Swelling, Structure, and Phase Behavior of Microgel Mixtures

Microgels are soft colloidal particles made up of cross-linked polymer networks that swell in the presence of a solvent. The compressibility of the particles allows them to swell and deswell in response to changes in surrounding conditions, such as concentration, temperature, and pH. Due to their responsiveness to the environment, microgels have applications as biosensors and drug delivery vehicles. Recent experiments indicate that mixtures of microgels exhibit unusual thermodynamic phase behavior, including self-assembly of equilibrium crystal structures under conditions where mixtures of hard particles would form amorphous solids. Motivated by such experimental results, we perform Monte Carlo simulations to model binary mixtures of microgels that interact according to Hertzian elastic forces and swell according to the Flory-Rehner free energy of cross-linked polymer gels. From our simulations, we obtain particle size distributions, osmotic pressure, radial distribution functions, and static structure factors. We also analyze the fluid-solid transition and stability of competing crystal structures. Our results help to interpret experimental observations and guide design of soft materials composed of tunable, responsive particles.