Soft ULC Microgels at the Interface Interact and Flow as Hertzian-Like Colloids

Soft pair potentials predict a reentrant liquid phase for high concentrations, a behavior not observed experimentally. Here, very soft microgels confined at an oil-water interface are used as a model system of particles interacting via a soft potential in 2D. Interfacial rheology measurements demonstrate the existence of different flow regimes that depend on the compression of the monolayer. Such a compression also leads to a non-monotonic variation of the elastic moduli and of the yield stress of the monolayer. These results, together with the equilibrium phase behavior of the monolayer, are reproduced in molecular dynamics simulations of a 2D system of particles interacting with a Hertzian-like potential. Remarkably, due to the non-monotonic variation of the elastic moduli, we observe \textit{isoelastic} points where the monolayer shows the same stiffness at very different concentrations. These points are the experimental manifestation of the predicted reentrant liquid phase.