Particle-Induced Viscous Fingering: Effects of Global Geometry

The displacement of air with a suspension inside a Hele-Shaw cell leads to the accumulation and fingering of particles near the oil-air interface. Termed "particle-induced viscous fingering", this surprising instability has been extensively investigated in radial geometry. In this study, we extend this fingering phenomenon to a rectangular geometry to remove the inherent time-dependent nature of the radial flow. To that end, we experimentally inject a mixture of silicone oil and non-colloidal particles into a confined rectangular cell. Our results demonstrate unexpected folding and merging of fingers over time, which is not observed in the radial geometry. We characterize this new nonlinear fingering regime via image processing and discuss the physical mechanism of this geometry-dependent behavior.