Contact and stress anisotropies in start-up flow of colloidal suspensions

Spatiotemporal correlations in start-up flows of attractive colloids are explored by numerical simulations as a function of their volume fraction and shear rate. The suspension is first allowed to flocculate during a time tw, then the stress necessary to induce its flow is computed. We find that, at low volume fractions, the stress is a universal function of the strain. On the contrary, at high volume fractions, this scaling behavior is no longer observed and a supplementary stress becomes necessary to induce flow. To better understand the physical origin of the supplementary stress, we examine the creation, disruption, and orientation of contacts between the particles and the corresponding contribution to stress as a function of strain. Our simulations show that the onset of flow is dominated by the creation of contacts between the particles at low shear rates and by their disruption at high shear rates. However, neither the evolution of the number of contacts with strain nor their orientation can fully account for the nonscaling behavior of the stress at high volume fractions. At small strains, the relative importance of forcing in the compression quadrant increases with volume fraction and with flocculation time. This mechanism of stress transmission through the compression quadrant is not accounted for in the usual description of yield stress, which considers the breaking of bonds oriented in the extension quadrant.