A thermomechanical coupling in cholesteric liquid crystals: Unidirectional rotation of double-twist cylinders driven by heat flux

Rotation and transportation of liquid crystal droplets for visualizing electric properties of microstructured electrodes

The spatial resolution of typical sensor probes is sufficient for measuring the average electric properties of microelectrical devices, but they are unable to measure the distribution with a spatial precision. Liquid crystal droplets (LCDs) are promising candidate for visualizing the distribution. When voltage is applied, the LCDs show rotational and translational behaviors which depend on the location of LCDs within the devices. We demonstrate that by comparing the experimental and numerical results, the electric field and electrostatic energy distribution are visualized by rotating and transporting LCDs, with a spatial resolution of 10 µm and a detection accuracy of 5 µV/µm. In addition, we produced an array of LCDs by designing periodic modulation of the electrostatic energy density in the model device. These findings show that the LCDs serve as a periodic modulator of the refractive index as well as a sensor for the observation of electric properties of microelectrical devices.

Temperature gradient sensing mechanism using liquid crystal droplets with 0.1-mK-level detection accuracy and high spatial resolution

We proposed the detection mechanism of the micro-levels of temperature gradient in a micro-electromechanical system using the unidirectional rotation of cholesteric-liquid crystal (Ch-LC) droplets. Ch-LC droplets in the presence of an isotropic phase subjected to a heat flux rotate with a speed proportional to the magnitude of the temperature gradient. We further quantified the temperature gradient-to-torque conversion efficiency to apply the thermomechanical cross-correlation to the detection of temperature gradient. Then, we observed the rotational behavior of Ch-LC droplets after introducing them onto model devices containing patterned Au thin-film electrodes. Direct electric current applied to these Au electrodes results in unidirectional rotation of the Ch-LC droplets in response to heat flux generated from the Au electrodes. By evaluating the possible temperature gradient detection resolution using Ch-LC droplet rotation, we show that Ch-LC droplets can achieve both high spatial resolution (~ 10 µm) and high detection accuracy (~ 0.1 mK/µm).

Steady rigid-body rotation of cholesteric droplets and their dumbbell-shaped aggregates driven by heat flux along the helical axes

We investigated the steady unidirectional rotation of cholesteric (Ch) droplets driven by a heat flux. The droplets coexisted with the isotropic (Iso) phase and possessed a helical molecular arrangement. When a heat flux was transported along the helical axis, the droplets and their dumbbell-shaped aggregates exhibited steady rigid rotation. Our results are in contrast with those of previous reports in which Ch droplets in the same geometry exhibited pure director rotation. The fact that Ch droplets and their aggregates prefer rigid rotation can be ascribed to the orientational elasticity combined with the anchoring force at the Ch-Iso interface, which locks the director to the rotational flow in the droplets.

Heat-Driven Rigid-Body Rotation of a Mixture of Cholesteric Liquid Crystal Droplets and Colloids

We show that cholesteric (Ch) liquid crystal droplets with cylindrically symmetric orientation dispersing in an isotropic (Iso) phase exhibited unidirectional rotation under a heat flux along the symmetry axis. By introducing colloidal particle adhesive to the Ch droplet surface, we traced the translational motion of the colloids and found that the colloids rotated unidirectionally around the center of each Ch droplet. The director configuration of the droplets was not distorted either spatially or temporally, while the colloids rotated constantly. The results suggest that the Ch droplets under the heat flux should rotate as a rigid body. Using this heat-driven rotation of the Ch droplets, we designed new geometries of various composites of Ch droplets and colloids and succeeded in driving intriguing complex dynamics.