Initial development of azimuthal instability in oxidized liquid metal drop impacts on solid surface

The splashing dynamics of droplets impacting dry solid surfaces have been widely investigated due to their relevance in applications such as additive manufacturing, inkjet printing, and spray coating. However, the very early-time instabilities that precede splashing remain poorly understood, largely due to limitations in ultra-high-speed imaging. In this work, we leverage the formation of an oxidized skin at the Galinstan–air interface to visualize the earliest undulations that give rise to later-stage radial ligaments. Using optical microscopy, we present what we believe to be the first experimental identification of the spatial onset of these early undulations for impact Weber numbers ranging from 150 to 600—coinciding with the formation of the spreading lamella. Lamella velocity at ejecation is scaled based on drop impact velocity (Boelens et al., 2018). Instead of assuming a global deceleration, we identify localized rim deceleration at the lamella formation time, which supports the relevance of Rayleigh–Taylor instability in triggering the initial undulations. Additionally, prior to the emergence of ligament structures, we observe the development of distinctive "triangular teeth" patterns—phenomenologically similar to observations in droplet impacts on liquid pool (Li et al., 2018)—suggesting a role for spanwise vortex shedding. Our study offers direct experimental insight into the onset of early-time undulations and quantitatively models their initiation locations and associated wavenumbers.