Prediction and measurement of initial flocculation rates in quiescent media

Abstract

Initial flocculation rate experiments were carried out with a dilute suspension of bidisperse polystyrene spheres (0.166 and 0.255 [mu]m in radius) under normal gravity conditions and, using a centrifuge, under conditions of an enhanced gravitational flow field. These experiments were designed to study the effect of differential creaming between small and large particles on the flocculation process and also to ascertain some of the limitations associated with the DLVO theory of electrostatic stabilization. A turbidity technique which accounts for the simultaneous effects of flocculation and creaming was developed to determine the initial flocculation rate. By comparing theoretically predicted rates with experimentally observed ones for each monodisperse system under normal gravity, the Hamaker constant was determined to be A = 2.87 +/- 0.36 x 10-14 erg for a retarded attractive potential, and the surface potential was found to be [Psi]0 = -12.7 +/- 0.5 mV. The corresponding [zeta] potentials ranged from -19 to -32 mV. However, the predicted stability factors for the bidisperse system were over 6 orders of magnitude greater than the measured values when the [zeta] potentials were substituted for [Psi]0 in the chosen electrostatic repulsion model, whereas, correct stability predictions were made with the characteristics potential of -12.7 mV. Under conditions of an enhanced gravitational flow field, the bidisperse flocculation rate increased with increasing gravitational force; in accordance with our previous theoretical predictions. For the larger gravitational forces studied, there appears to be a high degree of coupling between Brownian motion and gravitational forces.