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The temperature dependence of the helical pitch in a cholesteric liquid crystal

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Mazza,  Marco
Group Non-equilibrium soft matter, Department of Dynamics of Complex Fluids, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society;

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Citation

Skutnik, R. A., Eichler, J.-C., Mazza, M., & Schoen, M. (2021). The temperature dependence of the helical pitch in a cholesteric liquid crystal. Molecular Physics, 119(15-16): e1881638. doi:10.1080/00268976.2021.1881638.


Cite as: https://hdl.handle.net/21.11116/0000-0008-0096-E
Abstract
We investigate the temperature dependence of the helical pitch of a cholesteric liquid crystal by means of Monte Carlo simulations. We carry out both lattice and off-lattice simulations to assess the impact of geometric and modelling constraints on the properties of the cholesteric phase. For the off-lattice simulations we develop boundary conditions commensurate with the cholesteric phase and derive an analytic expression for the helical wavenumber q that works well qualitatively. We find that the common simplification of constraining the orientation of the mesogens to planes normal to the helical axis makes q temperature-independent, as predicted by a mean-field theory of van der Meer et al. [J. Chem. Phys. 65, 3935 (1976)]. However, if mesogens are allowed to rotate in three dimensions, q will increase with temperature, as the isotropic-cholesteric transition is approached from below, in agreement with experiments for a number of substances. Our simulations indicate that the temperature-independent q is merely a consequence of the overly restricted orientational degrees of freedom to points on the unit circle in the model on which the mean-field theory is based.