Phase Behaviour and Dynamics of Colloidal Liquid Crystals of Board-like Particles

Abstract

The phase behaviour and dynamics of colloidal board-like particles is a topic of interest due to their ability to form stable biaxial nematic liquid crystals, a phase whose existence is still questionable on the molecular scale. Although display technology based on colloids is currently beyond reach, they can be useful model systems to understand the laws that underpin the equilibrium and dynamics related to biaxial nematics. Investigating the phase behaviour and dynamics of colloidal board-like particles is achievable through the frameworks of statistical mechanics and computer simulations, particularly by stochastic methods such as Monte Carlo simulations which can mimic Brownian motion. The present thesis is divided into two parts. The first part concerns on the phase behaviour of board-like particles and studies the role of polydispersity and external fields to stabilise the biaxial nematic phase. Board-like particles exhibit very rich phase behaviour and can form isotropic, nematic, smectic and columnar phases of either uniaxial of biaxial symmetry. When polydispersity is present, it was found that biaxial nematic stability was enhanced through destabilisation of the smectic phase, in accordance to experimental and theoretical findings. If an external field is applied to monodisperse isotropic and uniaxial nematic phases, alignment of the particles' intermediate axis with the external field at sufficient field strengths can lead to biaxial nematic phase formation. In the second part, we address the dynamics of these system. When studying the response times in field-induced uniaxial-to-biaxial switching of board-like particles, we found an interesting feature related self-dual shape, an intermediate of rod-like and plate-like geometry. In particular, the self-dual geometry tends to show the slowest overall response despite being the most optimal shape for biaxial nematic stabilisation. We then studied the rheology of isotropic phases of board-like particles using active microrheology. Preliminary results found that the micron-scale rheological properties of these systems are influenced by the system packing and particle geometry. In the specific case of particle geometry, we suspect that an interplay between presence of nematic-like clusters and tracer-to-board particle ratio influences the overall effective friction of the systems. How these two factors play out is currently unclear and warrants future works.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Carlos Avendano (Supervisor), Daniel Corbett (Supervisor) & Alessandro Patti (Supervisor)