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A molecular model for the electroclinic effect in nematic liquid crystals

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The Mathematics of Liquid Crystals

The electroclinic effect (ECE) is an electro-optical effect that consists in a tilt of the optical dielectric tensor of a liquid crystal (LC) material upon application of an external electric field. The tilt is linear in the electric filed, and the proportionality coefficient, the electroclinic coefficient, is a property of the LC material. Originally observed in the orthogonal smectic-A phase made of chiral molecules (smectic-A phase)[1], the ECE effect was subsequently also measured in the helix-unwound nematic N phase [2], thus demonstrating that smectic layering is not essential for its appearance. Recently it was also measured in nematic LCs made of non-chiral molecules, upon imposition of an external mechanical twist [3]. The origin of the ECE in nematics is not obvious, and different molecular/environmental contributions have been proposed over the years. We have developed a molecular model for the ECE in nematics. Based on the molecular and phase symmetry, we hav e obtained expressions of the EC coefficient as a function of the relevant molecular properties (dipole moment, polarizability). The use of an atomistic representation of the molecular shape, charges and polarizability allows us to analyze the relationship between the ECE and the molecular structure. We will present some examples and discuss the role of the molecular and phase chirality.

[1] Garoff. S.; Meyer R. B. Phys. Rev. Lett. 1977, 38, 848-851. [2] Li, Z.; Petschek, R. G.; Rosenblatt, C. Phys. Rev. Lett. 1989, 62, 796-799. [3] Basu, R.; Pendery, J. S.; Petschek, R.G.; Lemieux, R. P.; Rosenblatt, C. Phys. Rev. Lett. 2011, 107, 237804: 1-4.

This talk is part of the Isaac Newton Institute Seminar Series series.

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