Georgetown University

 

In this talk I will first introduce soft materials by describing what makes soft matter soft, and hard. I will then discuss our recent results on the microscopic physical origins of shear thickening, a dramatic increase of the viscosity, for two vastly different materials. I will introduce a precision method we have developed that allows us to resolve how stresses propagate through sheared soft-materials. I will present our results on the existence of clearly defined dynamically localized regions of substantially increased stress within the fluid that look surprisingly similar to a liquid-solid phase transition. Surprisingly, we find that these spatially distinct and dynamic phases account quantitatively for the observed shear thickening seen in non-Newtonian fluids (e.g. Oobleck from the science fair). In the second part of my talk I will discuss our results on the flow of active matter. Our system is composed of microtubules and kinesin motor proteins that self-assemble to form complexes and propel themselves through the fluid. What results is a dramatic increase in the viscosity of the material with applied shear rate; due again to a dynamical solidification of the material.