  Soft colloids are increasingly used as model systems to address fundamental
issues such as crystallisation and the glass and jamming transitions. Among the
available classes of soft colloids, microgels are emerging as the gold
standard. Since their great internal complexity makes their theoretical
characterization very hard, microgels are commonly modelled, at least in the
small-deformation regime, within the simple framework of linear elasticity
theory. Here we show that there exist conditions where its range of validity
can be greatly extended, providing strong numerical evidence that microgels
adsorbed at an interface follow the two-dimensional Hertzian theory, and hence
behave like 2D elastic particles, up to very large deformations, in stark
contrast to what found in bulk conditions. We are also able to estimate the
Young's modulus of the individual particles and, by comparing it with its
counterpart in bulk conditions, we demonstrate a significant stiffening of the
polymer network at the interface. Finally, by analyzing dynamical properties,
we predict multiple reentrant phenomena: by a continuous increase of particle
density, microgels first arrest and then re-fluidify due to the high
penetrability of their extended coronas. We observe this anomalous behavior in
a range of experimentally accessible conditions for small and loosely
crosslinked microgels. The present work thus establishes microgels at
interfaces as a new model system for fundamental investigations, paving the way
for the experimental synthesis and research on unique high-density liquidlike
states. In addition, these results can guide the development of novel assembly
and patterning strategies on surfaces and the design of novel materials with
desired interfacial behavior.
