Dual Instability in Marangoni Flows during Drop Impact*

Marangoni flow, driven by spatial variations of surface tension, usually occurs where the temperature or concentration gradients are present. Here, we discovered a novel two-step instability phenomenon in the Marangoni flows when a water drop impacts a thin glycerol film. Through high-speed imaging, we observed the formation of a series of mushroom vortices at initial contact, which is determined by the undulations of the liquid interfaces and reinforced by the surface tension instability following the impact. After an intensive mixture through the initial mushroom vortices, we observed the formation of branching leaves due to Marangoni instability at the air-liquid interfaces where present unsteady concentrations of water and glycerol, and a flower pattern is formed at the final state. Through particle tracking velocimetry, we obtained velocity profiles in the mushroom vortices and later branching leaves. We found that the flow velocities in the initial mushroom vortices (~300 mm/s) are much higher than those in the branching leaves (~3 mm/s). These findings reveal that the water drop impacting a glycerol film has a dual Marangoni instability at different timescales, which provides new insight into Marangoni flows when a drop interacts with a miscible liquid film.