Highly Polydisperse Colloidal Gels

We experimentally study colloidal gels composed of highly polydisperse particles. Much prior work has focused on gels composed of mono- or bi-disperse particles as the backbone, perhaps with a third species acting as a depletant. Our samples are made from highly polydisperse particles (the ratio of sizes between largest and smallest particle being approximately 10:1) suspended in a density matched organic solvent, in addition to a polymer used to produce a depletion force. We study the effect polydispersity has on the dynamics and structures that form. Larger particles are more sensitive to the depletion force, and the depletion force acting on a small particle in proximity to a large particle is larger still (an effect similar to the enhanced depletion for particles near a wall). Thus, large particles act like nodes around which clusters of smaller particles will congregate. For our largest particles (~10 μm diameter) diffusive motion is reduced compared to that of our smallest particles (~1 μm diameter). Our work demonstrates the role that polydispersity plays in the behavior of colloidal systems.