To Swell or Not to Swell: Intermediate "Swelling Alloy" Phases in Microgel Crystals

Colloidal crystals made from soft microgel particles have a phase behavior that is distinct from hard particle colloidal crystals. Since these particles can be embedded with different core materials, there is interest in the creation of photonic and plasmonic metamaterials using different lattice structures. However, prior work has focused on particle shape or designing interactions to target certain lattice structures. Microgels such as Poly(N-isopropylacrylamide) can undergo a discontinuous swelling phase transition. We use this property to propose that phase coexistence between swollen and deswollen states can lead to new equilibrium lattice structures that are intermediate phases of microgel colloidal suspensions. We use Flory-Rehner theory with Hertzian contact mechanics as the basis for a lattice model of packed microgels. By assuming identical "reference" microgels, we numerically solve for the equilibrium states of the gel lattice. We find, at fixed volume, new equilibrium states corresponding to binary alloy configurations, where the unit cell consists of varying populations of swollen and deswollen microgels. This opens new questions about multiple metastable states for microgel crystals as well as phase transition/transformation kinetics.