Patchy Colloidal Systems

Over the past decade, the already vast possibilities offered by colloidal particles as building blocks for [the development of] new materials have been substantially widened by the advent of patchy colloids, i.e., colloidal particles with chemically or physically patterned surfaces. By virtue of the well-defined bonding geometries, patchy particles are nowadays regarded as ideal units of novel self-assembled materials with specific symmetries and physical properties. Even though there is no doubt about the great potentialities of such colloids as building blocks for a new generation of smart materials, experimental studies on the assembly of patchy particles (and related technological applications) are so far rather rare. This is mainly due to the fact that although many recent top-down fabrication techniques have been successfully developed to create fine tuned surface patterns on nano- and micro-scale colloids - some limitations still remain: one of them is the small yields of patchy particles in synthesis processes. Increasing the amount of synthesized particles is nothing but one of the open challenges: improving the fine control on the surface patterns (size, shape, positions and orientation of the patches) and increasing the richness of the pattern morphologies (number of patches per particle) are other serious issues in present-day top-down approaches developed so far. In contrast, recently developed bottom-up production routes based on the self-organization of appropriately chosen sub-units into nano- and micro-scale entities have opened the way to an entirely new class of particles with designed surface functionality. Preliminary studies have indeed shown that multi-block copolymers and polymer stars can self-aggregate into compact assemblies with a well-defined internal structure and a patterned (i.e., patchy) surface. The limitation encountered for particle yields of the top-down productions can be easily overcome by bottom-up methods once the fine control on the properties of the sub-units is achieved. The resulting self- assembled patchy aggregates constitute a completely new class of systems, which combine directionality and anisotropy with soft interactions and incessant fluctuations in the positions and in the size of the patches. These new features will largely extend the possibilities offered so far by rigid, hard patchy colloidal systems. The proposed research project aims to provide a comprehensive concept to describe- starting from a microscopic level - the macroscopic properties of patchy particles resulting from bottom-up production strategies. To be more specific, we will consider different, polymer-based systems, which show a self-organization process from polymeric disordered units to mesoscopic patchy entities. The first part of the project is dedicated to the development of suitable coarse-grained models (realized via both theoretical and numerical tools), in an effort to describe in a reliable way colloidal systems characterized by patchiness, softness and deformability. Subsequent investigations on the interplay between the directional bonding, the incessant rearrangements of the patches and the (ultra-)soft interactions will be carried out via suitably developed numerical techniques. In this way we will contribute to a deeper understanding of the role of the aforementioned features on the self-assembly scenarios of macroscopic ordered and disordered phases.

Materials with specific structures and physical properties are heavily sought after because of the broad spectrum of their technological applications in, e.g., electronics, photovoltaics, data-storage devices and biomimetic materials synthesis. Rather than relying on externally-controlled tools, many fabrication methods are nowadays based on self-assembly processes of carefully chosen/synthesized base units: the macroscopic counterpart would correspond to building a tower or a bridge just by choosing the appropriate bricks and letting them self-organize into the desired structure. The newest and most successful route to self-assembled materials relies on anisotropy: extra instructions for the assembly of target materials with desired architectures and properties can be imparted upon the particles if the interactions are no longer merely isotropic but rather depend on the relative positions and orientations of the particles with respect to each other. In the colloid realm, inter- particle interactions can be designed to be direction dependent by manipulating the shape and/or the surface properties of the particles. My project focused on a class of anisotropically interacting particles, referred to as patchy colloids, i.e., colloids with a surface divided into different regions characterized by distinct interaction properties. In particular, I have selected two very promising sub- classes of patchy systems: inverse patchy colloids, i.e., patchy particles with differently charged surface regions, and soft and flexible patchy colloids, i.e., polymer-based units characterized by soft effective interactions and incessant rearrangements of their bonding sites. Using state-of-the-art computational techniques and appropriately developed theoretical frameworks, I have explored the great potentialities offered by these classes of micro- and nano-units to produce responsive functional materials. While the first type of systems was shown to be very promising to produce ordered structures with non close-packed architectures, such as layered and porous phases, the second type of patchy entities was proven to favor the formation of gel-like disorder aggregates, whose properties could be tuned via the features of the self-assembling units. Materials with an ordered open architecture can offer tantalizing new perspectives in photonics, sensing and purification, while disordered materials with a tunable connectivity are precious for tissue engineering and drug delivery applications.