A distinctive feature of microgel particles is that of being soft colloids with an internal polymeric architecture. At liquid interfaces, in particular, they deform and flatten significantly due to the balance between surface activity and internal elasticity. For this reason, recent experimental studies have shown that they are a valuable choice to stabilize smart emulsions in contrast to common rigid colloids, along with the versatile patterning of surfaces and other high-end applications. Despite the clear potential of these systems, a fundamental investigation of their single-particle and collective properties at the interface is still lacking.
Thereby, within this project, I will investigate numerically the elastic properties and the effective interactions of microgels at liquid-liquid interfaces and compare the outcomes with experimental results, provided by our collaborators at ETH Zurich. To this aim, I will rely on a model I developed in the past months that has proven against AFM and FreSca imaging to reproduce correctly the morphology of flattened microgels [Camerin et al., ACS Nano 13 (2019)]. Most importantly, thanks to the realistic reproduction of the internal polymer network, this study will link for the first time the origin of the macroscopic behavior to the microscopic particle arrangement. This investigation is essential to develop predictive power for the use of microgels in a broad range of elastic materials as well as in bidimensional soft model systems for the accurate investigation of glassy and high-density states of thin microgel layers.
The research plan I propose ambitiously aim to link the microscopic properties of single particles to their collective behavior. Experiments and simulations go hand in hand to establish a sound framework for microgels adsorbed at a liquid-liquid interface. This study will provide the first evidence on how these polymeric colloids interact in a confined environment enabling the investigation of their phase diagram. This project will shed light on several aspects that up to this moment have been ignored and overcome. In fact, despite the huge experimental and industrial interest over the years for this system in bulk [Plamper17, Karg19], no one has dwelt on studying in depth the effect of adsorbing such particles at a fluid interface. Recent experimental work has envisaged their use in stabilizing smart emulsions or in the versatile patterning of surfaces [Xia17, Tang18], but the few hypotheses on the origin of their behavior have remained mostly speculative and uncertain.
With this work, I will boost the newborn research line on two-dimensional microgels by providing a microscopic justification for their effective interactions. The interest in understanding how such particles interact with each other has been recently demonstrated for 3D bulk microgels. Despite for long time, they were thought to interact as simple elastic spheres, their complex internal structure does not permit such a simple treatment and a multi-Hertzian description is needed [Bergman18]. At the interface, such investigations have not been carried out yet. By filling this gap, I will be able to uncover differences and similarities with the bulk case. As a matter of fact, a microgel at the interface retain a completely different structure with respect to its analogous in bulk. Moreover, the presence of the surface tension between two immiscible fluids induces in the polymeric network a deformation of the particle that has no counterpart also in other hard-particle-stabilized emulsions such as the Pickering ones.
Computationally, I will tackle effective interactions calculations that closely resemble the experimental conditions in terms of (i) presence of an explicit solvent, (ii) quantitative reproduction of the surface tension and (iii) realistic design of the microgel particle. The actual presence and correct modeling of the solvent itself is configured as an innovative aspect of the research. Indeed, most of the numerical work that relates to soft matter relies on implicit solvent approaches, especially owing to the high increase in computational cost. In this case, I will be able to correctly assess the effect that a liquid interface induces in the conformation and thus in the effective interactions of a soft polymeric network, such as the microgel. Indeed, as the analysis move from a microscopic model that reproduces experimental form factors and density profiles [Ninarello19], I will be able to link the effective potential with the structural features of the microgel at the interface. Our theoretical calculations will be compared to experimental results at a water/hexane interface via a Langmuir trough. Imaging via AFM and FreSca techniques will allow to determine the phases of this system for different compressions of the interfacial plane.
An important role for this system is played by the elasticity of the network. I will study for the first time the elastic moduli of a single particle by computing its equilibrium fluctuations and then linking to the Mooney-Rivlin theory. For such system we can expect a smaller response of the network as compared to the bulk case as the polymer chains are already stretched by the presence of the confining liquids. If actually confirmed, this could be indicative of the stiffening of the microgel in 2D with respect to the bulk case. This kind of analysis will be crucial for the future designing of innovative materials where the knowledge of the elastic response of the network is meaningful.
Overall, as a mid- and long-term perspective, this research project brings advantages both from a fundamental and applicative points of view. For instance, it will be possible to investigate the phase diagram of bidimensional microgels as well as the rheological properties of thin microgel monolayers for which the typical behavior of soft glassy materials has been lately observed [Huang17]. Other high-end applications will benefit. As an example, microgels' deformability at liquid interfaces may be exploited either for sensing and interferometry [Kim05] or as a valuable tool in biomedicine for noninvasive control over cell-adhesion [Uhlig18].
(refs.)
[Bergman18] Nat. Comms. 9 (2018)
[Camerin19] ACS Nano 13 (2019)
[Huang17] Macromolecules 50 (2017)
[Karg19] Langmuir 35 (2019)
[Kim05] J. Am. Chem. Soc. 127 (2005)
[Ninarello19] submitted, arXiv preprint 1901.11495 (2019)
[Plamper17] Acc. Chem. Res. 50 (2017)
[Tang18] ACS Omega 3 (2018)
[Uhlig18] Polymers 10 (2018)
[Xia17] ACS Appl. Mater. Interf. 9 (2017)