(524b) Marangoni-Driven Flow Oscillations during the Dissolution of Surfactant Powders

When particles of surface-active compounds are deposited on a liquid surface, they exhibit a variety of motions. A classic example is the erratic movement of camphor on water. While systems with one particle and several coupled particles have been previously studied, we examine the behavior of a water surface covered with soluble particles. We find that a rapid longitudinal oscillation occurs during their dissolution. This phenomenon happens with several common surfactant powders, but it is particularly striking with calcium propionate, an organic salt whose aqueous solutions have a lower surface tension than water. We characterize the oscillation of the surface of solutions with varying initial concentrations of the salt. Increasing the initial solute concentration decreases the available driving force for motion, which is the maximum difference in surface tension between regions with high and low surfactant concentration, and suppresses the oscillation. Due to the short period of the oscillation (on the order of 0.1 s) compared to the timescale of surfactant diffusion, we neglect diffusion and model the phenomenon by considering the evolution and stability of variations in surfactant concentration along a surface. This surfactant concentration is advected by the flow that is driven by Marangoni stresses due to the variations in surface tension. We examine the critical conditions for and characteristics of the oscillation in this model through theory and simulations.