Focusing patterns of spherical particles suspended in square tube flows of viscoelastic fluids

The particle focusing in square tube flows was investigated experimentally for spherical particles suspended in aqueous solutions of polyvinylpyrrolidone (PVP). The location of each particle center in the tube cross-section was detected near the outlet by an “enface” observation method, and the distribution of particles was obtained for various PVP concentrations, blockage ratios and Reynolds numbers (Re). In 4wt% PVP solutions, the particles were focused on the tube center and near the corners at low Re, whereas, at slightly elevated Re, all particles migrated toward the tube center. At elevated Re, the particles suspended in 1wt% PVP solutions were focused at the center of channel faces, in accord to those in Newtonian fluids. An increase in PVP concentrations at the same Re shifted the focusing position to the equilibrium position on the diagonal. These transitions of the particle focusing pattern were accounted for by the interplay between the inertial lift and elasticity-induced lift.