  Shear thickening is a widespread phenomenon in suspension flow that, despite
sustained study, is still the subject of much debate. The longstanding view
that shear thickening is due to hydrodynamic clusters has been challenged by
recent theory and simulations suggesting that contact forces dominate, not only
in discontinuous, but also in continuous shear thickening. Here, we settle this
dispute using shear reversal experiments on micron-sized silica and latex
colloidal particles to measure directly the hydrodynamic and contact force
contributions to shear thickening. We find that contact forces dominate even
continuous shear thickening. Computer simulations show that these forces most
likely arise from frictional interactions.
