Shear-driven diffusion in suspensions, studied via 1-d dynamic NMR projection

We have extended our earlier study of Couette flow using a dynamic 1-d NMR projection technique to suspensions of 50% polydisperse SiO2 powder in a po lybutadiene melt. The relatively narrow Couette gap and slow shaft rotation rate make the development of a concentration gradient from the migration o f the solids to low-shear regions unobservable in static projections during and immediately following the experiment. Using dynamic projection during shaft rotation we detect pronounced departures from uniform tangential flow of the host liquid over spatial scales well below the positional resolutio n of the experiment. Using numerical and Monte Carlo simulations of the NMR interferograms based on a superposition of smooth flow and pseudo-Brownian motion, we successfully interpret host molecular motion in terms of an eff ective "dynamic" diffusion coefficient which is found to be proportional to the shaft rotation rate. This description of the motion of the host fluid complements recent observation of dynamic diffusion of the suspended partic les in similar experiments. (C) 1999 The Japan Society of Fluid Mechanics a nd Elsevier Science B.V. All rights reserved.