Structure and dynamics of biphasic colloidal mixtures.

We investigate the structure and dynamics of biphasic colloidal mixtures composed of coexisting attractive and repulsive microspheres by confocal microscopy. Attractive gels formed in the presence of repulsive microspheres are more spatially homogeneous and, on average, are both more locally tenuous and have fewer large voids than their unary counterparts. The repulsive microspheres within these mixtures display heterogeneous dynamics, with some species exhibiting freely diffusive Brownian motion while others are trapped within the gel network during aggregation. Colloidal gels, which consist of attractive colloidal particles that stick together to form a system-spanning interconnected network, are widely used in ceramics processing 1, inks 2, catalyst supports 3, and membranes 4. To improve component performance, new strategies for designing concentrated gels with controlled structural and rheological properties are desired. The fundamental ties between local gel structure and bulk properties have received considerable attention, both theoretically and experimentally 5–7. The overall connectivity of the gel network provides finite bending and stretching elasticity 8, contributing to the system’s zero-shear elastic modulus. To date, most experimental studies have focused on colloidal gels composed of a single species with uniform interparticle interactions, in which the primary control parameters are the colloid volume fraction and the strength of the attractive interaction energy U 7,9,10. Surprisingly, mixtures in which interparticle interactions are independently tuned have not been systematically studied, even though such systems may open new routes for tailoring gel properties even when both and U remain fixed. In this Rapid Communication, we investigate the gel structure and suspension dynamics in biphasic colloidal mixtures using confocal laser scanning microscopy CLSM .W e selectively tune the interparticle interactions of attractive and repulsive microspheres, and create binary mixtures of these two species. In these mixtures, the attractive particles stick to one another yielding a gel network, while the repulsive particles are stable, sticking neither to the attractive particles nor to one another. This system differs from most colloidal dispersions in which attractive interactions are uniformly controlled via the addition of depletants 7,9,10, salt 11 ,o r acid or base to solution 2. By imaging and quantifying the two particle populations independently, we elucidate the role of the excluded volume of repulsive particles in mediating the local and the long-range structure of the gel phase as well as the effect of the arrested gel network on the dynamics of the repulsive particles. We use three separate particle batches of silica microspheres, all with similar diameters. Their average diameters, assessed from scanning electron microscopy photomicrographs, are 1 = 0.68 0.04 m, 2 = 0.72 0.02 m, and 3 = 0.62 0.03 m. The first batch diameter 1 is labeled with rhodamine dye, and rendered “attractive” by partially coating their surface with hydrophobic n-octyldimethylchlorosilane following a standard silanecoupling procedure 12. These particles rapidly flocculate when suspended in an index-matched polar mixture of H2O-dimethylsulfoxide. The second batch diameter 2 is labeled with fluorescein dye; these uncoated particles are “repulsive” due to their negatively charged surface 12. The third batch diameter 3 is synthesized without dye; however, their surfaces are rendered hydrophobic and thus they