Fingering instabilities in confined suspensions

We experimentally inject silicone oil into the mixture of oil and non-colloidal particles inside a Hele-Shaw cell, to investigate the connection between miscible fingering and the flow structure that develops inside the suspension. Previous studies with pure fluids have demonstrated that the onset of miscible fingering coincides with the transition from a smooth tongue-like structure to a sharp front between invading and defending fluids inside the thin gap. Our current experiments with suspensions reveal the same general behavior at the onset of miscible fingering in the continuum limit. However, beyond the onset, we observe the changes in morphologies of miscible fingering, as the channel confinement is systematically increased. Particularly, in the monolayer limit, we observe the emergence of fingering with two distinct wavelengths, which strongly depends on the local particle volume fraction. To qualitatively describe our experimental observations, we seek a kinetic theory that implements long-range hydrodynamic interactions.