Particle-induced miscible fingering: Continuum limit

We experimentally inject silicone oil into the mixture of oil and noncolloidal particles inside a Hele-Shaw cell, to investigate the connection between miscible fingering and the interfacial structure that develops inside the thin gap. Previous studies with pure fluids have demonstrated that the onset of miscible fingering coincides with the transition from a smooth tonguelike structure to a sharp front between invading and defending fluids inside the gap. Our current experiments with suspensions reveal the same general behavior at the onset of miscible fingering, which we capture qualitatively using a lubrication model. However, distinct from the pure liquid counterpart, we observe changes in the interfacial structures inside the gap as the ratio of the gap thickness to particle diameter is systematically varied. We demonstrate that the particle dynamics inside the thin gap may alter interfacial structures even within the continuum limit, which lead to changes in morphologies of miscible fingering. The results of our study suggest a potential use of the wall confinement to control hydrodynamic instabilities in suspensions.