"Shear Thickening of Brownian and non-Brownian suspensions: the Essential Role of Frictional Contact" Ryohei Seto

Speaker: Ryohei Seto is Research Associate of Benjamin Levich Institute at The City College of New York, working with Jeffrey F. Morris and Morton M. Denn. The use of modeling and numerical simulations, to investigate the rheological properties of colloidal suspensions, has been the focus of his research over the past several years. He was postdoctoral fellow working with Robert Botet, Martine Meireles, and Bernard Cabane in France, and also postdoctoral fellow working Heiko Briesen, Günter K. Auernhammer, and Hans-Jürgen Butt in Germany.

Abstract:  Particles suspended in a Newtonian fluid raise the viscosity, and there is generally also a shear rate dependence of the suspension rheology that can include shear thickening, in which the suspension becomes more viscous when sheared faster. Shear thickening can be a dramatic effect: a person can run on top of a pool filled with a cornstarch suspension, for example, because of the intense shear thickening.

Shear thickening of suspensions has historically been studied from a fluid mechanics perspective, and the emphasis has been on a description based on hydrodynamic interactions. The key point is that the hard-sphere particles are simply treated as boundary conditions for the Stokes equations describing the suspending fluid in the low Reynolds-number limit; the particles never directly generate forces through contacts in this limit, because the resistance coefficient for the squeezingmotion diverges and prevents particle-particle contact. Fluid-mechanics based models have two drawbacks in addressing shear thickening: (1) The lubrication singularity can lead only to continuous shear thickening with a logarithmic shear-rate dependence, which is much more moderate than observed experimentally, and (2) the shear-rate dependence of the microstructure is a consequence of the existence of two distinct time scales, one from Brownian motion and the other fromthe inverse of shear rate, but there is only a single time scale for non-Brownian suspensions. 

To address these issues, we have reconsidered the assumptions used in previous simulation models and introduced a new working hypothesis: the lubrication force remains finite at contact. As long as it is finite, the possibility of contact cannot be excluded, which means that we need to introduce a proper contact force model, i.e., friction. We showed such frictional contact forces to be essential for reproducing the shear thickening behavior in recent articles [1, 2], where we explain shear thickening of non-Brownian suspensions with possible repulsive forces acting between particles. The model also provides physical insight into the relation between shear thickening and a jamming transition.