(230x) Shear-Induced Clustering of Brownian Colloids in Associative Polymer Networks at Moderate Peclet Number

Shear-induced clustering of Brownian particles has been widely observed in viscoelastic polymer solutions, and has been hypothesized to arise from the influence of polymer normal stresses on particle-particle interactions. In principle, such interactions depend on two characteristic time scales: the relaxation time of the viscoelastic polymer network, t_r, and the Brownian time scale for relaxation of the suspension microstructure to equilibrium, t_Br. We have developed thermoresponsive colloid-polymer mixtures that form associative polymer-colloid networks, in which the relative magnitudes of these time scales can be tuned over several orders of magnitude relative. This allows for systematic study of the behavior of shear-induced clustering in various regimes of the two corresponding shear rate scales: the Weissenberg number, Wi, and the Peclet number, Pe. The steady shear behavior of these fluids exhibit two regimes of shear thinning: one atr small Pe where the rheology scales with Wi, and another above Pe~0.1 where the steady shear rheology scales with Pe.

Rheo-SANS measurements reveal that anisotropic scattering develops in both the flow-vorticity (1-3) and flow-gradient (1-2) planes above Pi ~ 0.1, and becomes enhanced with increasing Pe. The orientation of the anisotropic scattering indicates concentration fluctuations oriented along both the vorticity and compressional axes of shear. The degree of anisotropy over a wide range of material parameters and shear rates collapse onto a master curve, which follows a scaling of A_f ~ Pe^1/3. From this, we propose and validate a simple empirical model to capture the steady shear rheology of the fluid, in which the clustering of particles imparts a perturbative contribution to the shear stress that modifies the underlying viscoelastic shear stress. This model accurately captures the shear rheology of the system over a wide range of relative values of Wi and Pe, and shows that clustering exerts a shear-thickening contribution to the total stress. Finally, we report time-resolved rheo-SANS measurements to elucidate the mechanism(s) of cluster formation, which reveal that the clusters are dynamic, transient structures. These results provide clear evidence for the role of interparticle hydrodynamic interactions in the formation of anisotropic particle clusters in viscoelastic fluids, and elucidate means by which hydrodynamic interactions influence and control the shear-induced clustering of Brownian suspensions in viscoelastic liquids.