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We numerically investigate both single and multiple droplet dissolution withdroplets consisting of lighter liquid dissolving in a denser host liquid. Thesignificance of buoyancy is quantified by the Rayleigh number Ra which is thebuoyancy force over the viscous damping force. In this study, Ra spans almostfour decades from 0.1 to 400. We focus on how the mass flux, characterized bythe Sherwood number Sh, and the flow morphologies depend on Ra. For singledroplet dissolution, we first show the transition of the Sh(Ra) scaling from aconstant value to $Sh\sim Ra^{1/4}$, which confirms the experimental results byDietrich et al. (J. Fluid Mech., vol. 794, 2016, pp. 45--67). The two distinctregimes, namely the diffusively- and the convectively-dominated regime, exhibitdifferent flow morphologies: when Ra>=10, a buoyant plume is clearly visiblewhich contrasts sharply to the pure diffusion case at low Ra. For multipledroplet dissolution, the well-known shielding effect comes into play at low Raso 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 acollective plume enhances the mass flux, and remarkably the multiple dropletsdissolve faster than a single droplet. This has also been found in theexperiments by Laghezza et al. (Soft Matter, vol. 12, 2016, pp. 5787--5796). Weexplain this enhancement by the formation of a single, larger plume rather thanseveral individual plumes. Moreover, there is an optimal Ra at which theenhancement is maximized, because the single plume is narrower at larger Ra,which thus hinders the enhancement. Our findings demonstrate a new mechanism incollective droplet dissolution, which is the merging of the plumes, that leadsto non-trivial phenomena, contrasting the shielding effect.

Convection-dominated dissolution for single and multiple immersed sessile droplets