(19f) Self-Assembly of Bidisperse Colloidal Gels

Self-assembled colloidal depletion gels are important model systems for many industrial processes and applications, such as inks and pharmaceuticals. These nonequilibrium gels persist due to a dynamical arrest resulting in dense, percolated structures with cluster-level architectures. The self-assembly process is well studied with monodisperse colloids; however, the introduction of size polydispersity changes the network microstructure in ways that are not well understood. Here, we investigate the microstructure of three sets of fluorescent poly(methyl methacrylate) colloidal gels comprising sterically stabilized poly(methyl methacrylate) (PMMA) particles of size ratios α = 1, 0.7, and 0.62 (average diameter 2a_avg = 1.20 µm) mixed with polystyrene (M_w = 900,000 g/mol) as a depleting agent. Colloidal gels are synthesized at a range of volume fractions (0.1 < ϕ < 0.4) by suspending mixtures of bidisperse PMMA particles in a refractive index and density matched solvent. Confocal light scanning microscopy is used to obtain 3D image volumes from which particle centroids are detected. As the size disparity and volume fraction increases, gels exhibit increased signatures of reduced short-range and local order. We further investigate the origin of this reduced order through particle contact distributions, finding that increasing the size disparity leads to a decrease in the average contact number. Separating distributions into small-small, small-large, and large-large particle contact pairings show that small particle contacts are much less frequent compared to small-large and large-large particle contacts, due to differences in the available surface area and attractive potential. This disparity grows with increasing volume fraction, where the occurrence of large particle contacts increases linearly with ϕ. We hypothesize that differences in the behavior of different sized colloids across volume fractions may influence the mechanical integrity of the network structure underlying these gels.