On the nonlocality of dense frictionless suspensions: Experimental evidence and proposed theory*

We have recently characterized the rheology of viscous and inertial suspensions of frictionless particles across multiple lengths from nm to m by coupling advanced force microscopy, conventional rheometry, and lab-scale experiments. The well-resolved experimental results demonstrate that the transition from the viscous to inertial regime occurs at a surprisingly small value of the particle Reynolds number suggesting that a larger length scale may play a role. Therefore, the current local rheological laws based on binary collisions and transfer of momentum are inadequate in explaining the behavior of dense suspensions and their transition. These experiments suggest a hypothesis that the interplay of colloidal and hydrodynamic forces leads to the formation of clusters of particles which dominate the transmission of momentum across the material. In this talk, we discuss our hypothesis and put forward a proposed theory to address the nonlocality of dense frictionless suspensions accounting for the viscous stresses and inertial impacts acting on the clusters. We provide support for our hypothesis and proposed theory by presenting our preliminary experimental results of cluster formation in channel flows of dense suspensions.