Dissolution regimes of a horizontal channel in a gravity field

Buoyancy-driven dissolution of the solid phase is common in natural processes and subsurface applications, such as geomorphology, solution mining, and geological CO2 storage. When an external horizontal flow is imposed, the dissolution dynamics are controlled by the interplay between buoyancy-driven and forced convections. The reshaping of the solid surface due to this interplay is not well understood. Here, we fabricate a soluble microfluidic chip with a horizontal channel to investigate the pore-scale dissolution dynamics in a gravity field. We observe that the wave number and the surface roughness factor of the upper solid-liquid interface initially increases with the flow rate (Peclet number Pe) and then decreases, indicating two dissolution regimes, namely, Regimes I and II. Microparticle-image-velocimetry-based imaging reveals that the eddy evolution occurring in the flow-dissolution system controls the local dissolution rate and the geometry evolution of the solid-liquid interface. Through the quantification of the eddies (or troughs) for the whole channel, the number of the eddies increases with Pe in Regime I and decreases with Pe in Regime II, indicating that the buoyancy-driven convection is enhanced in Regime I and suppressed in Regime II by the forced convection. Finally, by performing a theoretical analysis of the density gradient, we obtain a scaled critical aperture, i.e., the critical aperture normalized by a characteristic length, for the onset of unstable dissolution of the solid-liquid interface. Such a scaled critical aperture is constant for both dissolution regimes. In this paper, we elucidate the crucial role of eddies in the flow-dissolution system in a gravity field. Moreover, we improve our understanding of the interplay between buoyancy-driven and forced convections in etching the solid surface.