Solid-Like Behavior of Concentrated Particulate Suspensions under Squeezing Flow

We have examined the solid-like behavior of concentrated suspensions with non-Brownian particles in squeezing film flows. The apparent yield stress has been estimated with a very weak squeezing flow between a flat plate and a spherical surface with a large radius of curvature. This instrument allowed us to explore the effect of repeated squeeze flow on these materials. It was found that there was a dramatic increase in solid-like properties as the number of cycles grew, especially for the higher volume fraction samples. This growth appeared to be accelerated if we used irregularly shaped particles, slower squeeze velocities, smaller final gap values or plates with a rougher surface. The fact that this occurred in cyclic squeeze flow, and did not occur in a corresponding cyclic shear flow, suggests that the highly non-uniform nature of the flow field in squeezing contributes to this behavior. Indeed, we suggest that there is a gradual build-up over many cycles of aggregates in the central region which leads to this behavior.