Experimental study on the migration of finite-size spherical particles in rectangular channels in Newtonian and viscoelastic fluids

We experimentally investigate the finite-size spherical particle migration in micro-rectangular channels in a Newtonian fluid (22 wt. % glycerol aqueous solution) and a viscoelastic fluid (8 wt. % polyvinylpyrrolidone aqueous solution) at various particle size to hydraulic diameter ratios a/Dh and channel aspect ratios H/W. Experiments are performed in rectangular channels at low Reynolds numbers (Re) and Weissenberg numbers (Wi) ranging from 0.09 to 1.53. Particles are almost mono-disperse neutrally buoyant polystyrene spheres with diameters of 20 μm, 30 μm, and 50 μm. The channel aspect ratios are 1, ½, and 1/3. The results show that the migration distributions differ significantly between the two fluids at the same mass flow rate. In square duct flow, we find that Re and a/Dh are key parameters governing particle migration in dilute suspensions. With increasing Re, both in Newtonian and in viscoelastic fluids, we observe a transition from a disordered to an ordered distribution in square duct flow. Particles in Newtonian fluid accumulate at the off-center long midline, while in the viscoelastic fluid, small and large particles accumulate at five points (four corners and the center) and at the center point of the duct, respectively. As the H/W decreases in rectangular channel flow, the focusing position of the particles shifts from four planes near the walls to two planes parallel to the longer walls and eventually to one center-plane in the Newtonian fluid. In the viscoelastic fluid, the focusing position transitions from five points to the center and then to the mid-plane-parallel to longer walls.