Role of gravity in coagulation of colloidal particles under low-shear environments

Shear rates play a critical role in the coagulation-flocculation-sedimentation processes of colloidal particles. Under high shear environments, it is widely accepted that the median floc size at equilibrium (D) decreases with an increase in shear rates (G). For low shear conditions, however, conflicting D-G relationships were measured in previous laboratory experiments, without a clear explanation of the reasons and a reasonable reproduction of the physical processes. In this study, the direct numerical simulation technique was used to mimic the flocculation of two typical colloidal particles (i.e., latex and silica) with different densities but under similar hydrodynamic conditions. Our results show that the previous different observations at low shear are mainly caused by particle gravity, which influences the particle residence time in a device system. They also confirm previous arguments about limited residence time due to gravitational settling being the reason for the observed peak, in accordance with Winterwerp (1998) [J. Hydraul. Res. Vol 36, pp. 309–326]. This study implies that to better investigate the shear-dominated flocculation of high-density particles such as sediments, settling processes should be addressed unless particles are always in suspension under specific conditions.