Jamming Distance Dictates Colloidal Shear Thickening

We report the steady state viscosity and contact microstructure of dense suspensions containing hard-particle poly(methyl methacrylate) (PMMA) colloids with tunable surface morphologies. Structural analysis of confocal micrographs shows that the contact number deficit scales as the jamming distance, where the scaling relations contain a range of exponents that describe the compactability of frictional packings with jamming fractions and jamming contact numbers Suspensions with rougher particles require fewer nearest neighbors than that of smoother particles to reach the jamming point. Agreement between model predictions from a mean-field theory and our rheological data shows that shear thickening is modeled by different types of frictional packings that form under applied shear stresses. The shear thickening strength, quantified by the slope of the viscosity-stress flow curves, scales with the jamming distance for a broad class of dense suspensions comprising PMMA smooth and rough colloids, silica smooth and rough colloids, and simulations with interparticle friction or surface asperities. Our results suggest that when jamming distance = 0.1 and contact deficit = 0.5, when scaled with respective values at jamming, is the point at which hydrodynamics, Brownian forces, and friction become equally important in colloidal shear thickening.