Elasto- and electro-capillary instabilities of a nematic-isotropic interface: Experimental results

Recently, we have shown the existence of an electro-capillary instability of a nematic-isotropic interface stabilized by a temperature gradient (P. Oswald, EPL 90, 16005 (2010)). This instability results from a competition between the destabilizing action of the electric Maxwell stress and the stabilizing action of the thermal and capillary forces. The control parameters are the temperature gradient G , the applied voltage V and the thickness h of the nematic layer. In this paper, we present new experimental results on this instability in both the linear and nonlinear regimes. In particular, very rich phase diagrams are mapped out in the (h, V) plane for three different values of G . The divergence of the growth time close to the onset of instability is also studied in detail. In addition, we show the existence at low voltages of another instability of the de Gennes type, where the elastic Ericksen stress is responsible for the destabilization. In this case, a hill-and-valley structure or a square array of umbilics develop at the interface depending on the values of h , V and G .