Front propagation into an unstable state in a forced medium: Experiments and theory

Spatially forced systems can exhibit coexistence and a rich interface dynamics between manipulable states. We show here how the propagation speed of a front into an unstable state can be modified through periodic space forcing. Based on optical feedback, we set up a quasi-one-dimensional forced experiment in a liquid-crystal cell. When changing the forcing parameters, fronts exhibit a ratchet motion. Unexpectedly, the average speed of fronts decreases when the strength of the forcing increases. Close to molecular reorientation transition, an amplitude equation allows characterizing analytically and numerically the observed dynamics.