  Solutions of microgels have been widely used as model systems to gain insight
into atomic condensed matter and complex fluids. We explore the thermodynamic
phase behavior of hollow microgels, which are distinguished from conventional
colloids by possessing a central cavity. Small-angle neutron and X-ray
scattering are used to probe hollow microgels in crowded environments. These
measurements reveal an interplay between interpenetration and deswelling, and
an unusual absence of crystals. Monte Carlo simulations of hollow microgel
solutions confirm that, due to the cavity, they form a supercooled liquid more
stable than in the case of microgels with a crosslinked core.
