Probing particle transport in closed-streamline flows with microfluidic devices

We use microfluidic devices to study the flow of neutrally-buoyant suspension around bluff-bodies. We use low-viscosity liquids and monodisperse particles of diameter below 10 $\mu $m at a constant concentration of 8.4 volume {\%}. Several bluff-body geometries are introduced and by using high-speed video imaging we observe a striking segregation of the particles and fluid in the wake region at elevated Reynolds numbers. Based on 2-D and 3-D flow field simulations, we interpret the migration of particles and their trajectories across the streamlines based on the geometry of the bluff-body. Experimental observations reveal that if particles are forced into an initially particle-depleted region, they will eventually leave and will bring the system to its original state.