Dr. Xiang Cheng worked on the physics of granular materials with Sidney Nagel and Heinrich Jaeger at Department of Physics, University of Chicago and received his PhD in 2009. After graduation, he joined Itai Cohen’s group at Cornell University as a postdoctoral fellow, working on the microstructure of colloidal suspensions under shear and its relation to their macroscopic flow behaviors (rheology). Dr. Cheng is an expert in experimental studies of granular materials and colloidal suspensions.

Abstract:
The viscosity of colloidal suspensions can vary by orders of magnitude depending on how quickly they are sheared. Although this non-Newtonian behavior is believed to arise from the arrangement of suspended particles and their mutual interactions, microscopic particle dynamics in such suspensions are difficult to measure directly. Here, by combining fast confocal microscopy with simultaneous force measurements, we systematically investigate a suspension's structure as it transitions through regimes of different flow signatures. Our measurements of the microscopic single-particle dynamics unambiguously show that viscoelasticity arises from the competition between Brownian dynamics and shear flows, that shear thinning results from the decreased relative contribution of entropic forces, and that shear thickening is due to the formation of particle clusters induced by inter-particle hydrodynamic lubrication forces. This combination of techniques illustrates an approach that complements current methods for determining the microscopic origins of non-Newtonian flow behavior in complex fluids.