  The shear-jamming of dense suspensions can be strongly affected by
molecular-scale interactions between particles, e.g. by chemically controlling
their propensity for hydrogen bonding. However, hydrogen bonding not only
enhances interparticle friction, a critical parameter for shear jamming, but
also introduces (reversible) adhesion, whose interplay with friction in
shear-jamming systems has so far remained unclear. Here, we present atomic
force microscopy studies to assess interparticle adhesion, its relationship to
friction, and how these attributes are influenced by urea, a molecule that
interferes with hydrogen bonding. We characterize the kinetics of this process
with nuclear magnetic resonance, relating it to the time dependence of the
macroscopic flow behavior with rheological measurements. We find that
time-dependent urea sorption reduces friction and adhesion, causing a shift in
the shear-jamming onset. These results extend our mechanistic understanding of
chemical effects on the nature of shear jamming, promising new avenues for
fundamental studies and applications alike.
