this early-time undulation. Phenomenologically similar to drop impacts on liquid surface, "triangular teeth" patterns are developed at Rc before the presence of ligament lines, potentially due to spanwise vortex shedding (Li et al, 2018). This work provides experimental evidence of cusp formation and models precisely the onset location of early-time undulation and its wavenumber at various drop impact velocities.