Soft glassy rheology model applied to stress relaxation of a thermoreversible colloidal gel

Measurements of the stress relaxation modulus of a thermoreversible colloidal gel are compared with the predictions of the soft glassy rheology (SGR) model of Sollich and co-workers [Sollich et al., Phys. Rev. Lett. 78, 2020-2023 (1997); Sollich, Phys. Rev. E 58, 738-759 (1998); Fielding et al., J. Rheol. 44, 323-369 (2000)] in the linear and nonlinear regimes. The material is a 20% volume fraction suspension of organophilic silica (diameter=80 nm) dispersed in the organic solvent tetradecane that gels below the critical temperature of T=23 degrees C. We perform measurements in which prior deformation history and slip are carefully controlled. The linear stress relaxation modulus shows a significant dependence on waiting time, t(w). SGR model predictions are applied to linear step strain measurements for a range of t(w) to assign a best fit estimate of the noise temperature, x= 1.05 +/- 0.01. We then test predictions of the SGR model for the nonlinear stress relaxation modulus, G(t-t(w),t(w); gamma(o)), based on the linear determination of the noise temperature. The measured gel nonlinear relaxation modulus obeys time-strain separability. For strains greater than the yield strain, gamma(y,) the experimentally determined gel damping function, h(gamma(o)/gamma(y))similar to(gamma(0)/gamma(y))(-1.36). The nonlinear stress relaxation response disagrees significantly with the SGR constitutive prediction. This discrepancy appears linked to the functional form of the strain-accelerated activated trap dynamics in the SGR model. (C) 2008 The Society, of Rheology.