#### Seminar 4/9/2014 - Ryohei Seto, City College of New York/Levich Institute: Shear Thickening - Introducing Friction to Suspension Rheo

Abstract:

Dense particle suspensions often exhibit shear thickening: suspension becomes more viscous when sheared faster. The dependence on shear rate is even discontinuous in some cases. Our work explains the actual mechanism of shear thickening [1,2].

Suspension rheology has historically been studied from a fluid mechanics perspective, and the emphasis has been on a description based on hydrodynamic interactions. Because of the universality of the phenomenon, researchers have focused on simple models to reproduce shear thickening: rigid spheres immersed in a Newtonian fluid, interacting through hydrodynamics (including Brownian forces) and sometimes an additional soft potential, e.g., of DLVO type. The key point is that the particles are treated as mere boundary conditions for the Stokes equations describing the fluid phase, and they never directly generate forces through contacts. This is self-consistently justified by the fact that the Stokes flow between two rigid surfaces leads to the singular lubrication force, preventing two particles from colliding. While these fluid-mechanics based models do predict shear thickening, the shear-rate dependence is much more moderate than observed experimentally. The discontinuous dependence has remained a mystery.

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. Our model also assumes a repulsive force (or a threshold force to activate frictional contacts), which introduces a shear rate dependence.

Frictional contacts restrict the smooth reorganization of particles to adapt to the shear strain. Once created, local disturbance propagates through a pileup effect; frictional contacts tend to become chains. Due to friction, these chains can temporary behave as rigid rods with compatible stresses. Upon increase of the shear rate (and shear stress), creating frictional contacts becomes easier, leading to more and longer chains. If the suspension is not concentrated enough, the viscosity continuously increases as chains grow. But, if the suspension is sufficiently concentrated, there is a point at which such rigid entities span the entire space and the system becomes a solid, i.e., (a kind of) jammed phase. At this point, one observes a discontinuity in the shear-rate dependence of viscosity.

[1] R. Seto, R. Mari, J. F. Morris, and M. M. Denn. Discontinuous shear thickening of frictional hard-sphere suspensions. Phys. Rev. Lett., 111:218301, 2013. doi: 10.1103/PhysRevLett.111. 218301.

[2] R. Mari, R. Seto, J. F. Morris, and M. M. Denn. Shear thickening, frictionless and frictional rheologies. arXiv:1403.6793, 2014.

Bio:

Ryohei Seto is Research Associate of Benjamin Levich Institute at The City College of New York. The use of modeling and numerical simulations, to investigate the rheological properties of particle suspension systems, has been the focus of his research over the past several years. He was postdoctoral fellow working with R. Botet at Laboratoire de physique des solides in Orsay, with Martine Meireles at Laboratoire de Laboratoire de genie chimique in Toulouse, with H. Briesen at Technische Universität München, and with G. K. Auernhammer at Max-Planck-Institut für Polymerforschung.

All seminars are held on Wednesdays from 12:00 noon-1:00 p.m. in the Bowen Hall Auditorium Room 222. A light lunch is provided at 11:30 a.m. in the Bowen Hall Atrium immediately prior to the seminar.

**Location:**
Bowen Hall Auditorium

**Date/Time: **04/09/14
at
12:00 pm
- 04/09/14
at
01:00 am

**Category: **PRISM/PCCM Seminar Series

**Department: **PRISM