Marangoni flow induced in a waterbody by the impact of a raindrop

The particle image velocimetry (PIV) technique is used to measure the transient flow induced near the surface of a waterbody due to the impact of a freshwater drop. The focus of this work is on the influence of surfactant and salt concentrations of the waterbody on the induced Marangoni flow. The latter arises due to the mixing of the drop's water with the surface water, which reduces the concentration of salt and surfactant on the surface, changing the surface tension of the water in the impact area. Since the surface tension of water increases with increasing salt concentration and decreases with increasing surfactant concentration, the net Marangoni flow depends on the salt and surfactant concentrations in the waterbody. Experiments show that when the surfactant-induced Marangoni flow dominates, the radially outward flow on the surface due to the drop's impact quickly reverses direction and becomes directed towards the point of impact. However, when the salt-induced Marangoni flow dominates, the radially outward flow on the surface due to the impact of the drop is enhanced and persists for a longer time interval.