Dynamics of a horizontal thin liquid film in the presence of reactive surfactants

We investigate the interplay between a stable horizontal thin liquid film on a solid substrate and an excitable or bistable reactive mixture on its free surface. Their coupling is twofold. On the one hand, flow in the film transports the reacting surfactants convectively. On the other hand, gradients in the surfactant concentration exert Marangoni stresses on the free surface of the film. A reduced model is derived based on the long-wave approximation. We analyze the linear stability of the coupled system as well as the nonlinear behavior, including the propagation of solitary waves, fronts, and pulses. We show, for instance, that the coupling of thin film hydrodynamics and surfactant chemistry can either stabilize instabilities occurring in the pure chemical system, or in a regime where the pure hydrodynamic and chemical subsystems are both stable, the coupling can induce instabilities. (c) 2007 American Institute of Physics.