Molecular Simulation of Biaxial Nematic Liquid Crystals

Colloids consist of particles dispersed in a solvent, whose size, between 10 and 10000 nm, is small enough to neglect sedimentation with respect to Brownian motion. Colloids incorporating anisotropic particles are able to form liquid crystals (LCs), which exhibit properties in between those of crystals and fluids and are classified in terms of positional and orientational order. This project will focus on nematic LCs, showing long-range orientational order, but lacking long-range positional order. In particular, uniaxial nematic (NU) LCs have a single director indicating the preferred direction of the particles, while biaxial nematic (NB) LCs exhibit 3 orthogonal directors.

Since its prediction in the 1970s, the NB phase has strongly attracted the interest of the LC community, especially for its appealing applications in LC displays. Unfortunately, the existence of the NB phase at temperatures generally suitable for most display applications is still highly debated. The recent experimental observation of a remarkably stable NB phase of colloidal boardlike particles (BPs) is stimulating new interest on this phase, justified by its potential use in displays with shorter response time. However, prior to exploring the feasibility of this challenging technology, it is crucial to acquire a full picture of the laws underpinning the equilibrium and dynamical properties of colloidal NB phases, whose fundamental understanding constitutes the aim of this project.

By molecular simulation, we will seek the fundamental know-how to rationally ponder the potential of NB phases in future display devices. In particular, we will map out the phase diagram of colloidal BPs and determine the stability range of the NB phase, being strongly enhanced by polydispersity as indicated by experiments and theory. This task is particularly challenging due to the extension of the phase space to explore, but will be addressed by a scientific workforce relying on a specific expertise in molecular simulation and significant human resources. To better understand the formation of the oblate and prolate NU phases, we will study their nucleation from the isotropic (I) phase and calculate the nucleation barriers and the morphology of the growing nuclei at different supersaturations. This is challenging because the I-N are weakly first-order transitions, but crucial to have an insight into the kinetics of reorientation of BPs. Phase equilibrium and transition are expected to be modified by an external field, whose effect will be addressed for its crucial role in practical applications involving the reorientation of the NB phase.

In the second part of the project, we will focus on the effect of dynamical heterogeneities, collective motion and formation of transient clusters on the relaxation decay of the NB phase when an external field is switched on. One of the aims is to follow the rearrangement of the NB phase and how the eventual occurrence of these phenomena might influence its response time and final structural order. Insight into the kinetics of reorientation of the NB phase and into its dependence on the presence of transient clusters upon the on/off switching of an external field, will be crucial to grasp the physics underpinning any potential use of biaxial nematics in display applications.

The project will benefit from collaborations with Prof. A. Cuetos (AC) from the Pablo Olavide University in Seville, and Dr. A. V. Petukhov (AVP) and Dr. G. J. Vroege (GJV) from Utrecht University. AC will bring key expertise in simulation and theory of the phase behaviour of colloidal LCs. AVP/GJV will provide experimental data to validate the simulation results on the reorientation dynamics of the NB phase. Finally, an internal collaboration with Prof. Masters will facilitate the modification of a mean field theory to describe the phase behaviour of polydispersed boardlike particles in the presence of an external field.

The present project aims at investigating the physical properties of colloidal biaxial nematics with focus on their phase behaviour and relaxation dynamics, constituting the preliminary knowledge underpinning their potential use in liquid crystal (LC) displays. Although the full impact of this fundamental research is expected outside the time scale of the First Grant, its outcome will be of remarkable benefit for society, industry and economy.

On a shorter time scale, the present project has a direct impact on the community directly involved in fundamental and practical research on LCs, and particularly on the biaxial nematic (NB) phase, which is still highly debated at a molecular level. Moreover, the dynamics of colloidal particles in crowded media is of common interest to several disciplines including Biology (diffusion of proteins through cells), Materials Science (mobility of fillers in polymer nanocomposites), and Medicine (drug release). These emblematic processes are generally characterised by dynamical heterogeneities with transient clusters of particles rearranging collectively. Therefore, studying these phenomena in the context of colloidal LCs will generate an impact on diverse research communities in Soft Matter. Additionally, understanding the effect of an external field on the reorientation of the particles and the rearrangement of the LC phase, has also an impact on, e.g., display applications where the dynamics of reorientation of the optical axes controls the response time of a given device; on the ability of self-healing nanomaterials to recover its initial properties in response to structural defects; and finally, in the area of plastic crystals for photonic applications.

From a fundamental point of view, the project aims at applying a simulation methodology recently developed by the PI and one of his collaborators in this project, Prof. A. Cuetos, that allows for the application of an efficient Monte Carlo algorithm to investigate the dynamics of dense polydispersed colloids. Due to the long relaxation dynamics of dense colloidal systems, such as gels, glasses, and supercooled liquids, this simulation methodology is a valid alternative to Brownian dynamics or Molecular Dynamics and will definitely be a beneficial tool for researchers interested in investigating the diffusion of generic nano-objects in crowded media. This simulation technique is expected to have an impact on their research by significantly reducing the computational time and efforts in programming.

On a longer time scale, a benefit for industries is expected through knowledge transfer to companies. Such a knowledge transfer will be favoured via the Knowledge Transfer Networks (KTNs) of the Innovate UK Network, which bring together academic and industrial partners. The PI is already a member of the Materials KTN and will attend at least one event per each year of the project to establish new interactions and linkages with companies. Due to their enhanced thermal stability as compared to molecular LCs, colloidal LCs can provide displays with longer average life. Companies producing LC-based devices might be interested in exploring the feasibility of displays incorporating biaxial nematic colloidal LCs and thus need an understanding of their phase behaviour and ability to respond to an external field. Additionally, the reorientation of the minor directors in the biaxial nematic phase is expected to be faster and require less energy than the reorientation of the main director in the uniaxial nematic phase. Displays lasting longer, requiring less energy, and improving the quality of videos due to the shorter response time, will be of significant societal benefit for both consumers and professionals working with HQ videos or communication applications. This will in turn benefit the overall UK economy by providing technical jobs and well-being.