(232h) Gel Formation and Aging in Concentrated Nanoparticle Suspensions

We report a combined x-ray photon correlation spectroscopy (XPCS) and rheometry study of the evolution of concentrated suspensions of nanometer-scale colloids undergoing gelation and aging. The suspensions are comprised of silica colloids, 45 nm in diameter, coated with octadecyl-hydrocarbon chains in decalin at colloidal volume fractions ranging from 0.20 to 0.43. At high temperatures, decalin is a good solvent for the chains, which form a solvated brush that stabilizes the colloids. At low temperature, the solvent quality is poor, leading to a weak, temperature-dependent, short-range attraction between the colloids that drives a reversible gel transition. Following a quench through this transition, the shear modulus grows at a rate that depends strongly on temperature. The intermediate scattering function measured with XPCS displays two features, a plateau value that provides information about constrained local dynamics in the gel and a terminal relaxation time that provides information about relaxation of residual stress. From the wave-vector dependence of the plateau value, a localization length can be extracted. At lower colloidal volume fraction, the relationship between the localization length and the shear modulus agrees quantitatively with the prediction of Chen and Schweitzer based on a simplified mode coupling theory [1], but deviations from the prediction are observed at early times for higher volume fraction.

[1] Y.-L. Chen and K. S. Schweitzer, J. Chem. Phys. 120, 7212 (2004).