Distinguishing between dynamic yielding and wall slip in a weakly flocculated colloidal dispersion

Silica spheres coated with grafted poly(butyl methacrylate) chains and dispersed in a poor solvent at low to moderate volume fractions display high elastic and loss moduli, together with viscosities that diverge at a dynamic yield stress and decrease by several orders of magnitude with increasing stress. The elastic modulus and dynamic yield stress increase rapidly with volume fraction, but remain in constant ratio. Longer grafted chains produce stronger interparticle attractions, resulting in a smaller yield strain and larger elastic modulus. An apparent Newtonian plateau at extremely low shear rates in creep experiments is argued to result from slip at the wall. Thus, attractions between these grafted layers in a poor solvent generate a pseudo plastic solid, a metastable state that persists indefinitely and demonstrates little affinity for the walls of the rheometer, hence the slip. In contrast, depletion flocculated dispersions with higher volume fractions but weaker attractions produce very similar rheology but with a true Newtonian viscosity and adhesion to the wall. (C) 2000 Elsevier Science B.V. All rights reserved.