(295f) Distance to Jamming Defines Shear Thickening Strength in Colloids

Microscopic mechanisms that describe colloidal shear thickening, the process where the viscosity (η) increases with increasing shear stress (σ), has been intensely debated for the past decade, both from experimental and simulation points of view. Here we report the strength, onset, and volume fraction of shear thickening using rough colloids that have variable surface bumpiness. We synthesize smooth and rough polyhydroxystearic acid (PHSA) grafted polymethylmethacrylate (PMMA) colloids with diameters 1 µm < 2a < 2 µm using free-radical dispersion polymerization. Rough particles have surface roughness ranging from tens to hundreds of nanometers and are inherently more frictional than smooth counterparts. The PMMA particles are suspended in the solvent squalene to provide near hard-sphere properties. We define the strength of shear thickening based on the slope of η vs. σ, on a logarithmic scale, defined as β. Colloidal suspensions at various ϕ below the shear jamming point (ϕ_J) are prepared by centrifuging the suspensions at Pe_g= 1500 and diluting subsequently. We use image processing to identify the particle centroids in 3D and obtain the average contact number <z> at any ϕ. We find that the rough particles jam at a lower ϕ_J than smooth particles and have a lower value of z_J. We use the experimental z_J scaling close to jamming and the Wyart-Cates model (M Wyart & M E Cates, Phy Rev Lett, 2014) to model the value of β. First, we find that the scaling Δz ~ (Δϕ)^αclose to jamming varies with the surface anisotropy of the particles. The scaling power (α) is close to unity for smooth spheres and decreases with increasing surface roughness. Second, we explore a universal behavior for Brownian suspensions where the strength of shear thickening changes from DST (β = 1) to a CST (β < 1) at Δϕ = ϕ–ϕ_J ≈ 0.1. The results show that while hydrodynamics is important at Δϕ > 0.1, the degree of shear thickening at Δϕ ≤ 0.1 is dominated by the contact mechanics of the smooth and rough colloids close to their jamming point.