Fabrizio Camerin, Nicoletta Gnan, José Ruiz-Franco, Andrea Ninarello, Lorenzo Rovigatti, and Emanuela Zaccarelli have published Microgels at Interfaces Behave as 2D Elastic Particles Featuring Reentrant Dynamics
The properties and the structure of colloids—in which particles of one substance are dispersed in another—are determined by the way those particles interact with each other. An easy guess might lead one to say that complex particles possess an equally complex interaction potential. However, this may not always be the case. Here, we consider microgels, which are colloids made of an intricate polymeric architecture, confined at a liquid-liquid interface. We show that this complex situation can be described in terms of a collection of simple 2D elastic objects.
This simplicity is not necessarily linked to the absence of intriguing behavior but actually reveals the presence of multiple reentrant transitions of the system at high densities. This means that, unlike most dense liquids, diffusion speeds up when the particle concentration increases. Remarkably, we demonstrate that this peculiar behavior is within accessible experimental conditions of small and soft microgels. In addition, the presence of the interface itself has consequences for the stiffness of the polymer network. Through our numerical study, we find that the polymer chains of the microgel are more stretched and less responsive than in bulk.
Altogether, our work suggests the use of microgels on the collective scale as an ideal system for fundamental investigations in two dimensions with unique reentrant dynamics. Besides, the a priori knowledge of the interactions that we provide here will allow for a clever design of microgel assemblies with desired properties.