Continuous rotation of achiral nematic liquid crystal droplets driven by heat flux

TL;DR

This study demonstrates that achiral nematic liquid crystal droplets can spontaneously rotate under heat flux due to a macroscopic twist in the director field, challenging the notion that chirality is necessary for Lehmann effect.

Contribution

It shows that macroscopic director twist alone can induce rotation in nematic droplets, removing the need for molecular chirality to explain Lehmann effect phenomena.

Findings

Achiral nematic droplets rotate in a temperature gradient.

Rotation speed is proportional to the temperature gradient.

Rotation speed inversely proportional to droplet radius.

Abstract

Suspended droplets of cholesteric (chiral nematic) liquid crystals spontaneously rotate in the presence of a heat flux due to a temperature gradient, a phenomenon known as Lehmann effect. So far, it is not clear whether this effect is due to the chirality of the phase and the molecules or only to the chirality of the director field. Here, we report the continuous rotation in a temperature gradient of nematic droplets of a lyotropic chromonic liquid crystal featuring a twisted bipolar configuration. The achiral nature of the molecular components leads to a random handedness of the spontaneous twist, resulting in the coexistence of droplets rotating in the two senses, with speeds proportional to the temperature gradient and inversely proportional to the droplet radius. This result shows that a macroscopic twist of the director field is sufficient to induce a rotation of the droplets, and that the phase and the molecules do not need to be chiral. This suggests that one can also explain the Lehmann rotation in cholesteric liquid crystals without introducing the Leslie thermomechanical coupling -- only present in chiral mesophases. An explanation based on the Akopyan and Zeldovich theory of thermomechanical effects in nematics is proposed and discussed.