  Shear thickening denotes the reversible increase in viscosity of a suspension
of rigid particles under external shear. This ubiquitous phenomenon has been
documented in a broad variety of multiphase particulate systems, while its
microscopic origin has been successively attributed to hydrodynamic
interactions and frictional contact between particles. The relative
contribution of these two phenomena to the magnitude of shear thickening is
still highly debated and we report here a discriminating experimental study
using a model shear-thickening suspension that allows us to tune independently
both the surface chemistry and the surface roughness of the particles. We show
here that both properties matter when it comes to continuous shear thickening
(CST) and that the presence of hydrogen bonds between the particles is
essential to achieve discontinuous shear thickening (DST) by enhancing solid
friction between closely contacting particles. Moreover, a simple argument
allows us to predict the onset of CST, which for these very rough particles
occurs at a critical volume fraction much lower than that previously reported
in the literature. Finally, we demonstrate how mixtures of particles with
opposing surface chemistry make it possible to finely tune the shear-thickening
response of the suspension at a fixed volume fraction, paving the way for a
fine control of the shear-thickening transition in engineering applications.
