Understanding the viscoelasticity of colloidal dispersions in nematic liquid crystals: a new route towards sustainable formulations

Abstract

Stiff, yet highly thinning, viscoelastic gels can be formed upon mixing colloids with homeotropic (normal) alignment into a nematic liquid crystalline medium. A nematic liquid crystal is composed of anisotropic rod shaped molecules or colloids which prefer to align along a preferred direction. This creates elasticity within the system resulting from a resistance to deformation. When particles are dispersed into a nematic liquid crystal, elasticity results due to the entangled network of defect lines that percolate throughout the sample. Preliminary investigations show that this gel has similar rheological behaviour to polymer gels, typically used in many formulations. Cellulose nanocrystals (CNCs) are anisotropic rods, which when dispersed in an aqueous solution and form a lyotropic (solvent-based) liquid crystals [Lagerwall et al., NPG Asia Materials, 2014]. CNCs are derived from bio-renewable sources such as wood and seaweed, and offer a sustainable option as the continuous liquid crystalline phase in defect-entangled gels. These materials present an environmentally friendly avenue for advancing the development of such systems.

Presented is a study of colloidal particles dispersed in liquid crystal phases, with the aim of developing a new class of soft matter that can be composed solely of bio-compatible materials. Three systems are studied: thermotropic (4-Cyano-4’-pentylbiphenyl), lyotropic chromonic (sunset yellow) and lyotropic (cellulose nanocrystals).

The ability to measure viscoelastic ratios, the nematic elastic constant divided by the viscosity for each type of distortion, will help us understand which dominate the composite behaviour. In future, this may help us design composites to achieve the desired behaviour from a formulation. Theoretical and experimental results have shown that the value of the elastic constant, K, determines the storage, G′, and loss, G′′, moduli of the gel by [Katyan et al., Journal of Rheology, 2021]. During this project, we set out to determine the range of viscoelastic ratios, K/η, using Differential Dynamic Microscopy (DDM) [Giavazzi et al, Soft Matter, 2014] for thermotropic and, for the first time, lyotropic liquid crystals. This approach offers an effective alternative to light scattering, while also avoiding the need to apply an electric field, as required in the Fréedericksz method. The latter induces movement of charged entities, which often form lyotropics. Dynamic analysis of the videos of different geometries is performed, providing the splay, twist and bend viscoelastic ratios.

We show, for the first time, that DDM is an effective technique for measuring the viscoelastic ratios of lyotropic nematic liquid crystals. Through performing rheology on nematic LCs we learn that behaviour of lyotropic chromonic systems differs significantly from the expected behaviour, with much lower scaling of the dynamic moduli with frequency. Through comparing the viscoelastic ratios measured using DDM and the rheology of nematic liquid crystal crystals we learn that bulk behaviour of thermotropic and lyotropic liquid crystals is closely linked to bend distortions. We have also discovered that CNCs form composites, presenting an attractive alternative to polymers for the formulation industry, with yielding behaviour going as G′′ ≃ ω^1/2 and G′ moduli ranging from ≈ 10^−1 Pa to 10^2 Pa. However, we find that the particles dispersed need to be bigger than 2 μm to achieve dispersions with sufficient stability due to the large correlation length of the nematic phase.