Do cavities matter? Suppression of crystallization in hollow microgel solutions

Microgels are crosslinked polymeric networks in the colloidal domain that exhibit phase behavior comparable to soft spheres. Solutions of microgels have been widely used as model systems to study self-assembly of condensed matter and complex fluids. Here we study the phase behavior of hollow microgels, characterized by a solvent-filled cavity in their center. Surprisingly, the phase behavior of these microgels, which lack a dense polymeric core, does not show crystal formation. The absence of crystals is independent of both the softness of the network and the size of the cavity. By means of small-angle neutron and X-ray scattering, both the form factors and the interparticle separations of hollow microgels are measured in overcrowded environments, revealing a complex interplay between interpenetration and deswelling. We confirm these findings by Monte Carlo simulations of microgel solutions modeled by the Flory-Rehner single-particle free energy and the Hertz pair potential.