  A consensus is emerging that discontinuous shear thickening (DST) in dense
suspensions marks a transition from a flow state where particles remain well
separated by lubrication layers, to one dominated by frictional contacts. We
show here that reasonable assumptions about contact proliferation predict two
distinct types of DST in the absence of inertia. The first occurs at densities
above the jamming point of frictional particles; here the thickened state is
completely jammed and (unless particles deform) cannot flow without
inhomogeneity or fracture. The second regime shows strain- rate hysteresis and
arises at somewhat lower densities where the thickened phase flows smoothly.
DST is predicted to arise when finite-range repulsions defer contact formation
until a characteristic stress level is exceeded.
