Elasticity in dense suspensions of geometrically frustrated colloids

The elastic component of the stress response in dense colloidal suspensions is determined by the evolution of local microstructure, especially for frictional particles with surface asperities. For frictionless particles interacting through a hard-sphere potential, the storage modulus (G’) of the suspension scales with the particle diameter (2a) as 1/a³. However, this scaling does not necessary hold for rough particles at high Φ. We suspend smooth and rough poly(methyl methacrylate) colloids with diameters 2a = 1.50 μm ± 6% and 2a_eff = 1.55 μm ± 5% respectively in a refractive index matched solvent, squalene. The aim is to study the effects of surface roughness and packing on the viscoelastic modulus. Frequency-sweep rheometry show that changes in the scaled modulus occurs as a function of the scaled jamming distance Φ* = Φ_J-Φ/Φ_J and with particle roughness. Close to jamming (Φ* < 0.2), the smooth particle suspensions exhibit a scaled modulus in the O(10 Pa) while rough particle suspensions exhibit a modulus in the O(10⁵ Pa). The data suggest that there is a direct link between the energy dissipation and the geometric frustration of particles that exhibit rotational constraints due to surface roughness.