Suspension dynamics: From accumulation to instability

In this talk, we examine how the presence of suspended particles can cause instabilities by generating changes in the local rheology. First, we observe a particle-induced instability when a mixture of non-colloidal particles and viscous oil displaces air inside a Hele-Shaw cell. Our experimental results show that particles tend to accumulate on the oil-air interface, which results in viscous fingering near the interface. We introduce the mechanism of particle accumulation and rationalize the critical wavenumber of instability based on a reduced mathematical model. In the second part of the talk, we delve further into the particle-induced fingering by experimentally injecting silicone oil into the mixture of the same oil and particles inside a Hele-Shaw cell. Here, the viscosity difference between the pure oil and the suspension is expected to generate miscible viscous fingering. As previously observed in pure liquids, our experiments reveal a connection between miscible fingering and the interfacial structure that develops inside the thin gap, However, distinct from the pure liquid counterpart, we observe particle-induced changes in the interfacial structures, as the ratio of the gap thickness to particle diameter is systematically varied. We discuss the physical mechanism behind these changes that are unique to suspensions and the role of channel confinement in controlling hydrodynamic instabilities in suspensions.