Measurement of an unexpectedly large shear-induced self-diffusivity in a dilute suspension of spheres

We report the measurement of unexpectedly large shear-induced diffusivities for various sized tracers in a dilute suspension of noncolloidal spheres in simple shear. The suspension was sheared in a narrow gap Couette device at low Reynolds number, and the tracer diffusivities parallel to the velocity gradient D were obtained using an orbit-time technique. It is shown that the presence of even a dilute concentration phi of particles renders an otherwise smooth tracer trajectory strongly stochastic and the resulting diffusivity is linear in phi in this limit. indicating irreversible displacements at the pair interaction level. The measured values of the diffusivity, however, are at least an order of magnitude larger than that predicted by current theories. In a previous study, Beimfohr et al. [Proc. DOE/NSF Workshop. Ithaca. NY (1993)] likewise obtained values for D larger than that predicted by theory. but the discrepancy was attributed to the large eccentricity (average aspect ratio = 1.19) of the particles used. In the present study. very nearly spherical ground acrylic particles were used. yet diffusivities of the same order of magnitude as that obtained by Beimfohr et al. were measured. Various possible causes for this anomalous diffusivity were explored and tested in the course of our investigation. including the effect of inertial lift and possible non-Newtonian properties of the base fluid. but none proved sufficiently large to account for the observed behavior. (C) 2002 American Institute of Physics.