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Thermoreversible Gel Formation and Aging In Concentrated Nanoparticle Suspensions

Subramanian Ramakrishnan1, James Harden2, Robert L. Leheny3, and Hongyu Guo3. (1) Department of Chemical and Biomedical Engineering, Florida A&M University - Florida State University, 2525 Pottsdamer Street, A131, Tallahassee, FL 32310, (2) Physics, University of Ottawa, Ottawa, ON K1N 6N5, Canada, (3) Johns Hopkins University, Department of Physics, 3400 N. Charles St., Baltimore, MD 21218

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 high temperatures the chains form a solvated brush that stabilizes the colloids. At low temperature, the brush collapses leading to a weak, temperature-dependent, short-range attraction between the colloids that drive a reversible ergodic to nonergodic transition in the suspensions. Following a quench through this transition, the shear modulus grows exponentially with a time constant 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 suspensions and a terminal relaxation time that provides information about relaxation of residual stress. Both the plateau value and the terminal relaxation time increase exponentially following the quench with a time constant that closely matches the value for the growing shear modulus. Thus, a comparison between XPCS and rheometry indicates how the arrest of the particle-scale dynamics correlates with the growth in elasticity. Further, a comparison of intermediate scattering functions for suspensions with colloidal volume fractions ranging from 0.20 to 0.43 shows a qualitative variation in the temporal evolution that indicates a crossover from gel-like to glass- like dynamical arrest with increasing volume fraction.