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

Silica spheres coated with grafted poly(butyl methacrylate) chains and disp ersed in a poor solvent at low to moderate volume fractions display high el astic and loss moduli, together with viscosities that diverge at a dynamic yield stress and decrease by several orders of magnitude with increasing st ress. The elastic modulus and dynamic yield stress increase rapidly with vo lume 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 she ar rates in creep experiments is argued to result from slip at the wall. Th us, attractions between these grafted layers in a poor solvent generate a p seudo plastic solid, a metastable state that persists indefinitely and demo nstrates little affinity for the walls of the rheometer, hence the slip. In contrast, depletion flocculated dispersions with higher volume fractions b ut weaker attractions produce very similar rheology but with a true Newtoni an viscosity and adhesion to the wall. (C) 2000 Elsevier Science B.V. All r ights reserved.