Experimental synthesis and characterization of rough particles for colloidal and granular rheology

We review the experimental synthesis of smooth and rough particles, characterization of surface roughness, quantification of the pairwise and bulk friction coefficients, and their effect on the rheology of wet particulate flows. Even in the absence of interparticle attraction or cohesion, such types of flows are broadly ubiquitous, spanning enormous length scales ranging from consumer and food products to earth movements. The increasing availability of model frictional particles is useful to advancing new understanding of particulate rheology. Although hard-sphere particles remain the most widely studied system due to their simplicity, their rigid and frictionless nature cannot predict many of the complex flow phenomena in colloidal and granular suspensions. Besides a myriad of interparticle forces, the presence of tangential interparticle friction arising from either hydrodynamics or solid contacts of asperities is now thought to be responsible for commonalities in shear thickening and jamming phenomena at high volume fractions and shear stresses. The overall richness of the suspension mechanics landscape points to the reunification of colloidal and granular physics in the near future: one in which it may become possible to apply a universal set of physical frameworks to understand the flows of model rough particles across multiple spatiotemporal scales. This can only be accomplished by properly distinguishing between microscopic and bulk friction and by decoupling hydrodynamics and contact contributions within the context of experimental observations.