Unidirectional rotation of cholesteric droplets driven by UV-light irradiation

Owing to recent technological advances, heat-driven unidirectional molecular rotation as soft active matter is rapidly gaining scholarly attention as it potential for application in flexible micro-machines is becoming a reality. Chiral liquid crystals have been known to regularly exhibit nontrivial cross correlations between thermodynamic forces and fluxes due to their broken mirror symmetry. A notable example is the distinguished unidirectional molecular rotation of cholesteric liquid crystals driven by a heat flux. The inherent poor efficiency exhibited can be ascribed not only to the weak thermomechanical coupling, but also to the large friction on the solid substrates. Unfortunately, it is not easy to strengthen the complex thermomechanical cross correlation of the liquid crystals materials, although friction on the surface can be reduced via surface lubrication.

In a recent research paper published in Soft Matter, researchers led by professor Yuka Tabe, at the Kagami Memorial Research Institute for Materials Science and Technology, Waseda University in Japan coated glass substrate with azobenzene polymers and then illuminated them using ultraviolet light. Their main objective was to investigate the novel photo-induced dynamics of azobenzene-doped cholesteric droplets coexisting with the isotropic phase. The research team expected that by coating the substrates with azobenzene and irradiating with ultraviolet light, the substrates would melt the cholesteric droplets near the surface and lubricate the rotation of the cholesteric droplets.

Briefly, the empirical procedure begun by preparing the samples where the chiral dopant was added to the host nematic in predetermined ratios. The researchers then determined the distribution of the cholesteric droplets by applying confocal reflection microscopy. Eventually, an in-depth analysis of the rotational behavior of the cholesteric droplets was conducted using polarized optical microscopy and confocal reflection microscopy.

The authors observed that upon ultraviolet light irradiation of the hemispherical cholesteric droplets stuck to glass substrates, the droplets were parted from the substrates due to the surface disordering caused by the photo-isomerization of azobenzene. The research team also noted that the spherical droplets floating in the isotropic phase exhibited an unexpected motion – a continuous and unidirectional rotation along the light propagation direction. Additionally, the team recorded that the rotational direction was reversed by the inversion of either the sample’s chirality or the ultra violet irradiation direction, and the rotational velocity increased with both the ultra violet-light intensity and the concentration of the doped azobenzene, the dependences of which were described by linear and relaxation functions, respectively.

Professor Yuka Tabe research team were successful in decreasing the surface friction. Astonishingly, they unveiled novel dynamics in that the cholesteric droplets were observed to display a steady rotation under the ultraviolet light even without a temperature gradient. The rotational direction was determined by the cholesteric chirality and the light incident direction, and the angular momentum of the droplets increased with both the concentration of the doped azobenzene and the intensity of the ultraviolet-light. To this regard, the unique ability of the cholesteric droplets to transform the non-polarized light into the torque may be used for soft liquid crystal devices.