Particle-induced viscous fingering: Review and outlook

Changes in bulk rheology of suspensions with the particle concentration are well documented, ranging from normal stress differences to shear-thinning and shear-thickening behaviors. However, relatively little is known about the impact this bulk rheology can have on the interfacial dynamics of free-surface, particle-laden flows. Recently, it has been demonstrated that particles suspended in liquid may cause viscous fingering by modifying the local rheology near the fluid-fluid interface, when a mixture of noncolloidal particles and viscous oil displaces air inside a Hele-Shaw cell. As the same flow configuration sans particles is inherently stable to viscous fingering, the simplicity of the baseline flow renders this a perfect model problem to study to gain insight into the coupling between the bulk rheology and fluid-fluid interface. To that end, we first summarize the recent experimental findings of this particle-induced viscous fingering, which highlights the effect of the particle volume fraction and the particle diameter relative to the gap size on the emergent finger geometries. We also present experimental measurements of the critical viscosity gradient at the fingering onset, as well as the surprising time-independent finger growth regime for sufficiently large volume fractions. These results-both old and new-together highlight the similarities as well as important differences between fingering in suspensions versus pure liquid, suggesting a direction for continued research at the intersection between suspension rheology and interfacial dynamics.