March 24, 2025, Daniel Blair, Georgetown University
4:00 p.m.
190 Goodwin Hall
"Local Rheological Probes of Stress Heterogeneities in Active and Passive Fluids"
Abstract: In this talk I will discuss our recent results on the microscopic physical origins of shear thickening in two vastly different materials: colloidal suspensions and active gels. In the first part of my talk, I will introduce a method we have developed that allows us to resolve the spatial distribution of stresses in sheared soft-materials, known as Boundary Stress Microscopy. We have applied this technique to suspensions undergoing shear thickening. I will present our results on the existence of clearly defined dynamically localized regions of substantially increased stress that appear intermittently at stresses well above the applied stress. Surprisingly, we find that these spatially distinct and dynamic phases account quantitatively for the observed shear thickening seen in sheared colloidal dispersions (e.g. Oobleck). In the second part of my talk I will discuss our results on the rheology of active matter. Our system is composed of microtubules and kinesin motor proteins that self-assemble to form complexes that propel themselves through the fluid. What results is a dramatic alteration in the measured viscosity. I will provide a simple physical model that fits the rheological response with zero fitting parameters.
Bio: Daniel Blair is a Professor of physics and a co-founding member of the Institute for Soft Matter Synthesis and Metrology I(SM)2 at Georgetown University. Prior to joining Georgetown in 2007, Professor Blair was a postdoctoral fellow at Harvard University in the Paulson School Of Engineering and the Department of Physics. In 2009, he was awarded the NSF Career Award and has done pioneering work into the structure and rheology of soft and biological materials. His group has developed a variety of tools and techniques centered on quantitative 4D microscopy coupled with microfluidic and rheometric methods.