'Living' Colloidal Liquid Crystals

Project Information

[LC]2

Grant agreement ID: 851196

DOI

10.3030/851196

Project closed

EC signature date 5 November 2019

Start date 1 December 2019

End date 31 May 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 402 345,00

EU contribution

€ 1 402 345,00

Coordinated by

THE UNIVERSITY OF EDINBURGH
United Kingdom

Periodic Reporting for period 4 - [LC]2 ('Living' Colloidal Liquid Crystals)

Reporting period: 2023-08-01 to 2025-05-31

Summary of the context and overall objectives of the project

Living organisms are composed of biological materials with striking physical properties. Biological matter can autonomously grow, self-organise, heal and move, while synthetically manufactured materials tend to be static, non-responsive and passive. This project aims to design a class of materials that are soft, self-assembled and self-motile—in other words, life-like. Such biomimetic materials have the potential to replicate the way living systems actively, adaptively and autonomously interact with their environment, and so transform the way we approach manufacturing and engineering our world. The goal of this specific research programme is to combine qualities of active liquid crystals and colloidal liquid crystals to design hierarchical and intrinsically out-of-equilibrium structures, which we call ‘Living’ Colloidal Liquid Crystals, or [LC]².

The first, ‘living’ part of [LC]² involves active liquid crystals, which are fluids that spontaneously flow without being driven from the outside. Many spontaneously flowing active fluids are biological, such as groups of bacteria or other cells that are self-motile and tend to align. This alignment is what makes them liquid crystals. The second, 'colloidal' part of [LC]² involves adding small particles, called colloids, to these fluids. The liquid crystal background then forces the colloids to arrange themselves into precisely controlled patterns, a phenomenon called self-assembly. Together, the colloidal aspect of [LC]² can generate self-assembly, while the active aspect can endow autonomous and functional dynamics. However, [LC]² materials are complex and so designing ‘living’ metamaterials requires new models of active liquid crystals that can deal with the complexities of adding freely moving colloids. The [LC]² project has developed multiscale coarse-grained simulations and new theories of liquid crystals that now allow us to study these dynamically complex, life-like fluids.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The [LC]² research programme is divided into three work packages, each of which has produced notable results. The first (WP1) concerned developing novel approaches to modelling active fluids. It successfully developed, implemented and quantified a coarse-grained algorithm for simulating spontaneously flowing active fluids, which is ideal for embedding colloids within. Furthermore, the project advanced methods for simulating active systems, including motile bacteria, epithelial tissues and layered liquid crystals. By advancing the ability of researchers to model different types of active materials, WP1 increased our capacity to design animate materials. These models have been employed to simulate how active fluids flow when confined in microfluidic geometries.

While WP1 advanced the ‘living’ aspect of [LC]², work package two (WP2) tackled the ‘colloidal’ facet of ‘Living’ Colloidal Liquid Crystals. WP2 developed an algorithm for simulating passive composite colloidal/liquid crystal materials. These include small particles, called colloids, embedded in nematic-type liquid crystals, which are fluids composed of aligned, rod-like molecules. The [LC]² team quantified the interactions between different colloids and how they entangle within the nematic liquid crystal. Flexible polymers chains in nematics were studied to discover how their sudden turns can untangle in time. The hybrid approach represents a methodological advance for studying composite material systems across length scales.

These ideas were brought together in work package three (WP3). The [LC]² team built simulations to model rotating 2D colloids above an active fluid and compared this directly to experiments, and designed particles with different north and south hemispheres, called Janus particles. Janus can steal surrounding activity to propel themselves, thus becoming self-propelled particles. The [LC]² team studied how the activity-driven dynamics of rods differs from spherical particles, the dynamics of flexible polymers in active turbulence and discovered an active version of Darcy's Law for flow through porous environments.

Thus, foundational work has been accomplished, results obtained and discoveries disseminated as a result of [LC]². The groundwork for achieving ‘living’ colloidal liquid crystals has been laid by developing codes for active nematics and colloidal liquid crystals, which have been quantified and merged into a single simulation package. The work resulted in 24 peer-reviewed articles, over 100 conferences, workshops and colloquia presentations and 18 collaborations, including with 7 experimentalists.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The [LC]² project has advanced the state of the art in multiple ways.

Most importantly, [LC]² has developed a first truly mesoscopic algorithm for simulating active nematics, and numerically implemented and quantified the algorithm. This approach is particle-based, which gives it a number of advantages, especially for simulating the dynamics of embedded particles, such as colloids, filaments or polymers ⁠—making it ideal for simulating ‘Living’ Colloidal Liquid Crystals. The success of this approach allowed the method to be extended to simulate other types of active fluids and variations that suppressed giant number fluctuations. The [LC]² project also extended the state of the art for modelling other soft materials, which may prove to be potential active fluids in which colloids can be embedded. These include new approaches to modelling layered liquid crystals, numerically modelling tissues using mesoscale approaches with a degree of detail not been done before, and simulating bacterial microcolonies and polymicrobial communities consisting of bacteria and fungi within lung-like environments.

Additionally, [LC]² resulted in multiple discoveries that advanced the field of active matter. The team has worked with international experimental collaborators to discover new methods for controlling active flows. These include using submersed micropatterned structures below active fluids to guide flows, and driving disk-shaped colloids to cause topological transitions to vortices that do not otherwise exist. These collaborations will continue to provide the potential to experimentally realise [LC]² designs. The project also produced predictions that have yet to be experimentally realised, including discovering persistent helicity, which is a first instant of spontaneous but steady-state helicity in 3D active nematics. Furthermore, [LC]² led to predictions for the behaviour of engineered Janus colloids and the rheology of clusters of active particles.

Unexpected findings of [LC]² include multiple discoveries about the fundamental nature of active nematic fluids. [LC]² discovered a spontaneous self-constraint in active flows that reveals a new organising principle in active matter. Similarly, the [LC]² project identified structures in active fluids that are analogous to fundamental particles called Majorana fermions — a discovery that emerged from 3D simulations of active nematics. These unexpected breakthroughs have implications extending beyond our original [LC]² goals, suggesting new mechanisms for controlling and harnessing active matter.

This work has laid an essential foundation for future work to consider non-spherical colloids in active nematics and multiple colloids interacting through the active nematic fluid, producing further advancements of the state of the art.

Pairs of colloids can become entangled in liquid crystals

A coarse-grained simulation of spontaneously flowing active liquid crystals,

A design for a passive colloid that becomes self-motile in active fluids