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The flow of dense suspensions, glasses, and granular materials is heavilyinfluenced by frictional interactions between constituent particles. However,neither hydrodynamics nor friction has successfully explained the full range offlow phenomena in concentrated suspensions. Particles with asperities representa case in point. Lubrication hydrodynamics fail to completely capture two keyrheological properties - namely, that the viscosity increases drastically andthe first normal stress difference can switch signs as volume fractionincreases. Yet, simulations that account for interparticle friction are alsounable to fully predict these properties. Furthermore, experiments show thatrheological behavior can vary depending on particle roughness anddeformability. We seek to resolve these apparent contradictions bysystematically tuning the roughness of model colloids, investigating theirviscosity and first normal stress differences under steady shear, and finallygenerating a rheological state diagram that demonstrates how surface roughnessinfluences the transition between shear thickening and dilatancy. Oursimulations, which are in good agreement with the experiments, suggest thatfriction between rough particles is significant. In addition, we find thatroughness progressively lowers the critical conditions required for the onsetof shear thickening and dilatancy. Our results thus provides a majorcontribution in the field of suspension rheology with broad relevance togranular and particulate materials. For instance, particle geometry can betuned to increase the efficacy of materials that turn solid-like on theapplication of stimuli. On the other hand, engineers who work with concentratedslurries can now use images of the constituent particles to estimate optimalflow processing conditions.

A rheological state diagram for rough colloids in shear flow