Nonlinear stages of the recently uncovered instability due to insoluble surfactant at the interface between two fluids are investigated for the case of a creeping plane Couette flow with one of the fluids a thin film and the other one a much thicker layer. Numerical simulation of strongly nonlinear longwave evolution equations which couple the film thickness and the surfactant concentration reveals that in contrast to all similar instabilities of surfactant-free flows, no amount of the interfacial shear rate can lead to a small-amplitude saturation of the instability. Thus, the flow is stable when the shear is zero, but with non-zero shear rates, no matter how small or large (while remaining below an upper limit set by the assumption of creeping flow), it will reach large deviations from the base values-- of the order of the latter or larger. It is conjectured that the time this evolution takes grows to infinity as the interfacial shear approaches zero. It is verified that the absence of small-amplitude saturation is not a singularity of the zero surface diffusivity of the interfacial surfactant.
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