Modeling Crystallization in Concentrated Suspensions of Compressible Microgels

Microgels are soft colloidal particles, made of crosslinked polymer gels. Their ability to swell by absorbing solvent makes them responsive to changes in temperature and concentration, enabling applications, e.g., to drug delivery and photonic crystals. In concentrated suspensions, mutual crowding can cause microgels to deswell, facet, and interpenetrate [1], which can influence thermodynamic properties. To explore the influence of particle softness on phase behavior, we adapt a coarse-grained model, based on the Hertz elastic pair potential and the Flory-Rehner theory of polymer networks [2]. In our model, faceting reduces the swelling volume, while interpenetration costs polymer-solvent mixing entropy. By performing Monte Carlo simulations with trial changes in particle size [2], we compute liquid-solid and solid-solid phase boundaries over a range of microgel crosslink fractions. In contrast to the phase behavior of incompressible Hertzian spheres [3], we find that compressible microgels crystallize at significantly lower volume fractions, when accounting for deswelling, faceting, and interpenetration. Our results may help to interpret experiments on microgel phase behavior [4].

[1] F. Scheffold, Nat. Commun. 11, 4315 (2020).

[2] M. Urich and A. R. Denton, Soft Matter 12, 9086 (2016).

[3] J. C. Pàmies et al., J. Chem. Phys. 131, 044514 (2009).

[4] M. Pelaez-Fernandez et al., PRL 114, 098303 (2015).