EXCITABILITY AND DEFECT-MEDIATED TURBULENCE IN NEMATIC LIQUID-CRYSTAL

We report the observation of one-arm spiral waves and of turbulence states in a nematic slab subjected to an external forcing. These one-arm spiral waves constitute a close analog to the spiral waves observed in chemical and biological excitable media. Those results were obtained by adding a small vertical magnetic field on the experimental apparatus described in [1], in which double-arm spiral waves were observed in a sample with homeotropic alignment. We also describe a Ginzburg-Landau dynamical model accounting for the main kinds of experimental behavior and patterns obtained. In particular, the experiments and the simulations of this model reveal the appearance of a new instability of the 2 pi walls which is associated with the spontaneous formation of +1, -1 vortex, with the undoubling of the wavefronts, and which also leads to 2D turbulence.