Viscoelasticity of a colloidal gel during dynamical arrest: Evolution through the critical gel and comparison with a soft colloidal glass

We consider the gelation of colloidal particles in suspension after cessation of shear flow. Particle aggregation is driven by a temperature-tunable attractive potential which controls the growth of clusters under isothermal conditions. A series of frequency resolved time sweeps is used to systematically reconstruct the frequency dependent dynamic moduli as a function of time and temperature or attraction strength. The data display typical hallmarks of gelation with an abrupt transition from a fluid state into a dynamically arrested gel state after a characteristic gelation time t(g) that varies exponentially with temperature and serves to collapse the evolution of the system onto a universal curve. We observe the viscoelastic properties of the critical gel where we find that G'(omega) approximate to G '' (omega) similar to omega(nc), where n(c) = 0.5 in a narrow time window across all attraction strengths. We measure a dynamic critical exponent of kappa = 0.25 which is similar to that observed in cross-linked polymer gels. The approach to the critical gel is therefore governed by zero-shear viscosity eta(0) similar to -epsilon(-s) and plateau modulus G(e) similar to epsilon(z) with s = z = 2, where epsilon = p/p(c) - 1 is the distance to the gel point in appropriate reaction coordinates. Remarkably, the relaxation moduli of the near critical gels are identical across the temperatures considered, with G(t) approximate to 0.33 t(-0.5). This suggests an underlying strong similarity in gel structure in the regime of attraction strengths considered, despite the differences in aggregation kinetics. We contrast these findings with the behavior of a colloidal glass undergoing dynamical arrest where no critical state is observed and where the arrest time of the system displays a marked frequency dependence. These findings highlight the underlying structural differences between colloidal gels and glasses which are manifest in their dynamic properties in the vicinity of the liquid-to-solid transition. (C) 2014 The Society of Rheology.