  Thermoresponsive microgels are one of the most investigated class of soft
colloids, thanks to their ability to undergo a Volume Phase Transition (VPT)
close to ambient temperature. However, this fundamental phenomenon still lacks
a detailed microscopic understanding, particularly regarding the presence and
the role of charges in the deswelling process. Here we fill this gap by
combining experiments and simulations to show that the microgel collapse does
not happen in a homogeneous fashion, but through a two-step mechanism, entirely
attributable to electrostatic effects. The signature of this phenomenon is the
emergence of a minimum in the ratio between gyration and hydrodynamic radii at
the VPT. Thanks to simulations of several microgels with different cross-linker
concentrations, charge contents and charge distributions, we build a unifying
master-curve able to predict the two-step deswelling. Our results have direct
relevance on fundamental soft condensed matter science and on microgel
applications ranging from materials to biomedical technologies.
