  Dispersing small particles in a liquid can produce surprising behaviors when
the solids fraction becomes large: rapid shearing drives these systems out of
equilibrium and can lead to dramatic increases in viscosity (shear-thickening)
or even solidification (shear jamming). These phenomena occur above a
characteristic onset stress when particles are forced into frictional contact.
Here we show via simulations how this can be understood within a framework that
abstracts details of the forces acting at particle-particle contacts into
general stress-activated constraints on relative particle movement. We find
that focusing on just two constraints, affecting sliding and rolling at
contact, can reproduce the experimentally observed shear thickening behavior
quantitatively, despite widely different particle properties, surface
chemistries, and suspending fluids. Within this framework parameters such as
coefficients of sliding and rolling friction can each be viewed as a proxy for
one or more forces of different physical or chemical origin, while the
parameter magnitudes indicate the relative importance of the associated
constraint. In this way, a new link is established that connects features
observable in macroscale rheological measurements to classes of constraints
arising from micro- or nano-scale properties.
