Complex Wave Packet Dynamics Induced by Marangoni Stresses.

New features emerge during Marangoni spreading of a surfactant-laden drop on a thick film, where inertia plays a significant role in the hydrodynamics relative to the more commonly studied low inertia spreading phenomenon. We uncover these features using high-speed imaging and understand their dynamics using numerical simulations on subphases of varying viscosity and depths. Deposition of a drop of surfactant solution drives the formation of a packet of waves moving across the surface. Waves closest to the deposition point are partially covered with the surfactant layer and are directly affected by Marangoni stresses; waves at larger distances are not. A previously unobserved event, merging of peaks close to the deposition point, is detected at early times in experiments and replicated in simulation. Such an event does not occur if a pure fluid drop of the same composition as the subphase is deposited on the surface. At later times, the difference in speed between the surfactant front and the innermost wave causes splitting of the innermost wave, creating a wave covered with surfactant and another wave not covered with surfactant. These features are detectable for thicker subphases of low viscosity liquids, commonly found in laboratory studies and technological settings where Marangoni spreading is present.