Anomalous Lehmann Rotation of Achiral Nematic Liquid Crystal Droplets Trapped under Linearly Polarized Optical Tweezers

Jarinee Kiang-Ia,Nattaporn Chattham,Rahut Taeudomkul,Pongthep Prajongtat,Apichart Pattanaporkratana,Padetha Tin

doi:10.3390/molecules26144108

Jarinee Kiang-Ia, Nattaporn Chattham + Show 4 more

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https://doi.org/10.3390/molecules26144108

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Abstract

Continuous rotation of a cholesteric droplet under the heat gradient was observed by Lehmann in 1900. This phenomenon, the so-called Lehmann effect, consists of unidirectional rotation around the heat flux axis. We investigate this gradient heat effect using infrared laser optical tweezers. By applying single trap linearly polarized optical tweezers onto a radial achiral nematic liquid crystal droplet, trapping of the droplet was performed. However, under a linearly polarized optical trap, instead of stable trapping of the droplet with slightly deformed molecular directors along with a radial hedgehog defect, anomalous continuous rotation of the droplet was observed. Under low power laser trapping, the droplet appeared to rotate clockwise. By continuously increasing the laser power, a stable trap was observed, followed by reverse directional rotation in a higher intensity laser trap. Optical levitation of the droplet in the laser beam caused the heat gradient, and a breaking of the symmetry of the achiral nematic droplet. These two effects together led to the rotation of the droplet under linearly polarized laser trapping, with the sense of rotation depending on laser power.

Highlights

Chiral nematic liquid crystal (NLC), composed of rod-like molecules with symmetry breaking caused by the handedness of the molecules, can form a helical structure with molecular directors twisted around the axis in the chiral nematic phase
We show that it is possible to exhibit Lehmann rotation of achiral nematic droplets subjected to heat flux under linearly polarized optical tweezers
Nematic liquid crystal droplets were prepared by constant stirring of a mixture of

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. There have been reports of Lehmann rotation in achiral molecules [5], which had been suggested theoretically by Brand et al [6] This discovery opens the door for liquid crystal scientists to search for more possibilities of finding the Lehmann effect in other macroscopic structures of achiral molecules. Optical levitation is useful in many optomechanical systems [16], especially in ultrasensitive force detection [17] In this experiment, we used linearly polarized optical tweezers to create an asymmetric macroscopic structure in achiral nematic droplets [18,19]. We show that it is possible to exhibit Lehmann rotation of achiral nematic droplets subjected to heat flux under linearly polarized optical tweezers

Materials and Methods

Results and Discussion

Plot speed with laserlaser power of a radial hedgehog defect under linearly