Unidirectional rotation of cholesteric droplets driven by UV-light irradiation

Abstract

We investigated the novel photo-induced dynamics of azobenzene-doped cholesteric (Ch) droplets coexisting with the isotropic (Iso) phase. When the hemispherical Ch droplets initially stuck to glass substrates were irradiated by UV-light, they were parted from the substrates due to the surface disordering caused by the photo-isomerization of azobenzene. Then, the spherical droplets floating in the Iso phase exhibited an unexpected motion - a continuous and unidirectional rotation along the light propagation direction. The rotational direction was reversed by the inversion of either the sample's chirality or the UV irradiation direction, and the rotational velocity increased with both the UV-light intensity and the concentration of the doped azobenzene, the dependences of which were described by linear and relaxation functions, respectively. We proposed a possible scenario based on Leslie's theory combining mass fluxes and torques, which well explained the photo-driven rotation of the Ch droplets.

Lehmann rotation of cholesteric droplets driven by Marangoni convection

“…The reason for eliminating this mechanism as the main explanation for the Lehmann effect observed in the coexistence region is that the rotation velocity of the droplets is independent of the chemical nature and the concentration of the impurities present in the samples. On the other hand, it is not excluded that this mechanism played a role in the experiment of Bono et al 17 on the reversal of the sense of rotation of the droplets under strong UV illumination.…”

“…In addition they do not always predict the good sense of rotation of the droplets 14 . Another explanation suggested by a recent work by Bono et al 17 but not confirmed experimentally 7 -lies on the existence of a flux of dissolved impurities (solute) across the droplets that would be responsible for a chemo-mechanical torque analog to the thermomechanical one. This torque, first predicted by de Gennes 18 , has already been observed in Langmuir monolayers [19][20][21] , freestanding films of Smectic C liquid crystals 22 and cholesteric phases 23 .…”

Stimuli‐Driven Control of the Helical Axis of Self‐Organized Soft Helical Superstructures

“…The droplets with helical axis parallel to the temperature gradient exhibit pure director rotation whereas the droplets with helical axis perpendicular to the temperature gradient rotate as a single body . Bono et al found photodriven rotation of cholesteric droplets . Color displays have been demonstrated by dispersing CLC droplets into polymer matrices .…”

A Two-Fluid Model for the Macroscopic Behavior of Nematic Fluids and Gels in a Chiral Solvent

“…In other cases two phase regions involving a cholesteric phase and an isotropic liquid phase were studied to bring out effects specific for systems with macroscopic handedness such as Lehmanntype effects [2,3]. When studying those an external force such as temperature and concentration gradients or electric fields give rise to rotations of the director field in suitably chosen geometries, for example, with the helical axis in the cholesteric drops perpendicular to the bounding plates of the sample [7][8][9][10][11]. We note that the inverse effect [12] has also been observed [13].…”