(464g) The Effect of Suspended Colloidal Particles on the Stability of High-Density Particles Against Sedimentation

The stability against sedimentation of 1 wt% TiO₂ particles, with diameters of 300±200 nm and density of 4.2 g/cm³, in 0.5 wt% (10.8 mM) or 2.0 wt% (43.4 mM) aqueous vesicular dispersions of the cationic surfactant DDAB (didodecyldimethyl-ammonium bromide) was examined; see Yang, Y.-J., D. S. Corti, and E. I. Franses, Langmuir, 31, 8802-8808, 2015. Whereas at the low DDAB concentrations the particles settled completely by 1 cm over two months, at the high DDAB concentrations the particles remained suspended for over 18 months. A variety of methods, including cryo-TEM (transmission electron microscopy), dynamic light scattering, and electrophoretic mobility measurements, were used to probe the microstructure, size, surface potential, and mobility of vesicles [Yang et al., 2015]. The hollow vesicles were shown to have diameters of about 400-600 nm. At 0.5 wt% DDAB, the vesicles were quite mobile, with an estimated volume fraction of less than 0.2. At 2.0 wt% DDAB, the vesicles were quite immobile, probably owing to their high volume fraction of over 0.5, making them to arrange into a close-packed formation. The close-packed vesicles provide strong resistance to the sedimentation of the dense particles, but the dispersions are nonetheless highly shear-thinning, and free flowing, with moderate bulk viscosities.

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 = Ï?d³Î?ρgL/6k_BT, Pe is high; Pe is defined as the ratio of the sedimentation flux, v_sedc, to the macroscopic diffusion flux, Dc/L, over the scale L of the sample, where d is the particle diameter, Î?ρ is the particle-dispersion medium density difference, c is the particle concentration, D is the diffusion coefficient, v_sed is the sedimentation velocity, g is the gravitational acceleration, k_B is Boltzmannâ??s constant, and T is the absolute temperature. These particles, of diameter d₁ and volume fraction Φ_¹, will settle rapidly on their own. For the second type of particles, Pe is zero, to represent non-settling â??rigid vesiclesâ?. The effects of the particle size d₂ and volume fraction Φ_² of the â??vesiclesâ? 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.