(257g) Creep and Recovery in Dense Suspensions of Rough Colloids

We report the effect of particle surface roughness on viscoelasticity, yielding and elastic recovery in dense colloidal suspensions. The suspensions are composed of poly (methyl methacrylate) colloids exhibiting hard sphere interactions. Rheological results are compared between particles without surface roughness (smooth, ‘S’) and particles with micron-scale surface roughness (‘rough’, R) at volume fractions Φ^S = 0.64 ± 0.01 and Φ^R = 0.56 ± 0.01, respectively. These volume fractions are 3.0% and 3.4% away from the jamming volume fractions (Φ_J,S= 0.66 ± 0.01, Φ_J,R = 0.58 ± 0.01). Yielding and flow responses of the suspensions are studied by applying constant shear stresses (0.01 Pa - 0.07 Pa) above and below the yield stress (σ_y,S = 0.035 Pa, σ_y,R = 0.02 Pa) through creep and recovery rheology. During creep, suspensions of rough colloids exhibit 4 - 5 times higher strain deformation compared to smooth colloids, irrespective of the applied stress. We hypothesize that interlocking of surface asperities in rough colloids generates a heterogeneous microstructure enhancing dynamic particle activity and percolation of strain heterogeneities, thereby resulting in higher strain deformation and early onset of steady flow. During strain recovery, suspensions of rough colloids exhibit non-monotonic trend in recoverable strain as a function of applied stress, where the peak recoverable strain (~ 100%) is observed near the yield stress, followed by a steep decline with increasing stress. It is likely that the frictional constraints between the interlocking contacts function as particle bonds capable of higher elastic recovery, but only near the yield stress. This study improves our understanding of the effects of surface roughness on yielding in dense suspensions which can be useful in designing yield stress fluids for additive manufacturing and novel product formulations.