Observations of shear-induced particle migration for oscillatory flow of a suspension within a tube

Suspensions of noncolloidal, neutrally buoyant, spherical particles were subjected to oscillating displacements at low Reynolds number along the axis of a circular tube. Using nuclear magnetic resonance imaging (NMRI), the phase distribution of a suspension with a particle volume fraction 0.4 was assessed for a variety of conditions. The variables studied included ratio of particle to tube diameter, amplitude of oscillation, and number of oscillations. Consistent with macroscopic theories of shear-induced particle migration, the particles preferentially moved away from the walls and to the center of the pipe for amplitudes of oscillation much greater than the particle diameter when the ratios of particle radius to tube radius were 6.4x10(-3) and 1.48x10(-2). However, for a ratio of particle radius to tube radius of 6.4x10(-3), the images showed that the suspension was not uniform along the tube length for an amplitude of oscillation equivalent to one pipe diameter. For a larger ratio of particle radius to tube radius of 1.48x10(-2), the suspension remained uniform along the pipe for similar conditions. For the smaller ratio of particle to tube radius of 6.4x10(-3) and an amplitude of oscillation of five particle radii, the particles migrated to the wall of the pipe as predicted by the Stokesian dynamics simulations of Morris ["Anomalous particle migration in oscillatory pressure-driven suspension flow," presented at the 1997 Annual Meeting of the AICHE (unpublished)]. These phenomena, which have not previously been observed experimentally, are not described by any existing theories of shear-induced particle migration. (C) 1999 American Institute of Physics. [S1070-6631(99)03910-0].