Abstract
Soft colloids are increasingly used as model systems to address fundamental issues such as crystallization and the glass and jamming transitions. Among the available classes of soft colloids, microgels are emerging as the gold standard. Since their great internal complexity makes their theoretical characterization very hard, microgels are commonly modeled, at least in the small-deformation regime, within the simple framework of linear elasticity theory. Here we show that there exist conditions where its range of validity can be greatly extended, providing strong numerical evidence that microgels adsorbed at an interface follow the two-dimensional Hertzian theory, and hence behave like 2D elastic particles, up to very large deformations, in stark contrast to what found in bulk conditions. We are also able to estimate Young’s modulus of the individual particles and, by comparing it with its counterpart in bulk conditions, we demonstrate a significant stiffening of the polymer network at the interface. Finally, by analyzing dynamical properties, we predict multiple reentrant phenomena: By a continuous increase of particle density, microgels first arrest and then refluidify due to the high penetrability of their extended coronas. We observe this anomalous behavior in a range of experimentally accessible conditions for small and loosely cross-linked microgels. The present work thus establishes microgels at interfaces as a new model system for fundamental investigations, paving the way for the experimental synthesis and research on unique high-density liquidlike states. In addition, these results can guide the development of novel assembly and patterning strategies on surfaces and the design of novel materials with desired interfacial behavior.
- Received 15 January 2020
- Revised 9 April 2020
- Accepted 15 May 2020
DOI:https://doi.org/10.1103/PhysRevX.10.031012
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
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.