Suppressing fingering instabilities using gradients in porous media

Drainage, the displacement of a wetting fluid from a porous medium by an immiscible non-wetting fluid, arises in key technological problems including oil recovery, groundwater contamination, and waste CO₂ sequestration. When the displacing non-wetting fluid is less viscous than the displaced fluid, Viscous Fingering occurs. By contrast, when the displacing fluid is more viscous than the displaced fluid, Capillary Fingering occurs. These two processes lead to disordered finger-like displacement pathways. For both cases, we find that a pore size gradient stabilizes the flow--the fluid flows uniformly without fingering. For Viscous Fingering, this stabilization only occurs below a threshold flow rate [1], while for Capillary Fingering, it only occurs when the gradient exceeds a static geometric criterion. Knowing these two suppression criteria provides a new way to control fingering instabilities in porous media.
[1] The Viscous Fingering work was done previously in collaboration with H. S. Rabbani, D. Or, Y. Liu, C-Y Lai, H. A. Stone, and N. Shokri; see our recent publication, "Suppressing viscous fingering in structured porous media." PNAS 115, 4833 (2018).