Director/barycentric rotation in cholesteric droplets under temperature gradient

When a chiral liquid crystal is given a transport current, a unidirectional molecular motion is known to take place, which is called the Lehmann effect. In this paper, we study the mysterious heat-current-driven Lehmann effect using two types of hemispherical cholesteric droplets using polarizing, reflecting, confocal and fluorescent microscopies. Both the droplets, coexisting with the isotropic phase and contacting on a glass substrate, are characterized by the concavo-convex modulated surface and the inside orientational helix. Further, the only difference between them is the helical axis direction; i.e., one is perpendicular and the other is parallel to the substrate. Under the temperature gradient perpendicular to the substrate, the droplet whose helical axis is parallel to the heat current exhibited pure director rotation, while that with the axis perpendicular to the current rotated independently as a rigid body. In the two droplets, the rotational conversion efficiency from the temperature gradient into the angular velocity showed very different dependences on the chirality strength and on the droplets' size, suggesting that the rotations of the two droplets may be driven by independent torques with different origins. This is the first observation that the cholesteric droplets under the temperature gradient exhibit the two rotational modes, the pure director rotation and the molecular barycentric motion, which can be switched to each other by changing the heat current direction parallel and perpendicular to the helical axis.