  The dissolution or growth of a droplet in a host liquid is an important part
for 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 with numerical simulations. 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., 2016, J. Fluid Mech.) 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 non-monotonic, 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.
