Using translational shear to smooth the interface between pairs of miscible fluids and delay the onset of the viscous fingering instability.

We study the viscous fingering instability occurring in miscible fluids confined between the plates of a Hele-Shaw cell. In this case, the inter-fluid interface becomes unstable if the lower viscosity fluid displaces the one with higher viscosity. We are particularly interested in finding how the shape of the interface, spanning the gap between the plates, affects the onset and subsequent growth of fingers. Using experiments and simulations, we perturb the interface shape by applying uniaxial translational shear during fluid injection. In the direction parallel to the shear axis, shear begins to smooth the interface shape when the shear speed approaches the speed of the interface propagation. At higher shear speeds, increasing the shear amplitude leads to smoother interfaces. We measure the onset radius and finger growth rate and find that smoother interfaces correlate with delayed onset and slower growth. Uniaxial shear also breaks azimuthal symmetry. We find that fingers growing in the direction perpendicular to the shear axis are wider than those growing in the parallel directions.