Director/barycentric rotation in cholesteric droplets under temperature gradient

Abstract

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.

Lehmann rotation of cholesteric droplets: Role of the sample thickness and of the concentration of chiral molecules

“…In this case, the bands disappear and the droplet presents a texture made of concentric circles between crossed polarizers (CC droplets in the following). Droplets of this type spontaneously form in the mixtures studied by Yoshioka et al [11]. Nonetheless, we noted that they easily transform in our samples into banded droplets when their size decreases, which indicates that the helical axis easily tilts from vertical to horizontal position.…”

“…[8]) to the Leslie thermomechanical coupling of microscopic origin. These conclusions were recently confirmed by the crucial observation that each droplet with a banded texture seems to rotate as a rigid body in contradiction with the Leslie explanation [11]. Other types of droplet exist.…”

Continuous Rotation of Achiral Nematic Liquid Crystal Droplets Driven by Heat Flux

“…Perhaps the best known among these intriguing non-equilibrium thermodynamic phenomena is the thermomechanical coupling, known as Lehmann effect, described in 1900 by Lehmann [14], who reported the continuous rotation of the internal texture of CLC droplets in equilibrium with the isotropic phase, when they were subjected to a temperature gradient. In spite of the apparent simplicity of these observations, they remained experimentally elusive until recently [15][16][17][18].…”

Untwisting of the helical structure of cholesteric droplets with homeotropic surface anchoring

“…2 капли ХЖК применяли в качестве резонаторов для создания микролазеров [14,15], фото-механических [16] и термо-механических устройств [17]. Оптические и диэлектрические свойства ХЖК материалов, а также их отклик на внешние воздействия зависят от того, какая конфигурация директора n реализуется внутри капель [18], которая, в свою очередь, определяется граничными условиями (ориентацией директора на границе капли), материальными параметрами ХЖК, а также соотношением шага спирали ХЖК p 0 и размера капли d. Так, например, при тангенциальных граничных условиях в каплях ХЖК формируется закрученная биполярная конфигурация при d/p 0 < 1.5, структура с радиальным дефектом при d/p 0 > 3 и переходная при промежуточном соотношении [19].…”

“…Так, в [10] было показано, что шаг спирали в структуре с радиальным дефектом близок к собственному шагу спирали p 0 . В каплях ХЖК со слабым сцеплением, например в каплях холестерика, окруженных своей изотропной фазой, закрученная структура оказывается недеформированной с шагом спирали p 0 [17]. По аналогии с плоским слоем [24,25] можно предполагать, что в каплях с сильным гомеотропным сцеплением межфазная граница окажет существенное влияние на шаг спирали.…”