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Suspension flows past bluff bodies: Investigation in a microfluidic environment

This sequence of fluid dynamics videos illustrates the behavior of a suspension of noncolloidal particles flowing past various bluff body obstacles within a microfluidic device. The polystyrene particles, of 7 μm diameter and volume fraction of 8.4%, are carefully made neutrally buoyant with the suspending liquid composed of a mixture of water and a small fraction of glycerol. The channel depth is 60 μm and the typical length of the obstacles normal to the flow direction is 200 μm. The flow rate is varied to generate Reynolds numbers based on the scale of the obstacle in the approximate range 60 < Re < 500; the narrow dimension in the depth direction suppresses onset of unsteadiness and vortex shedding, so that the flows studied are found to be steady (aside from particle-scale fluctuations). Particles are observed to be depleted in the wake region of the obstacle. In certain cases, the entire wake is clear of particles; in other cases there is a portion of the wake in which particles recirculate while a portion of the wake is completely devoid of particles. Experimental observations reveal that if particles are forced into an initially particle-depleted region, they will eventually leave and will bring the wake to its original state, implying these are steady-state distributions.