Shear thinning and microstructures of attractive non-Brownian suspensions

Multiple interparticle forces play a crucial role in determining the rheological behavior of particle suspensions. In dense non-Brownian particle suspensions, weak van der Waals attraction between particles is conceived to be responsible for inducing non-linear rheology. This study investigates the effects of attraction using numerical simulations that account for hydrodynamic, attractive, and frictional contact forces. The results reveal that the shear-thinning behavior becomes increasingly pronounced in steady shear with the increasing strength of attraction. Although this attraction is relatively weak compared to dominant contact forces, it indirectly modulates shear-thinning by controlling the size of particle clusters. Based on this mechanism, we propose a renormalized stress to account for the shear-thinning curves of attractive suspensions with varying attraction strengths. By imposing oscillatory shear on attractive particle suspensions, we demonstrate another frequency-dependent mechanism of shear-thinning behavior, which results in a deviation from the Cox-Merz law. As the frequency increases, the attractive suspensions undergo a transition from a contact-dominated state to a hydrodynamic-dominated state, where the motion of the particles is confined to small regions, forming hydrodynamic pairs that contribute to the complex viscosity in an unignorable way.