Modeling crystallization in concentrated suspensions of compressible microgels

Soft colloidal particles, such as microgels, made of cross-linked polymer gels that can swell by absorbing a good solvent, have internal degrees of freedom that allow them to respond to external stimuli by changing size. Sensitive responses to variations in temperature and concentration have inspired many practical applications, e.g., to drug delivery, biosensing, filtration, and photonic crystals. At sufficiently high concentrations, mutual crowding can drive microgels to interpenetrate or deform by faceting. To explore the influence of particle softness and compressibility on thermodynamic phase stability, we develop a coarse-grained model in which spherical microgels interact via the Hertz elastic pair potential and can respond to crowding by deswelling and faceting or interpenetrating, as governed by the Flory-Rehner theory of polymer networks. In our model, faceting reduces the volume available for swelling, while interpenetration affects polymer-solvent mixing entropy. By performing Monte Carlo simulations that incorporate novel trial changes in particle size and shape, we compute equilibrium swelling ratios and Lindemann ratios to determine the fluid-solid phase stability boundary over a range of particle softness, tuned by varying cross-link fraction. Predictions from our facet model are in qualitative agreement with experiments and molecular-scale simulations. In contrast to the phase behavior of incompressible Hertzian spheres, we find that suspensions of compressible microgels crystallize at significantly lower volume fractions when accounting for the free energy costs of faceting and interpenetration. Our results extend previous studies of microgel phase behavior and can help to interpret experiments.