Phase behavior of equilibrium linker gels

Gels made from networks of colloids linked by physical bonds are an important class of soft materials. A standard route to produce a gel is to rapidly cool a suspension of isotropically attractive colloids, with gelation resulting from kinetic arrest during spinodal decomposition. However, such gels are inherently nonequilibrium, and the gel’s properties inevitably change as it ages. Alternatively, “equilibrium” gels with open, homogeneous structures that are resilient to aging can be created by restricting the number of bonds that form between particles. Here, we report on equilibrium gelation controlled by the addition of a secondary “linker” macromolecule that mediates bonding between colloids. The phase diagrams of such mixtures were predicted using Wertheim’s thermodynamic perturbation theory (TPT) and compared to molecular dynamics simulations. Good agreement was obtained between the predictions and simulations, with the spinodal region depressed to lower colloid densities by increased linker length at fixed linker-to-colloid number ratio. However, the presence of looping in the colloid–linker networks inhibited percolation compared to TPT predictions at low linker concentrations.