CBE Spring Seminar Series Speaker: Jeffrey Urbach

Description

The spatiotemporal dynamics of sheared dense suspensions:  Does continuous shear thickening exist

Dense suspensions often undergo dramatic shear thickening, where the apparent viscosity can increase by an order of magnitude or more upon increasing stress or shear rate.  Rheological measurements of average properties typically reveal two regimes:  at moderately high particle concentrations, continuous shear thinking (CST)  is observed, where the viscosity increases smoothly as a function of stress, while at very high concentrations suspensions display discontinuous shear thickening (DST), with the viscosity increasing abruptly at a critical shear stress.  Both of these regimes can be quantitatively described by a mean field model attributing shear thickening to  a  transition from primarily hydrodynamic interactions at low stress to primarily frictional interactions when the stress is high enough to overcome inter-particle repulsion. We have been searching for CST using Boundary Stress Microscopy, a technique for measuring spatially resolved surface stresses with high spatial and temporal resolution, with no success.

Instead, we find that sheared dense suspensions bifurcate into local regions of high or low stress, with a length scale determined by the suspension confinement, which is typically very large compared to the size of the particles.

We have observed similar behavior in Brownian monodisperse colloids, non-Brownian irregular particles (Cornstarch and calcium carbonate), and colloidal rods.  The smooth viscosity increase reported in average rheological measurements obscures these local discontinuous transitions.  I will also describe observations showing the transitions are associated with changes in particle concentration, producing directly observable fluid migration, behavior that cannot be captured by a mean field model.   These results highlight the importance of moving beyond measurements of average properties and mean field models in order to develop a mechanistic understanding of shear thickening.

Bio: 
Dr. Jeffrey Urbach received his Ph.D in Physics at Stanford University, and after a Postdoctoral Fellowship at the University of Texas at Austin joined the Physics Department Georgetown University as an Assistant Professor in 1996 and was promoted to Professor of Physics in 2006. He served as chair of the Physics Department for a total of 8 years, as the co-Director of the Program on Science in the Public Interest from its founding in 2006 until 2011, and the Director of the Institute for Soft Matter Synthesis and Metrology from its founding in 2011 until 2015. From 2009-10, he served as a AAAS Science and Technology Policy Fellow at the US Department of Energy., and in 2024 was appointed to Vice Provost for Research at Georgetown.   Dr. Urbach’s research interests include complex dynamical systems, biophysics, and soft matter physics. His diverse interdisciplinary collaborations have produced impactful publications in materials physics, cellular biophysics, biomaterials, optical imaging and statistical physics. He received a Sloan Foundation Fellowship and research funding from the National Science Foundation, the National Institutes of Health, the Air Force Office of Scientific Research and other government agencies and private foundations. In 2000, Dr. Urbach received the Presidential Early Career Award for Scientists and Engineers and in 2016 he was elected Fellow of the American Physical Society.