Rotation of a liquid crystal by the Casimir torque

TL;DR

This paper calculates the Casimir torque on a liquid crystal near a birefringent crystal, showing how the torque causes measurable rotation depending on the separation and material properties, with potential experimental detection methods.

Contribution

It provides a theoretical calculation of the Casimir torque effect on liquid crystals near birefringent materials, including the impact of layer thickness and retardation effects.

Findings

A barium titanate slab induces a 10° rotation of 5CB liquid crystal at 35 nm separation.

The torque depends on the thickness of the intervening layer, affecting retardation.

An optical measurement technique for detecting the twist is proposed.

Abstract

We present a calculation of the Casimir torque acting on a liquid crystal near a birefringent crystal. In this system, a liquid crystal bulk is uniformly aligned at one surface and is twisted at the other surface by a birefringent crystal, e.g. barium titanate. The liquid crystal is separated from the solid crystal by an isotropic, transparent material such as SiO$_2$. By varying the thickness of the deposited layer, we can observe the effect of retardation on the torque (which differentiates it from the close-range van der Waals torque). We find that a barium titanate slab would cause 5CB (4-cyano-4$'$-pentylbiphenyl) liquid crystal to rotate by 10$^\circ$ through its bulk when separated by 35 nm of SiO$_2$. The optical technique for measuring this twist is also outlined.