  We numerically investigate both single and multiple droplet dissolution with
droplets consisting of lighter liquid dissolving in a denser host liquid. The
significance of buoyancy is quantified by the Rayleigh number Ra which is the
buoyancy force over the viscous damping force. In this study, Ra spans almost
four decades from 0.1 to 400. We focus on how the mass flux, characterized by
the Sherwood number Sh, and the flow morphologies depend on Ra. For single
droplet dissolution, we first show the transition of the Sh(Ra) scaling from a
constant value to $Sh\sim Ra^{1/4}$, which confirms the experimental results by
Dietrich et al. (J. Fluid Mech., vol. 794, 2016, pp. 45--67). The two distinct
regimes, namely the diffusively- and the convectively-dominated regime, exhibit
different flow morphologies: when Ra>=10, a buoyant plume is clearly visible
which contrasts sharply to the pure diffusion case at low Ra. For multiple
droplet dissolution, the well-known shielding effect comes into play at low Ra
so that the dissolution rate is slower as compared to the single droplet case.
However, at high Ra, convection becomes more and more dominant so that a
collective plume enhances the mass flux, and remarkably the multiple droplets
dissolve faster than a single droplet. This has also been found in the
experiments by Laghezza et al. (Soft Matter, vol. 12, 2016, pp. 5787--5796). We
explain this enhancement by the formation of a single, larger plume rather than
several individual plumes. Moreover, there is an optimal Ra at which the
enhancement is maximized, because the single plume is narrower at larger Ra,
which thus hinders the enhancement. Our findings demonstrate a new mechanism in
collective droplet dissolution, which is the merging of the plumes, that leads
to non-trivial phenomena, contrasting the shielding effect.
