Differential rotation in cholesteric pillars under a temperature gradient

Abstract

Steady rotation is induced in cholesteric droplets dispersed in a specific liquid solvent under a temperature gradient. In this phenomenon, two rotational modes have been considered: (1) collective rotation of the local director field and (2) rigid-body rotation of the whole droplet structure. However, here we present another rotational mode induced in a pillar-shaped cholesteric droplet confined between substrates under a temperature gradient, that is, a differential rotation where the angular velocity varies as a function of the radial coordinate in the pillar. A detailed flow field analysis revealed that every pillar under a temperature gradient involves a double convection roll. These results suggested that the differential rotation in the cholesteric pillars was driven by the inhomogeneous material flow induced by a temperature gradient. The present experimental study indicates that the coupling between the flow and the director motion plays a key role in the rotation of the cholesteric droplets under the temperature gradient.

Slip of a liquid crystal droplet rotator in viscous fluids

“…Due to fluidity of LC, the disagreement between the direction of the LC director and flow velocity inside the LC has been reported in a rotating cholesteric LC pillar. 25 To verify these possibilities, we used optically cured solid particles (a mixture of E7 and 15 wt% RM257) that showed no internal fluidization in water. 16 We compared v θ / aω for the droplet and a cured particle as shown in Fig.…”

Morphogenesis of a chiral liquid crystalline droplet with topological reconnection and Lehmann rotation

“…The flow field was measured using the previously described fluorescence photobleaching method [21,27,33]. The fluorescent LED illumination system D-LEDI (Nikon Co., Ltd.) was used as the light source for photobleaching.…”

“…Subsequently, this phenomenon was reproduced by several research groups [18,[27][28][29][30][31][32][33][34][35]. As the mechanism of the rotational phenomenon, two possibilities have been proposed.…”

“…The first is based on Leslie's theory, which predicts the existence of direct coupling between director rotation and the temperature gradient in Ch LC [23,36]. The other mechanism is attributed to Marangoni convection resulting from the surface tension gradient; the rotation is driven by the material flow in the twisted director field [31,33]. To date, the rotational phenomenon has mainly been observed in droplets dispersed in specific liquid solvents [18,[26][27][28][29][30][31][32][33][34][35].…”

“…The other mechanism is attributed to Marangoni convection resulting from the surface tension gradient; the rotation is driven by the material flow in the twisted director field [31,33]. To date, the rotational phenomenon has mainly been observed in droplets dispersed in specific liquid solvents [18,[26][27][28][29][30][31][32][33][34][35]. In contrast, we used a system without solvents; a droplet directly exposed to air was used in this study.…”