3,164 research outputs found
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
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