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For many applications of colloidal dispersions, the particles must be suspended for a long time. This is often accomplished by preventing agglomeration, which generates aggregates of increasing size. Nevertheless, many colloidal dispersions of dense particles may settle even without agglomeration. Preventing sedimentation without significantly increasing the bulk dispersion viscosity is difficult and has received little attention in the literature. However, settling can be drastically reduced through the novel use of close-packed vesicular dispersions at high enough concentrations, which are non-Newtonian shear-thinning fluids. Such dispersions have much higher viscosities at the low shear stresses "felt" by sedimenting colloidal particles than at the high shear stresses relevant to bulk dispersion flow. In a practical example, dense TiO 2 nanoparticles which normally would settle rapidly can remain suspended for at least 6 months without any observable sedimentation when they are introduced into a close-packed vesicular dispersion, while the dispersion retains its flowability. Cryo-TEM images reveal that the vesicles in these dispersions are tightly close-packed. Dynamic light scattering and electrophoretic mobility data also confirm that the vesicles in such dispersions have very low mobilities.

The sedimentation half-times t s of initially monodisperse dispersions of 750, 505, and 350 nm silica microspheres were measured in water, in ethanol, and in aqueous NaBr solutions of concentration c NaBr ranging from 50 to 1000 mM, where the particles may have formed clusters. In water and in ethanol, t s was about 8, 18, and 33 h for the 750, 505, and 350 nm particles, respectively. These values were the same as the ones predicted by Stokes’ law, suggesting that the particles were monodisperse and remained so during sedimentation; t s values remained the same with increasing particle weight fraction up to 0.03, indicating no hydrodynamic interactions. Three regions of NaBr concentrations with different settling behavior were found for each size. In region I or at lower c NaBr, the t s values were the same as at no salt conditions, implying that there was no significant agglomeration before particles settled. In region II, t s decreased with increasing c NaBr, suggesting that the agglomeration and sedimentation half-times of medium-size clusters were comparable. In region III, the t s values were quite similar for all particles, and independent of the NaBr concentration, indicating that at short times the particles formed large clusters which settled rapidly. The zeta potentials of the particles in water or in NaBr solutions were measured and used to predict the corresponding Fuchs–Smoluchowski stability ratios, which were sensitive to the chosen Hamaker constant values and the NaBr concentrations. Two models, based on the Smoluchowski steady-state and the more general unsteady-state agglomeration rates, were developed for obtaining the agglomeration times t an for forming clusters of size 2 N m , where N m = 1, 2, 3, ..., and the net predicted sedimentation half-time t s * for these clusters. The clusters were described by a fractal model with a fractal dimension d f. Diffusion-limited clusters (d f = 1.8) were compared to the coalescence-limit clusters (d f = 3). The models provide some useful and accurate upper bounds of t an and t s *. Moreover, the effective sizes, density differences, and volume fractions of the clusters were obtained as a function of time. The predicted trend of t s * was consistent with the experimental data. The predictions supported the inferences that the particles were unagglomerated in region I, formed medium size clusters in region II, and rapidly formed large clusters in region III.

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A novel method in which vesicular dispersions of the double-chain cationic surfactant DDAB (didodecyldimethylammonium bromide) stabilize suspensions of high density titania particles was recently presented (Yang, Y.-J; Corti, D.S.; Franses, E. I. Langmuir 2015, 31, 8802–8808). At high enough DDAB concentration, the vesicles form a close-packed structure, providing strong resistance to the sedimentation of the titania particles, while the dispersions remain highly shear-thinning with moderate limiting viscosities. Here, to elucidate the key factors of the mechanism by which vesicles or other nonsettling particles stabilize high density particles against sedimentation, we use Brownian dynamics simulations (BDS) to examine the sedimentation behavior of mixtures of “dense particles” that settle rapidly on their own and “light particles” that represent nonsettling “rigid vesicles”. BDS confirm that for large enough values of the volume fraction ϕ2 of the light particles, the dense particles should remain suspended. The rheological behavior of the mixtures is also computed with BDS. The observed shear-thinning behavior of the light particle dispersion suggests that the suspensions of the dense particles are still flowable at high shear stresses. Furthermore, the local viscosity of light particles around the dense particles significantly increases with increasing ϕ2, particularly when the same gravitational force applied in the BDS is exerted on a dense particle. The arrangement of light particles around the moving dense particles is an important factor in determining the stability of the dense particles against sedimentation. The BDS results indicate that dispersions of nonsettling particles provide a general method for the stabilization against sedimentation of high density particles.