  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.
