Heat or mass transfer around an evaporating drop or condensing vapor bubble is a complex issue due to the interplay between the substrate properties, diffusion- and convection-driven mass transfer, and Marangoni effects, to mention but a few. In order to disentangle these mechanisms, we focus here mainly on the convective mass transfer contribution in an isothermal mass transfer problem. For this, we study the case of a millimetric carbon dioxide bubble which is suspended under a substrate and dissolved into pure liquid water. The high solubility of ${\mathrm{CO}}_2$ in water makes the liquid denser and promotes a buoyant-driven flow at a high (solutal) Rayleigh number ($\text{Ra}\sim {10}^4$). The alteration of $p\mathrm{H}$ allows the concentration field in the liquid to be imaged by laser fluorescence enabling us to measure both the global mass flux (bubble volume, contact angle) and local mass flux around the bubble along time. After a short period of mass diffusion, where the boundary layer thickens like the square root of time, convection starts and the ${\mathrm{CO}}_2$ is carried by a plume falling at constant velocity. The boundary layer thickness then reaches a plateau which depends on the bubble cross section. Meanwhile the plume velocity scales like ${\left(dV/dt\right)}^{1/2}$ with $V$ being the volume of the bubble. As for the rate of volume loss, we recover a constant mass flux in the diffusion-driven regime followed by a decrease in the volume $V$ like $V^{2/3}$ after convection has started. We present a model which agrees well with the bubble dynamics and discuss our results in the context of droplet evaporation, as well as high Rayleigh convection.

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• Convection
• Drop & bubble phenomena
• Rayleigh-Bénard convection
• Drops & bubbles