  The dissolution and subsequent mass transfer of carbon dioxide gas into
liquid barriers plays a vital role in many environmental and industrial
applications. In this work, we study the downward dissolution and propagation
dynamics of CO2 into a vertical water barrier confined to a narrow vertical
glass cylinder, using both experiments and direct numerical simulations.
Initially, the dissolution of CO2 results in the formation of a CO2-rich water
layer, which is denser in comparison to pure water, at the top gas-liquid
interface. Continued dissolution of CO2 into the water barrier results in the
layer becoming gravitationally unstable, leading to the onset of buoyancy
driven convection and, consequently, the shedding of a buoyant plume. By adding
sodium fluorescein, a pH-sensitive fluorophore, we directly visualise the
dissolution and propagation of the CO2 across the liquid barrier. Tracking the
CO2 front propagation in time results in the discovery of two distinct
transport regimes, a purely diffusive regime and an enhanced diffusive regime.
Using direct numerical simulations, we are able to successfully explain the
propagation dynamics of these two transport regimes in this laterally strongly
confined geometry, namely by disentangling the contributions of diffusion and
convection to the propagation of the CO2 front.
