469442 The Effect of Suspended Colloidal Particles on the Stability of High-Density Particles Against Sedimentation
The mechanism of the stabilization of the dense particles against sedimentation with the use of close-packed vesicles was also examined in general by using Brownian dynamics simulations (BDS) for the first time. BDS were performed for mixtures of two types of colloidal spherical particles interacting via DLVO potentials. For one type, the Peclet number, Pe = πd3ΔρgL/6kBT, Pe is high; Pe is defined as the ratio of the sedimentation flux, vsedc, to the macroscopic diffusion flux, Dc/L, over the scale L of the sample, where d is the particle diameter, Δρ is the particle-dispersion medium density difference, c is the particle concentration, D is the diffusion coefficient, vsed is the sedimentation velocity, g is the gravitational acceleration, kB is Boltzmann’s constant, and T is the absolute temperature. These particles, of diameter d1 and volume fraction Φ1, will settle rapidly on their own. For the second type of particles, Pe is zero, to represent non-settling “rigid vesicles”. The effects of the particle size d2 and volume fraction Φ2 of the “vesicles” on the sedimentation stability of the high-Pe particles were examined. Stability maps of d2 and Φ2 ranges were generated, to provide general guidelines on preventing, or slowing down, the settling of the high-Pe particles. In addition, the local microviscosities around the dense particles and the viscosities of the vesicular dispersions were computed from the simulations as a function of the relevant shear rates or shear stresses. The resulting viscosities are consistent with the sedimentation observations and the measured shear-dependent viscosities of the TiO2-DDAB suspensions. Hence, the close-packed vesicular dispersions provide a high local resistance to the dense particles gravity-induced flow, which corresponds to low local shear stresses, but a much smaller resistance to the suspension bulk flow, which corresponds to high shear stresses. The experiments and the BDS results indicate that close-packed vesicular dispersions provided a general method for long-term stabilization against sedimentation of high-Peclet number particles of a wide range of diameters and for a wide range of volume fractions.