Synchrotron x-ray study of the smectic layer directional instability

We have investigated the phenomenon of field-induced smectic layer instabil ity, as monitored by synchrotron x-ray scattering. This instability means t hat, upon application of time-asymmetric electric fields to chiral smectics , the layer direction seems to "rotate" locally around an axis given by the direction of the applied field. For moderate values of field amplitude and asymmetry, domains with a favored layer inclination grow at the expense of unfavored ones, while larger fields and asymmetries generally lead to a ch aotic flow behavior. At moderate amplitudes, we have followed the process o f the horizontal layer folding (or horizontal chevron domain formation) and the smectic C* layer reorientation of ferroelectric Liquid crystals by app lying symmetric and asymmetric wave forms, respectively, and performing tim e resolved x-ray measurements. The studies unambiguously show the formation of a horizontal (in-plane, i.e., in a plane parallel to the cell substrate s) chevron domain structure from a nonoriented sample by application of a s ymmetric electric field of sufficient amplitude. It is then demonstrated th at a transition from the horizontal chevron domain structure to an in-plane uniform smectic layer direction takes place on application of asymmetric e lectric wave forms. Reversal of the field asymmetry reverses the inclinatio n direction and selects the other layer normal direction as the uniform end state. The in-plane smectic layer reorientation process is followed here a s it evolves, and analyzed directly by means of x-ray scattering.