(686f) Gelation Versus Phase Separation: Gravitational Effects On Adhesive Hard Sphere Colloidal and Nanoparticle Dispersions

Tuning either the volume fraction or the interparticle potential of colloidal dispersions can lead to a diverse spectrum of dynamically arrested states ranging from repulsive driven- and attractive driven- glasses [1], nonergodic gels [2, 3], and to self-similar fractals [4]. The most popular model systems for studying the microstructure, properties, and formation kinetics of colloidal gels are colloid-polymer mixtures [2, 5] and thermoreversibly gelling octadecyl silica particles [3, 6]. Differences in the gel strength, microstructure, and phase behavior are reported in the literature between these systems, which can be related in part to the differences in the nature of the attractive interactions.  In this work, we report experimental results on thermoreversible gelling systems with varying particle size and fixed coating, such that the range of attraction is systematically varied. Rheology, small angle neutron scattering, and fiber optic quasi-elastic light scattering are used to determine the gel point, microstructure and strength of the attractive interaction and thereby develop a state diagram.  The gel boundary is found to depend systematically on particle size, thus violating the proposed principle of corresponding states by Noro and Frenkel [7]. A density-mismatch is suspected to be the possible reason for this unexpected behavior based on prior experimental and theoretical studies [8, 9] on the effect of gravity on the competition between gelation and phase separation of colloidal dispersions. To test our hypothesis, we studied gelation in the presence of increased gravitational acceleration by centrifugation. To understand this behavior we calculate the gravitational Péclet number (Pe_g), a comparison of a relative magnitude between Brownian diffusion and gravitational settling of the aggregates. We find that Pe_g can predict the interplay between gravity, particle size, and volume fraction and the occurrence of the gravitational settling.  Our experimental findings are corroborated by Monte Carlo simulations, which display the same quantitative separation boundary and densification of the particle-rich phase when the attraction is turned on at a given Pe_g. Thus, the density-mismatch is responsible for the particle size-dependence of the gel transition of octadecyl silica particles.  These results provide guidance for the rational design of materials based on colloidal gels and show that gelation is not necessarily preceded by phase separation at low to intermediate particle concentrations.

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