Measuring shear-induced self-diffusion in a counterrotating geometry - art. no. 021403

The novel correlation method to measure shear-induced self-diffusion in con centrated suspensions of non-colloidal hard spheres which we developed rece ntly [J. Fluid Mech. 375, 297 (1998)] has been applied in a dedicated count errotating geometry. The counterrotating nature of the setup enables experi ments over a wider range of well-controlled dimensionless time ((gamma) ove r dot Deltat in the range 0.03-3.5, compared to 0.05-0.6 in previous experi ments; here (gamma) over dot denotes the shear rate and Deltat the correlat ion time). The accessible range of timescales made it possible to study the nature of the particle motion in a more detailed way. The wide radius geom etry provides a well-defined flow field and was designed such that there is optical access from different directions. As a result, shear-induced self- diffusion coefficients could be determined as a function of particle volume fraction phi (0.20-0.50) in both the vorticity and velocity gradient direc tion. A transition could be observed to occur for (gamma) over dot Deltat o f O(1), above which the particle motion is diffusive. The corresponding sel f-diffusion coefficients do not increase monotonically with particle volume fraction, as has been suggested by numerical calculations and theoretical modeling of Brady and Morris [J. Fluid Mech. 348, 103 (1997)]. After an exp onential growth up to phi = 0.35, the diffusion coefficients level off. The experiments even suggest the existence of a maximum around phi = 0.40. The results are in good agreement with experimental literature data of Phan an d Leighton [J. Fluid Mech. (submitted)], although these measurements were p erformed for much larger values of the dimensionless time (gamma) over dot Deltat.