Creep and recovery of frictional colloids in dense suspensions*

We report the effect of particle surface roughness on rate-dependent creep deformation and slow recovery in dense colloidal suspensions. The colloidal suspensions are composed of hard sphere like poly (methyl methacrylate) particles suspended in squalene. Surface roughness is induced by adding a crosslinking agent, ethylene glycol dimethyl acrylate, during the nucleation step. The study compares rheological creep and recovery responses for suspensions containing particles with and without surface roughness at volume fractions corresponding to their respective jamming fractions. The suspensions are loaded onto a stress-controlled rheometer with parallel plate geometry which applies constant shear stress above and near the yield stress of the suspension. Interestingly, suspensions of rough colloids exhibit 4-5 times higher creeping strain deformation as compared to the smooth colloids. Unlike suspensions of smooth colloids, suspensions of rough colloids show a non-monotonic trend during recovery, with peak recoverable strains observed at 0.02 Pa (slightly below the yield stress) followed by a steep decline with increasing stresses. Such a response has previously been reported for attractive colloidal glasses. At short times (i.e., in the caging regime), higher strain deformation in suspensions of rough colloids may be due to the comparatively lower storage modulus. At longer times, rough suspensions may exhibit rapid deformation and yielding at lower stresses due an increase in the effective void fraction in the suspension because of particle clustering due to roughness induced interlocking. This may facilitate easier cage rearrangements at short times and consequent cage breaking at the end of creep. During recovery, the peak recoverable strains observed near the yield stress may indicate a resistance to the shear induced flow by the geometrically frustrated interlocking contacts between rough colloids.