MULTIPLE FRONT PROPAGATION INTO UNSTABLE STATES

The dynamics of transient patterns formed by front propagation in exte nded nonequilibrium systems is considered. Under certain circumstances , the state left behind a front propagating into an unstable homogeneo us state can be an unstable periodic pattern. It is found by a numeric al solution of a model of the Freedericksz transition in nematic liqui d crystals that the mechanism of decay of such periodic unstable state s is the propagation of a second front which replaces the unstable pat tern by another unstable periodic state with a larger wavelength. The speed of this second front and the periodicity of the new state are an alytically calculated with a generalization of the marginal stability formalism suited to the study of front propagation into periodic unsta ble states.