Anisotropies and non equilibrium in soft matter: routes to the self assembly of advanced materials

This research project is a joint experimental, theoretical and computational effort in the field of soft matter, with the aim of identifying and exploring new

self-assembly

routes of elementary constituents to create materials with novel properties[1]. This objective will be achieved by exploiting mainly out-of-equilibrium pathways[2] and through the use of particles with an anisotropy in the shape or in the interactions[3].





Strategies for the

bottom-up

construction of materials, based on a judicious choice of constituent macromolecules, are crucial for their potential applications in fields as diverse as biomedicine, nanotechnology, cosmetics, food science. These strategies often mimic those used by Nature[4] to obtain improved materials and enhance the quality of life, welfare and health of the humans and of the environment.





Up to now researchers focused mainly on model systems in which the particles are either spherical or interact isotropically, mostly via equilibrium processes. However, many systems of technological and industrial interest very often exhibit anisotropies or are the result of out-of-equilibrium processes. For example, non-spherical macromolecules are widely used for the production of high-definition displays due to their ability to organize into liquid crystalline phases; clays like bentonite are employed for their ability to form gels in various applications ranging from clumping cat litters to oil extraction; wormlike micelles are currently used as components of detergents and in the cosmetic industry, in particular for shampoos.





The reason for this apparent disinterest of basic research for anisotropic systems is to be found mainly in the limited availability of synthetic model systems of sufficient quality to perform accurate and repeatable experiments. Paradoxically, the gaps in chemical synthesis have often been filled by using natural anisotropic materials such as fibers (actin, myosin, ..), fragments of nucleic acids (DNA, RNA, ...), rod-shaped viruses (mosaic virus, bacteriophage fd, ...) that, owing to their rich phase behavior, have been used in the past for instance to test theoretical models on liquid crystals or for the study of their rheological properties.





It is only in recent years, thanks to the advances in chemical synthesis, that the zoology of synthetic anisotropic particles has been greatly enhanced, offering now a wide range of building blocks that will allow:

(i) the design and construction of the materials of the future[3] such as photonic crystals, metamaterials, cements or cosmetic products with high biocompatibility; (ii) the development of model systems for the understanding of important processes such as protein aggregation, which is responsible for diseases such as sickle cell disease or the most common cataract.





Our aim in this project is to improve the knowledge on self-assembly in soft matter, exploiting the anisotropy of the constituent particles and the non-equilibrium phenomena, in order to drive research and its potential applications in entirely new and unexplored directions.





We choose a gradual approach, based on three different lines of inquiry with a close synergy between experiments, theory and simulations.





(1) OUT-OF-EQUILIBRIUM ROUTES OF GEL FORMATION

This theme gathers some studies on the dynamic behavior of model systems with known phase diagram that, through appropriate temperature jumps, will form a gel phase with desired properties. Part of this study aims also to characterize the properties of the non-equilibrium processes taking place in an arrested phase separation[5].





2) ANISOTROPIC DEPLETANTS: A NEW CLASS OF INTERACTION POTENTIALS

Here we will deal with a new class of systems with arbitrarily-shaped depletants. In particular we will focus on the role played by shape anisotropy on depletion forces, on critical Casimir forces or on those interactions generated by aggregation phenomena such as percolation or polymerization.





3) ANISOTROPIC COLLOIDS: PHASES, STATES AND APPLICATIONS

Finally, a major direction of research will concern the phase behavior of anisotropic particles and in particular the competition between equilibrium and non-equilibrium states, such as gels, isotropic glasses and nematic phases. The study will focus on anisotropic particles synthesized ad-hoc for the project, and will start from the systems that are already available, such as rods, DNA nano-assemblies and reversible inverse patchy colloids.





The previous three work packages will be complemented by a more technical work package aimed at developing innovative experimental and computational tools to tackle an ambitious project like ANISOFT, which introduces a paradigm shift in the soft matter panorama.













The project will be addressed by three complementary units in terms of skills and competences: one unit with theoretical and numerical skills and two experimental

















units, at the forefront in the development of tools and techniques for the study of soft matter. The tasks of the units can be summarized as follows:





1) the CNR-ISC unit, coordinating the project, will investigate the behavior of anisotropic and complex colloids in and out of equilibrium. This will be done with the already available theoretical and computational tools, but also with newly developed algorithms and theoretical techniques, specifically tailored for the different systems under study;





2) the UNIMI unit will perform experiments on anisotropic and complex colloids in and out of equilibrium, by refining and applying existing techniques and methodologies but also by developing new ones. In particular, we want to implement tools for the space/time resolved study of intermittent and heterogeneous dynamics and for rheomicroscopy measurements based on particle tracking and on adaptations of the Differential Dynamic Microscopy technique;





3) the POLIMI unit will bring in ANISOFT its competences in producing colloids with shape anisotropy and functionalized colloids. POLIMI will also perform experiments on anisotropic and complex colloids in and out of equilibrium with innovative optical space/time resolved correlation techniques and will develop a holographic microscope for 3D particle tracking and for the reconstruction of their spatial orientation.





The units will be supported by prestigious international collaborators in the field of soft matter, and in particular by (see attached letters of intent):

* Harvard University and NVIDIA corporation to implement GPU numerical codes for simulations and image-processing and to develop new methods of confocal microscopy

* University of Utrecht for the synthesis of well-characterized anisotropic and functionalized particles, and for the study of the relevant phase diagrams with confocal microscopy

* University of Fribourg for the rheology of anisotropic colloids and for the space/time resolved study of intermittent and heterogeneous dynamics

* University of Montpellier for the space/time resolved study of intermittent and heterogeneous dynamics

* SISSA for the theoretical study of the effective forces in the presence of anisotropic depletants close to the critical point or to percolation.





[1] A. Travesset, Science 334, 183 (2011)

[2] P. A. Korevaar et al, Nature 481, 492 (2012)

[3] S. C. Glotzer, M. J. Solomon, Nat. Mater. 6, 557 (2007) [4] S. Lee and N. D. Spencer, Science 319, 575 (2008)

[5] P. J. Lu, E. Zaccarelli et al, Nature 453, 499 (2008)