  Dense suspensions of colloidal or granular particles can display pronounced
non-Newtonian behaviour, such as discontinuous shear thickening (DST) and shear
jamming (SJ). The essential contribution of particle surface roughness and
adhesive forces confirms that stress-activated contacts can play a key role in
these phenomena. By employing a system of microparticles coated by responsive
polymers that allow friction, adhesion, and surface roughness to be selectively
and independently tuned as a function of temperature, we offer a way to
disentangle these contributions. We find that DST occurs at lower shear rates
when friction and adhesion between particles are enhanced at high temperatures.
Additionally, the temperature-responsive polymers provide lubricity at low
temperatures that can mask surface roughness. The link between single-particle
properties and macroscopic rheology is elucidated via lateral force microscopy,
which reveals the nature of rheologically relevant contact conditions. In situ
temperature tuning during shear allows contact conditions to be modified, and
DST to be switched on and off on demand. These findings strengthen our
understanding of the microscopic parameters affecting DST and identify new
routes for the design of smart, non-Newtonian fluids.
