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

Suspensions of noncolloidal, neutrally buoyant, spherical particles were su bjected to oscillating displacements at low Reynolds number along the axis of a circular tube. Using nuclear magnetic resonance imaging (NMRI), the ph ase distribution of a suspension with a particle volume fraction 0.4 was as sessed for a variety of conditions. The variables studied included ratio of particle to tube diameter, amplitude of oscillation, and number of oscilla tions. Consistent with macroscopic theories of shear-induced particle migra tion, the particles preferentially moved away from the walls and to the cen ter of the pipe for amplitudes of oscillation much greater than the particl e 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 o f 6.4x10(-3), the images showed that the suspension was not uniform along t he tube length for an amplitude of oscillation equivalent to one pipe diame ter. For a larger ratio of particle radius to tube radius of 1.48x10(-2), t he suspension remained uniform along the pipe for similar conditions. For t he 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 o f the pipe as predicted by the Stokesian dynamics simulations of Morris ["A nomalous particle migration in oscillatory pressure-driven suspension flow, " presented at the 1997 Annual Meeting of the AICHE (unpublished)]. These p henomena, which have not previously been observed experimentally, are not d escribed by any existing theories of shear-induced particle migration. (C) 1999 American Institute of Physics. [S1070-6631(99)03910-0].