Impact of an oxidized liquid metal on solid surface: Revealing the onset of instability due to later-time restoring retraction

While splashing of an impact droplet on dry solid surfaces has been attracted numerous studies since Renaissance, one involving liquid metal drops has been studied only recently, despite of their exhibited distinctive dynamics due to high surface tension and tendency to form an oxidized skin at surface. Such a layer, albeit only nano thin, alters splashing patterns significantly and provides a valuable opportunity to visualize the initial formation and development of the instability: at a much larger time scale compared to the impacting one, the film prevents liquid metal from further oxidization and enables retraction of the enclosed metal, revealing aesthetic streaks tracing back to the entrained central bubble at center. The restoring dynamics, splash instability, as well as presence of entrained bubble are of fundamental interests as well as industrial applications, leading to delamination-induced defects in additive manufacturing with liquid metals. Conducting with impacting Galinstan drops of Weber numbers from 145 to 500, this study systematically approaches to both dynamics of restoring retraction and instability evidenced via the revealed splash patterns of an oxidized liquid metal drop impinging on solid surface, where the retraction mechanism is further investigated, and wavenumber of instability and its features (i.e., bubbles and microjets) reported via optical microscope with scaling analysis.