Electro-capillary instability of a nematic-isotropic interface

We have studied the morphological instability under AC electric field of a nematicisotropic interface confined between two electrodes set at different temperatures. The electrodes are treated for homeotropic anchoring so that, when the field is increased, the Ericksen elastic stress vanishes. In this limit, the instability resembles that observed in ordinary liquids. However, significant differences were observed. Copyright c © EPLA, 2010 Introduction. – In 1974, de Gennes [1] showed that the free surface of a nematic liquid crystal can destabilize into a hill-and-valley structure when the director field is distorted by a strong enough magnetic field. In the example treated by de Gennes, the critical field and the wavelength at the onset of the instability resulted from the competition between the destabilizing action of the Ericksen elastic stress and the stabilizing effect of the capillary and gravitational forces. This instability and another one of the same type, which led to an undulation of the interface, were observed a few years later at the nematic-isotropic (N-I) interface [2–4]. Similarly, the interface between two isotropic fluids having different electrical properties can become unstable under the action of an electric field. In this case, the Maxwell electric stress (absent in the de Gennes calculation, but taken into account in the work of Yokoyama et al. [3]) is destabilizing, whereas gravity and surface tension are stabilizing [5,6]. We found that an instability similar to that observed in usual liquids can develop at a N-I interface of a liquid crystal of positive dielectric anisotropy when the Ericksen elastic stress vanishes. A sketch of our experiment is shown in fig. 1. The liquid crystal is sandwiched between two parallel electrodes that had been treated for homeotropic anchoring. The N-I interface is prepared in a temperature gradient and is subjected to a strong AC electric field. “Strong” means that the field is large enough to break the (a)E-mail: [email protected] V Iso