(463e) The Structure and Rheology of Bimodal Attractive Colloidal Systems

Size disparity in colloidal suspensions represents intriguing questions in colloidal physics with promising implications for practical applications. Specifically, it leads to varied particle motion, heterogeneous clustering when attractive interparticle interactions are present, and selective gravitational settling. Together, these phenomena can affect gelation and coarsening of attractive suspensions in ways that are quite different from monodispersed systems, and much less understood. Our ongoing research focuses on investigating how bimodal size disparity influences the microstructure and mechanical properties of attractive colloidal suspensions. Our study utilizes two populations of fluorescent core-shell spherical particles that are synthesized with an approximate size ratio of R = 2 between them. The two populations are colored with different fluorescent dyes and suspended in index- and nearly density-matching mixtures of glycerol and water. Through multi-channel confocal imaging, we are capable of independently identifying and quantitatively tracking each population in 3D with single-particle resolution. Attractive interparticle interactions are induced by the addition of a non-adsorbing polymer (depletion systems) and also by screening of the particles’ electrostatic double layer (salted systems). We performed ground experiments in conjunction with computer simulations to study both classes of these bimodal attractive suspensions across a range of particle compositions and varying attraction strengths. These studies will be followed by a round of experiments at the International Space Station and corresponding simulations in the absence of gravitational forces. Our findings have revealed distinct microstructural and rheological differences between the bimodal and monodispersed systems. Our results to date indicate that by introducing small particles to a spanning network dominated by large particles, the elasticity of the network increases significantly compared to a large particle-only network of the same total particle volume fraction. We will demonstrate how size disparity may enable the small particles to fill in gaps between large-large particle contacts, increasing the local packing efficiency and the number of interparticle bonds, thereby enhancing the mechanical strength of the structure. In the opposite scenario where the network is mainly constructed by small particles, the large particles can serve as nucleation sites for the aggregation of small particles, leading to structured growth around these seeds and a sample-spanning network that again deviates from its monodispersed counterpart. In this presentation, we will explore the effects of these microstructural scenarios on the gelation and coarsening behaviors, along with the linear and nonlinear rheology in attractive bimodal colloidal gels. Through these endeavors, we aim to better understand how size disparity can impact the behavior of attractive colloidal suspensions and facilitate their applications in real-world scenarios.