The fuzzy sphere morphology is responsible for the increase in light scattering during the shrinkage of thermoresponsive microgels

Thermoresponsive microgels undergo a volume phase transition from a swollen state under good solvent conditions to a collapsed state under poor solvent conditions. The most prominent examples of such responsive...

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Thermoresponsive microgels are one of the most investigated types 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. This is particularly important for the widely used poly(N-isopropylacrylamide)–based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here, we address this point combining experiments with state-of-the-art 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 microgels with different cross-linker concentrations, charge contents, and charge distributions, we provide evidence that peripheral charges arising from the synthesis are responsible for this behavior and we further build a universal master curve able to predict the two-step deswelling. Our results have direct relevance on fundamental soft condensed matter science and on applications where microgels are involved, ranging from materials to biomedical technologies.

Mitotic Chromosome Condensation Driven by a Volume Phase Transition

Procedures were devised for the reversible decondensation and recondensation of purified mitotic chromosomes. Computational methods were developed for the quantitative analysis of chromosome morphology in high throughput, enabling the recording of condensation behavior of thousands of individual chromosomes. Established physico-chemical theory for ionic hydrogels was modified for application to chromosomal material and shown to accurately predict the observed condensation behavior. The theory predicts a change of state (a "volume phase transition") in the course of condensation, and such a transition was shown to occur. These findings, together with classical cytology showing loops of chromatin, lead to the description of mitotic chromosome structure in terms of two simple principles: contraction of length of chromatin fibers by the formation of loops, radiating from a central axis; and condensation of the chromosomal material against the central axis through a volume phase transition.