  Thermoresponsive microgels find widespread use as colloidal model systems,
because their temperature-dependent size allows facile tuning of their volume
fraction "in situ". However, an interaction potential unifying their behavior
across the entire phase diagram is sorely lacking. Here we investigate microgel
suspensions in the fluid regime at different volume fractions and temperatures,
and in the presence of another population of small microgels, combining
confocal microscopy experiments and numerical simulations. We find that
effective interactions between microgels are clearly temperature dependent. In
addition, microgel mixtures possess an enhanced stability compared to hard
colloid mixtures - a property not predicted by a simple Hertzian model. Based
on numerical calculations we propose a multi-Hertzian model, which reproduces
the experimental behaviour for all studied conditions. Our findings highlight
that effective interactions between microgels are much more complex than
usually assumed, displaying a crucial dependence on temperature and the
internal core-corona architecture of the particles.
