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Modelling and simulation of chiral and non-chiral nematic liquid crystals between cylinders Nikzad, Arash

Abstract

Liquid crystallinity defines a state between a crystal solid and a liquid. Liquid Crystal (LC) molecules have the flowing properties of liquids while they keep the orientational order of solids with no positional order. To study the characteristics of disc-like LCs, initially, an analytical method to calculate the viscosity coefficients and rheological properties of discotic nematic liquid crystals (DNLCs) was proposed. The method was illustrated on nematic Graphene oxide (GO) dispersions, the most processable graphene derivative, as an example of DNLCs. GO dispersions have attracted enormous attention due to their unique liquid crystal (LC) characteristics. In the second step, the calculated Landau and Leslie viscosity coefficients were implemented in the Ericksen-Leslie (EL) theory to simulate the flow of GO dispersions. GO aqueous suspensions, with a concentration range of 15 mg/mL to 30 mg/mL, were simulated as a lubricant between two cylinders with a small gap size, which is the preliminary geometry for journal bearings. The anisotropic feature of LCs leads to a preferred direction of the molecules close to the solid surfaces, making them an outstanding candidate for the lubrication problem. Flow properties of GO dispersions at different concentrations were calculated numerically using the EL theory and compared with the respective theoretical values, which were within 1% error. Lastly, the Landau-de Gennes theory was applied to investigate the behaviour of chiral liquid crystals (CLCs) between concentric and eccentric cylinders under different flow conditions. This theory was implemented using dynamic finite element simulations (COMSOL Multiphysics) to solve the evolution of the microstructure of CLCs and coupled with a linear momentum balance equation to capture the structure of CLCs. This section focused on the microstructure formation of CLCs and their performance as lubricants under various chirality strengths (θ), Deborah numbers (De), and eccentricity of eccentric cylinders. The hexagonal structure of the CLCs at low De, where the chiral term predominates, was observed, while at higher De, the hexagonal pattern vanished. The eccentricity ratio impact on the performance of CLCs was also considered, and it was concluded that CLCs as lubricants perform well at high θ and eccentricity and low De.

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Attribution-NonCommercial-NoDerivatives 4.0 International