Gel formation and aging in weakly attractive nanocolloid suspensions

We present combined x-ray photon correlation spectroscopy (XPCS) and rheometry studies of the evolution of concentrated suspensions of nanometer-scale colloids undergoing thermo-reversible gelation and aging. After a quench through the gel point, suspensions display a protracted latency period in which they remain fluid followed by a gelation regime in which the shear modulus grows rapidly. The XPCS intermediate scattering function displays two features, a plateau value that provides information about constrained local dynamics and a terminal decay related to relaxation of residual stress. From the wave-vector dependence of the plateau value, a localization length can be extracted. At intermediate colloidal volume fractions ($\phi\simeq 0.20$), the relationship between the localization length and the shear modulus agrees quantitatively with a prediction based on a simplified mode coupling theory, while deviations from the predicted scaling at a higher volume fraction ($\phi\simeq 0.43$) are observed near the gel point. While some features of slow strain from stress relaxation correlate with the evolving rheology, others appear decoupled from the macroscopic behavior.