Effects of medium elasticity on the inertial focusing of spherical particles in square tube flows

The inertial focusing of rigid spherical particles suspended in aqueous polyvinylpyrrolidone (PVP) solutions of various concentrations was investigated experimentally for square tube flows. The PVP solution has a nearly constant viscosity independent of shear rate, but has elastic properties dependent on the PVP concentration. A dilute suspension of spherical particles in PVP solutions was injected into a straight tube with a square cross-section. The position of each particle center in the tube cross-section was detected near the outlet by an enface observation method, and the evolution of the particle distribution from the tube inlet was obtained by using tubes of various lengths. In the solution with very low PVP concentrations, suspended particles migrated towards four points located at the center of channel faces, as in Newtonian fluids. As the PVP concentration increased, the particle focusing position was shifted to equilibrium positions on the diagonals, on the tube centerline as well as on the diagonal, and on the tube centerline. This variation of the particle focusing pattern can be accounted for by increasing the elasticity-induced lift which is directed inward in the radial direction and toward the diagonal in the azimuthal direction.