Viscoelasticity and rheology of depletion flocculated gels and fluids

The flow properties of high volume fraction hard sphere colloid-polymer suspensions are studied as a function of polymer concentration, depletion attraction range, and solvent quality up to, and well beyond, the gelation boundary. As the gel boundary is approached, the shear viscosity tends to diverge in a critical power law manner at a polymer concentration that is a function of the polymer radius of gyration and solvency condition. The shear viscosity for different polymer size suspensions can be collapsed onto a master curve motivated by mode coupling theory (MCT). The low frequency elastic modulus grows rapidly with increasing depletion attraction near the gel boundary, but becomes a dramatically weaker function of polymer concentration as the gel state is more deeply entered. A simplified version of MCT with accurate, no adjustable parameter polymer reference interaction site model (PRISM) theory structural input has been applied to predict the gelation boundaries and elastic shear moduli. The calculated gel lines are in semiquantitative agreement with experiment at high volume fractions, but increasingly deviate upon particle dilution. Calculations of the dependence of the gel elastic shear moduli on particle-polymer size asymmetry and scaled polymer concentration are in excellent agreement with experiment, and deep in the gel follow a power law dependence on polymer concentration. Quantitatively, MCT-PRISM elastic moduli are higher than experiment by a nearly constant large factor. This discrepancy is suggested to be due to the heterogeneous nature of the gel structure which small angle scattering experiments show consists of dense clusters and voids of characteristic length scales similar to4-7 particle diameters. A simple idea for correcting the particle level MCT modulus by employing cluster network concepts is proposed. (C) 2003 American Institute of Physics.