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

Abstract

In this study, the Particle Image Velocimetry (PIV) technique is employed to investigate the transient flow that arises when a freshwater drop impinges onto the surface of a water body. Our measurements show that in addition to the impact of the drop itself, surfactant and salt concentration play a crucial role in determining the resulting flow on and near the surface. The latter contributions arise from surface tension gradients that arise from merging the drop’s water with the water on the surface, which reduces the concentrations of salt and surfactant. As the surface tension of water falls with decreasing salt concentration, the decrease in salt concentration leads to a Marangoni flow, which moves radially outward from the impact area. On the other hand, the reduction in surfactant concentration causes a reverse Marangoni flow, which flows towards the impact point as the surface tension of water increases with decreasing surfactant concentration. The overall direction and strength of the flow depend on the Weber number and the surfactant and salt Marangoni numbers. Experiments indicate that when the surfactant Marangoni number dominates, the radial flow due to the drop’s impact quickly reverses direction giving rise to a jet-like flow from the impact point in the downward direction. In contrast, when the salt Marangoni flow dominates, the radial flow persists for a longer duration, and the diameter of the region free of PIV particles on the surface is larger than for the freshwater case. The height of the water column increases with increasing Weber and surfactant Marangoni numbers but decreases with salt Marangoni number. The strength and direction of the surface flow are crucial for enhancing the mixing of dissolved gases in the near-surface layer into the water body.