Self-organization of passive and active virus-based colloids: theory and experiments – VIRO-Lego

Confocal microscopy, theoretical modelling

An experiment-theoretical collaboration between two partners (LPS and CRPP) has been established on understanding complex intralamellar structure of charged viral rods both without and with patchy interactions. A particle-based density-functional theory (DFT) has been set up that enables to probe transition from simple SmA with long-range intralamellar fluidity to ones with crystalline signatures (be they long-ranged or quasi-long ranged) such as SmB and other smectic subphases. The DFT will be numerically implemented and the predicted results, in particular regarding the role of ionic strength and tip-functionalization, will be compared to experimental findings of fd rods.
A generic theoretical framework dealing with the effect of reversible polymerization of partially flexible rod-shaped monomers with double tip-functionalized adhesive ends in the presence of non-adsorbing discs has been developed using statistical mechanical theory. This project has been developed entirely at the LPS and highlights interesting opportunities arising from controlling the reversible association of tip-functionalized fd rods through the presence of non-adsorbing clay nanosheets.
An experimental project has been set up at the CRPP that focuses on tuning the mobility of swimming Bacillus Subt. bacteria through interaction with a matrix of fd rods arranged in both isotropic liquid and nematic phases. Important advances have been made in terms of visualizing the individual fd rods as well the flagellum and cell body of the bacterium which allows one to monitor the active Brownian dynamics of the bacterium as well as the passive diffusive motion of the host rods.

The partners have made important strides in advancing the use of filamentous fd viruses in complex media both in an experiment and theoretical context. Experimentally, fd rods turn out to be instrumental in elucidating the mechanisms of steering bacterial microswimmers through complex liquid crystalline media. Even richer phenomenologies could be expected if fd rods themselves could be rendered self-propelling as well, a goal that we are sincerely hope to achieve before the end of the project. Furthermore, modelling work (LPS) has pioneered the use of reversibly polymerizing fd rods equipped with double-tip adhesive spots in controlling the molecular-weight distribution and phase behavior of living polymers though templating with a matrix of non-adsorbing nanosheets. This scenario could be experimentally achievable though the use of fd rods mixed with non-adsorbing clay nanosheets. Last but not least, a fruitful experimental-theoretical collaboration is envisaged on providing a first-principles understanding of complex intralamellar order of smectic phases based on fd rods, and the role of tip-specific interactions.

Four-fluid coexistence of shape-persistent living polymers templated by discs (under review J. Chem. Phys.)

Hierarchical self-assembly of nanoparticles is a key process in manufacturing complex functional soft materials for use in a variety of applications within photonics and optical display and sensor technology. With most research efforts focussing on spherical nanoparticles, the
self-assembly mechanisms and the rich repertoire of possible superstructures formed by non-spherical building blocks receives far less
attention in current condensed matter research. An even richer phenomenology of self-assembled structures and symmetries can be expected by endowing the particles with regio-specific "patchy" interactions that fundamentally influences the way they self-organize on the meso- and macroscale.

This project aims to design novel anisotropic responsive colloidal particles based on fd virus and introduce reversible and tunable "patchy"
interactions through the creation of hybrid virus-metal rod-shaped nanoparticles consisting of end-functionalized fd rods each grafted with a metallic
nanobead coated with a thermo-responsive polymer with variable thickness. The presence of the beads breaks the fore-aft symmetry of the rod thus
generating polarity on the microscale while the polymer coating ensures the bead-bead interactions to be temperature-controlled and reversible.
The presence of tip-grafted metallic nanobeads also enables the viral rods to be rendered self-propelled through the action of thermophoresis or
through site-specific catalytic reactions, thus creating a new class of active rod-shaped microswimmers with tunable, polar interactions.

We will explore the self-assembly properties of passive and active tip-functionalized rods by combining experimental techniques with modeling tools that allow us to study these rods both at the single particle level (by addressing their dynamical properties by means of fluorescence microscopy) as well as on the macroscopic scale (by studying their phase behavior). This will be done both in thermal equilibrium as well as out-of-equilibrium conditions such as the case for emergent phenomena in systems involving active rods. A microscopic underpinning of the observed phenomena is sought through the use of computer simulation and use coarse-grained potentials matching the effective interactions between the modified virus rods and the experimental conditions, for instance, by mimicking phoretic rod self-propulsion of active virus rods.

For both passive and active systems, the combination of shape anisotropy, polarity and temperature-controlled regio-specific interactions present at the single-particle level is expected to exhibit a rich variety of self-assembled superstructures with a much higher level of complexity and functionality than could be achieved with common rod-shaped or spherical nanoparticles.

The project has a strong interdisciplinary character and is based on a combination of biological engineering (fabrication of functionalized virus particles), experimental soft matter physics (self-organization at many-particle level) and theoretical modeling (predicting and tuning structure-property relations). The results following from this research project are expected to open up perspectives within the realm of rational design where site-specific post-modification of anisotropic building blocks can be used to fabricate novel soft self-assembled materials or control structure formation in existing complex fluids by the presence of active rod-shaped dopants. Detailed knowledge of the mechanisms of self-assembly of rod-shaped building blocks with tunable, site-specific interactions (imparting e.g. local polarity) will not only contribute to our fundamental understanding of self-organization in (passive) liquid crystalline and active matter, the functionalized fd systems also have a high potential to position themselves at the forefront of materials science by generating a novel class of hybrid liquid crystals with bespoke optical properties for use as functional materials.