Tale of two sides: Origin of patterns in particle-induced viscous fingering

When displacing air inside a Hele-Shaw cell, non-colloidal particles suspended in the viscous oil induce viscous fingering (VF) owing to their gradual accumulation near the oil-air interface. In contrast to classical VF, this particle-induced instability gives rise to two distinct patterns: one at the interface in the form of interfacial deformations and the other within the suspension as dense particle clusters, which we call plumes. The dynamics of these patterns are closely intertwined, yet their interaction remains unexplored. To investigate this relationship, we inject a suspension of silicone oil and polyethylene particles into a rectangular Hele-Shaw cell by systematically varying the initial particle concentration, interface velocity, and oil viscosity. Confining the flow in a rectangular configuration amplifies the interplay between the evolving patterns, resulting in three distinct regimes. In Regime I, neighboring plumes grow independently while remaining attached to the advancing interface. In Regime II, the adjacent plumes merge as the interface deforms. Eventually, in Regime III, merged plumes detach from the interface and create islands that do not move with the flow. The resulting pattern is reminiscent of classical vortex shedding behind a blunt body.