RHEOLOGY OF WEAKLY FLOCCULATED SUSPENSIONS - EXPERIMENT AND BROWNIAN DYNAMICS SIMULATION

Theoretical treatment of depletion flocculation is extended to molecular computer simulation using Brownian dynamics. This study is the first dynamic simulation performed (by molecular or Brownian dynamics) to investigate the dynamical (rheological) properties of binary mixtures grossly different in diameter. With a basic model for the interparticle forces and simplification of the many-body dynamics we have reproduced and elucidated the broad trends of depletion flocculation. A binary mixture of model colloidal and polymer particles with varying extents of interpenetration has been included in a full shear flow simulation. In representative simulations, the osmotic pressure imbalance established by the added polymer causes an increase in the Newtonian viscosity by several orders of magnitude to ∼105 Pa sec. The resulting simulated flocs shear thin as η ∼ Pe-1, where Pe is the Peclet number, in agreement with experiment. The floc morphology was such that phase separation for diameter ratios equal to ∼2 was observed, which became more pronounced under shear thinning flow. More severe aggregation for size ratios ∼ 11 was clearly seen. In both cases a banded or striated pattern along the streaming direction at high Pe is evident. Further aspects of the phase transitions are revealed with the component pair radial distribution functions. The dynamics of the binary mixtures are revealed in time-dependent diffusion coefficients, auto and newly discovered cross time correlation functions. The latter are consistent with the predictions of group theory statistical mechanics. © 1991.