Molecular Simulation of Biaxial Nematic Liquid Crystals

Lead Research Organisation: University of Manchester
Department Name: Chem Eng and Analytical Science

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description 1. The main key finding so far is that colloidal particles with an intrinsic biaxial geometry (such as board-like particles) might not be able to self-assemble into a biaxial nematic liquid crystal. We have mapped out the phase diagram of different geometries and have not observed this phase. We are now investigating the effect of polydispersity. 2. We have now investigated the effect of size dispersity and found that binary mixtures of hard cuboids are not able to form stable biaxial nematic phases. However, it seems that a larger degree of size dispersity can stabilise these phases. 3. We have successfully developed a theoretical framework that allows us to describe the out-of-equilibrium dynamics of colloids with our dynamic Monte Carlo simulation technique. 4. We are now studying the effect of an external field to the time taken for uniaxial-to-biaxial phase switching. 5 The application of an external field can stabilise the biaxial nematic phase. We have publiched this result in a communication in Soft Matter. 6 We are now studying the dynamics of switching from uniaxial to biaxial phase (February 2019, ongoing).
Exploitation Route It is important to assess the feasibility of stabilising biaxial nematic phases for liquid crystal display applications. So far, it seems that cuboidal particles, despite their intrinsic biaxial geometry, cannot form stable biaxial nematic phases unless an external field is applied. We have also further developed a powerful simulation method that will allow to use Monte Carlo simulations to investigate the dynamics of out-of-equilibrium colloidal dispersions, for instance when an external field is applied. This work, published on PCCP, is one of the 2018 HOT articles selected the editors of this journal. This technique is extremely efficient as compared to Molecular Dynamics or Brownian Dynamics. The group of Prof Dijkstra in Utrecht is currently applying our method to study the dynamics of cholesteric phases.
Sectors Digital/Communication/Information Technologies (including Software),Education,Electronics,Manufacturing, including Industrial Biotechology,Other

URL http://personalpages.manchester.ac.uk/staff/alessandro.patti/default.htm
 
Description Newton Mobility Grants 2017 Round 2
Amount £11,810 (GBP)
Funding ID NMG\R2\170137 
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 03/2018 
End 02/2020
 
Description Research Project Grant
Amount £231,771 (GBP)
Funding ID RPG-2018-415 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 05/2019 
End 04/2022
 
Description Dr Matthew Dennison (MD) 
Organisation Technical University Berlin
Department Institute of Theoretical Physics
Country Germany 
Sector Academic/University 
PI Contribution - My simulations of the phase behaviour of board-like particles were useful to validate the theoretical predictions by Dr Dennison (results included in the manuscript for publication in Soft Matter)
Collaborator Contribution - MD developed a modification of the Onsager theory to describe the phase transitions of freely-rotating biaxial particles.
Impact - Modification of the Onsager theory extended to freely-rotating biaxial particles - Validation of this theory by simulations
Start Year 2016
 
Description Prof Alejandro Cuetos (AC) - Pablo de Olavide University, Seville, Spain 
Organisation Pablo de Olavide University
Department Department of Physical, Chemical and Natural Systems
Country Spain 
Sector Academic/University 
PI Contribution - I updated an existing home-made molecular simulation software to investigate the effect of particles' size dispersity and external field on the phase behaviour of hard board-like particles. - I have studied the phase behaviour of monodisperse board-like particles and, particularly, the formation of biaxial phases - I have mapped out the phase diagram of short oblate and prolate board-like particles. - I have written a manuscript, ready to be sent for publication to Soft Matter, on the phase behaviour of board-like particles.
Collaborator Contribution AC is providing key expertise in assessing the phase behaviour of colloidal liquid crystals, and the microscopic mechanism driving the collective motion of board-like particles. In particular: - AC tested the new version of the software in the computer cluster available at his university and provided crucial input to optimise it. - AC updated the dynamic Monte Carlo code to study the relaxation dynamics of board-like particles. - AC provided additional computational resources and is currently running simulations to characterise the equilibrium dynamics of oblate and prolate board-like particles. - AC has mapped out the phase diagram of long oblate and prolate board-like particles (input included in the above mentioned manuscript). - AC will host the PDRA associated to this project in September 2017 for a week and provide the appropriate workspace, computer facilities, and network access for him/her to work efficiently during this stay.
Impact - Molecular simulation software to investigate the phase behaviour of monodisperse and polydisperse board-like particles - Molecular simulation software to investigate the effect of an external field on the dynamics of reorientation of monodisperse and polydisperse board-like particles - One paper, ready to be submitted to Soft Matter, entitled "Phase Behaviour of Hard Board-like Particles", in collaboration with Prof Andrew Masters (University of Manchester) and Dr Matthew Dennison (Institute of Theoretical Physics, Berlin).
Start Year 2016
 
Title Molecular simulation code 
Description Sofware to simulate the phase behaviour and dynamics of (polydisperse) board-like particles under an external field. The software already works, but needs additional cleaning to be made available for public use. 
Type Of Technology Software 
Year Produced 2016 
Impact The software has been used to produce the results of the current work and by final year students working in my group. 
URL https://personalpages.manchester.ac.uk/staff/alessandro.patti/default.htm
 
Description Conference - CCP5 AGM 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Preliminary results presented at the CCP5 annual general meeting. Poster contribution: "Molecular Simulation of Board-like Colloidal Particles".
Year(s) Of Engagement Activity 2016
 
Description Conference - Wurzburg 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Oral contribution in the German-British Liquid Crystal Society Conference: "Phase Behaviour of Hard Board-like Particles". To be presented on 03/04/2017 in Wurzburg
Year(s) Of Engagement Activity 2017
 
Description Invited talk - Seville 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other audiences
Results and Impact Invited talk "On the phase and aggregation behaviour of colloidal cuboids". Department of Atomic, Molecular and Nuclear Physics, University of Seville. Talk attended by PDRAs, PhD students, and academic staff members.
Year(s) Of Engagement Activity 2017
 
Description Royal Society course 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact To acquire the necessary communication skills and confidence to disseminate this research to a non-specialist audience or to build collaborations across disciplines, I have taken part to the Media and Writing Skills course offered by The Royal Society and tutored by communications professionals. Such a course focused on a number activities strictly related to (i) how to write press releases and articles for online social media and (ii) how to communicate with the general public.
10 participants attended this course, who were trained in answering questions by radio and TV journalists. Specifically, we have been trained on how to communicate science to non-specialists in short interviews.
Year(s) Of Engagement Activity 2016
 
Description Thermodynamics 2017, Edinburgh, UK 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk on "Self-assembly of colloidal cuboids".
Year(s) Of Engagement Activity 2017
 
Description UK Colloids 2017, Manchester, UK 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact talk on "Phase behaviour of hard board-like particles"
Year(s) Of Engagement Activity 2017