[Paper Review] Ferroelectric nematic liquid crystal thermo-motor

Marcell Tibor Máthé, Ágnes Buka+2 more|arXiv (Cornell University)|Jan 19, 2022

Liquid Crystal Research Advancements35 references61 citations

TL;DR

This paper demonstrates a thermal gradient-driven circular motion of tracer particles on ferroelectric nematic liquid crystal droplets, driven by pyroelectricity and tangential ferroelectric polarization. The rotation arises from a vertical temperature gradient inducing a radial charge separation and a tangential electric field that exerts a force on charged particles, resulting in laminar, stable vortex-like flow without turbulence.

ABSTRACT

A thermal gradient-induced circular motion of particles placed on ferroelectric nematic liquid crystal sessile drops is demonstrated and explained. Unlike hurricanes and tornadoes that are the prime examples for thermal motors and where turbulent flows are apparent, here the texture without tracer particles appears completely steady indicating laminar flow. We provide a simple model showing that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become electrically charged in the fluid. These observations provide a fascinating example of the unique nature of fluid ferroelectric liquid crystals.

Motivation & Objective

• To investigate the emergence of spontaneous circular flow in ferroelectric nematic liquid crystal droplets under a thermal gradient.
• To determine the physical mechanism behind the laminar, vortex-like motion of tracer particles without turbulent flows.
• To explore the role of pyroelectricity and spontaneous polarization in generating electric fields that drive particle motion.
• To examine the influence of external electric fields on defect core position and flow direction reversal.

Proposed method

• Used RM734 ferroelectric nematic liquid crystal in sessile droplets with controlled thermal gradients via a hot stage.
• Employed polarizing and confocal microscopy to visualize director orientation and particle motion.
• Applied tuneable liquid crystal retarders to map spatial distributions of the director and optical retardation.
• Modelled the system using Poisson's equation and force balance between electric and viscous drag on charged particles.
• Measured angular velocity of particles and related it to pyroelectric coefficient, temperature gradient, and polarization magnitude.
• Applied in-plane and vertical DC electric fields to manipulate defect positions and flow direction.

Experimental results

Research questions

• RQ1 What physical mechanism drives the circular motion of particles on ferroelectric nematic droplets under a thermal gradient?
• RQ2 Why is the flow laminar rather than turbulent, despite being driven by a thermal gradient like in hurricanes?
• RQ3 How does the tangential arrangement of spontaneous polarization contribute to the generation of a tangential electric field?
• RQ4 What role do ionic contaminants play in charge separation and particle charging?
• RQ5 Can external electric fields control the direction and position of the topological defect and thus the flow?

Key findings

• Tracer particles with diameters of 8 µm exhibited stable clockwise circular motion at 131°C in the ferroelectric nematic phase.
• The angular velocity of particles was experimentally measured at approximately 0.16 s⁻¹, matching theoretical estimates.
• A vertical temperature gradient of 23 mK/µm was inferred from the phase transition temperature difference across a 47 µm droplet.
• Defect cores were displaced at a 45° angle relative to the applied in-plane electric field, with direction dependent on field polarity and flow helicity.
• Applying a vertical DC field occasionally reversed the circulation direction, indicating switching of the pyroelectric bound charge sign.
• In large droplets (1.4 mm diameter), defect lines exhibited circular motion, indicating stationary rotational flow without continuous director rotation.