  Dense suspensions of fine particles are significant in numerous biological,
industrial, and natural phenomena. They also provide an ideal tool to develop
statistical mechanics description for out-of-equilibrium systems. Predicting
the bulk response of such materials has been challenging since these systems
often undergo liquid-solid transitions upon a small change in solid
concentration or applied loading. Developing an understanding of the mechanisms
that drive these phenomena has over the last several years led to a surge in
research activity at the intersection of fluid mechanics, granular materials,
driven disordered systems, tribology, and soft condensed matter physics. One
central aspect that emerged is that these phenomena are due to a
shear-activated or deactivated network of contacts between particles. The
perspective briefly presents the current state of understanding and challenges
associated with relating the flow of material at the bulk scale with the
microscopic physics at the particle scale.
