  Control of frictional interactions among liquid-suspended particles has led
to tunable, strikingly non-Newtonian rheology via the formation of strong flow
constraints as particles come into close proximity under shear. Typically,
these frictional interactions have been in the form of physical contact,
controllable via particle shape and surface roughness. We investigate a
different route, where molecular bridging between nearby particle surfaces
generates a controllable "sticky" friction. This is achieved with
surface-functionalized colloidal particles capable of forming dynamic covalent
bonds with telechelic polymers that comprise the suspending fluid. At low shear
stress this results in particles coated with a uniform polymer brush layer.
Beyond an onset stress the telechelic polymers become capable of bridging and
generate shear thickening. Over the size range investigated, we find that the
dynamic brush layer leads to dependence of the onset stress on particle
diameter that closely follows a power law with exponent -1.76. In the shear
thickening regime, we observe an enhanced dilation in measurements of the first
normal stress difference and reduction in the extrapolated volume fraction
required for jamming, both consistent with an effective particle friction that
increases with decreasing particle diameter. These results are discussed in
light of predictions for suspensions of hard spheres and of polymer-grafted
particles.
