Dielectrowetting of Sessile Droplets of Smectic Liquid Crystals

We report on the dielectrowetting of sessile droplets of two common liquid crystals, 4-cyano-4'-pentylbiphenyl (5CB) and 4-cyano-4'-n-octylbiphenyl (8CB), deposited on interdigidated electrodes that were treated to induce homeotropic anchoring. We found a pronounced hysteretic response of the contact angle to the applied voltage caused by the pinning and depinning of the droplet contact line. Depinning occurred as the voltage exceeded a threshold value that increased from the nematic to the isotropic phase, whereas the smectic phase showed an intermediate value. Above the threshold, the contact angle decreased linearly and rapidly as a function of the voltage square, as expected from the dielectrowetting equation originally formulated for dielectric and isotropic liquids, with a slope larger in the anisotropic liquid crystal phases than in the isotropic phase. Observation between crossed polarizers showed that the molecular director realigned along the applied field in the anisotropic phase near the surface between the electrodes, thereby increasing the effective dielectric constant and strengthening the dielectrophoretic force compared to the isotropic phase. Director realignment involved the nucleation of topological defects in the nematic phase and was inhibited by large energy barriers in the smectic phase, which weakened the dielectrowetting response.