Localized Stress Fluctuations Drive Shear Thickening in Dense Suspensions

The mechanical response of solid particles dispersed in a Newtonian fluid exhibits a wide range of nonlinear phenomena including a dramatic increase in the viscosity \cite{1-3} with increasing stress. If the volume fraction of the solid phase is moderately high, the suspension will undergo continuous shear thickening (CST), where the suspension viscosity increases smoothly with applied shear stress; at still higher volume fractions the suspension can display discontinuous shear thickening (DST), where the viscosity changes abruptly over several orders of magnitude upon increasing applied stress. Proposed models to explain this phenomenon are based in two distinct types of particle interactions, hydrodynamic\cite{2,4,5} and frictional\cite{6-10}. In both cases, the increase in the bulk viscosity is attributed to some form of localized clustering\cite{11,12}. However, the physical properties and dynamical behavior of these heterogeneities remains unclear. Here we show that continuous shear thickening originates from dynamic localized well defined regions of particles with a high viscosity that increases rapidly with concentration. Furthermore, we find that the spatial extent of these regions is largely determined by the distance between the shearing surfaces. Our results demonstrate that continuous shear thickening arises from increasingly frequent localized discontinuous transitions between coexisting low and high viscosity Newtonian fluid phases. Our results provide a critical physical link between the microscopic dynamical processes that determine particle interactions and bulk rheological response of shear thickened fluids

Evidence for a Common Representation of Decision Values for Dissimilar Goods in Human Ventromedial Prefrontal Cortex

To make economic choices between goods, the brain needs to compute representations of their values. A great deal of research has been performed to determine the neural correlates of value representations in the human brain. However, it is still unknown whether there exists a region of the brain that commonly encodes decision values for different types of goods, or if, in contrast, the values of different types of goods are represented in distinct brain regions. We addressed this question by scanning subjects with functional magnetic resonance imaging while they made real purchasing decisions among different categories of goods (food, nonfood consumables, and monetary gambles). We found activity in a key brain region previously implicated in encoding goal-values: the ventromedial prefrontal cortex (vmPFC) was correlated with the subjects' value for each category of good. Moreover, we found a single area in vmPFC to be correlated with the subjects' valuations for all categories of goods. Our results provide evidence that the brain encodes a "common currency" that allows for a shared valuation for different categories of goods