Soft colloidal matter: A phenomenological comparison of the aging and mechanical responses with those of molecular glasses

Highly concentrated colloidal suspensions are often considered to exhibit behavior similar to that of glass-forming systems. While there is considerable rheological information in the literature concerning the flow behavior of such systems, there is little that has examined the mechanical response in a fashion that makes explicit comparisons with the relaxation behavior of molecular or polymeric glasses. On the other hand there is a significant literature that looks at shear melting and subsequent aging of such glasslike or pasty liquids. Here we report results for a polymer latex particle system at different concentrations near to the glassy or pasty regimes. Stress relaxation experiments and aging after shear melting experiments were performed. Single step stress relaxation results presented as isochrones of stress vs strain show behavior similar to that of polymers at the lower concentrations (50% and 53%). That is, there is a linear regime of behavior (generally less than a deformation of 1%) followed by a nonlinear response and subsequent yield. At the higher concentration, while the linear behavior is retained, the yieldlike response is much more abrupt. Interestingly, we find an effect of loading sequence on the isochrones. Aging behavior subsequent to large sinusoidal shear melting histories was also investigated. Importantly, in all instances where aging occurred we found that there was no time-aging time superposition, which is contrary to the postulate that the shear melting has the effect of changing the effective temperature. This may be due to the overlap of the alpha and beta relaxations. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3092933]