Nonlinear rheology and yielding in dense suspensions of hard anisotropic colloids

Glasses of hard spheres show a single yield stress or strain associated with deforming particle cages to an extent that stress is rapidly relaxed by exchange of nearest neighbors. In this first of two articles on the rheological behavior of suspensions of anisotropically shaped particles (particle diameter ~300 nm), we demonstrate that particles experiencing volume exclusion interactions at high volume fractions exhibit two maxima in G", which are interpreted as multiple yielding events. We interpret these results through naïve mode coupling theory (NMCT) where one yield stress is associated with exceeding the entropic barrier constraining rotational motion and the other exceeding the barrier for exchanging nearest neighbors. These results are qualitatively compared to theoretical state diagrams predicting double glass states for hard anisotropic particles. In the second of the two articles, we explore the effects of shape anisotropy and weak attractions on yielding and the onset of shear thickening. Furthermore, this second data set is undertaken with particles of similar shape but larger size (~1.2 μm) and under conditions where the particles feel sufficient attractions to reduce the gel point below the glass transition observed for purely volume exclusion interactions explored here.