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Abstract

The dissolution or growth of a droplet in a host liquid is an important part of processes like chemical extraction, chromatography, or emulsification. In this work we look at the dissolution of a pair of vertically aligned droplets immersed in water, both experimentally and numerically. The liquids used for the droplets are long chain alcohols with a low but finite solubility in water and a significantly lower density than that of the host liquid. Therefore, a solutal plume is formed above of the bottom droplet and natural convection dominates the dissolution process. We monitor the volume of the droplets and the velocity field around them over time. When the liquids of the two droplets are the same, our previously found scaling laws for the Sherwood and Reynolds numbers as functions of the Rayleigh number [Dietrich et al., J. Fluid Mech. 794, 45 (2016)] can be applied to the lower droplet. However, remarkably, when the liquid of the top droplet is different than that of the bottom droplet the volume as function of time becomes nonmonotonic, and an oscillatory Marangoni flow at the top droplet is observed. We identify the competition between solutal Marangoni flow and density-driven convection as the origin of the oscillation and numerically model the process.