The Rheology of Strongly Flocculated Gels

The power-law exponent /x is around 4.0-5.0. A similar result is obtained for the yield stress in shear cr, except the exponent is smaller, Ty a 0 , and yy is a couple of decades smaller in magnitude than Py. Chen and Russel (1991) found a similar power law for flocculated octadecyl-coated silica particles. Such power laws have been derived from theories that model the gel as a network of interconnected fractal clusters (BuscaU et al. 1988 Shih et al. 1990 Potanin et al. 1995). 

The influence of particle size on the yield stresses Py and cry have also been measured BuscaU et al. (1988) found the power-law dependencies 

Data for Oy versus a, corroborating the above scaling, are reported in Fig. 7-14. 

ihe yield stress ay is maximized at the isoelectric point. Note, however, in Fig. 7-20 that when the i.e.p. is shifted by the binding of ions to the particle surfaces, the maximum yield stress is reduced. As noted in Section 7.2.1, the binding of ions to the particle surfaces is likely to increase the thickness of the hydration layers on the particles that keep the particle surfaces from coming closer than a few nanometers from each other. Leong et al. (1993) have found a correlation between the size of the adsorbed anion and the magnitude of the decrease in the yield stress. However, the size of the hydrated ion, rather than the size of the ion itself, should, in principle, control the closest approach of the particles (Israelachvili 1991). 

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