Modeling Liquid-Solid Phase Transitions in Suspensions of Compressible Microgels

Microgels are soft colloidal particles, made of crosslinked polymer gels, whose internal degrees of freedom allow them to respond to external stimuli by changing size. Their sensitive responses to changes in temperature and concentration inspire practical applications, e.g., to drug delivery and photonic crystals. To explore the influence of particle compressibility on thermodynamic phase behavior, we modeled suspensions of microgels that interact via the Hertz pair potential and swell/deswell according to the Flory-Rehner theory of polymer networks. From extensive Monte Carlo simulations that incorporate novel trial changes in particle size [1], we determined the liquid-solid phase boundary (microgel density vs. crosslink density) by computing free energies, osmotic pressures, and chemical potentials of both liquid and solid phases via thermodynamic integration methods [2]. Our results significantly extend previous studies of the phase behavior of (incompressible) hertzian spheres [3]. We further computed latent heats of melting, radial distribution functions, and static structure factors.

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

[2] C. Vega et al., J. Phys.: Condens. Matter 20, 153101 (2008).

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