Nonlinear dynamics of surfactant-laden two-fluid Couette flows in the presence of inertia

The nonlinear stability of immiscible two–fluid Couette flows in the presence of inertia is considered. The interface between the two viscous fluids can support insoluble surfactants and the interplay between the underlying hydrodynamic instabilities and Marangoni ef- fects is explored analytically and computationally in both two and three dimensions. Asymptotic analysis when one of the layers is thin relative to the other yields a coupled system of nonlinear equations describing the spatiotemporal evolution of the interface and its local surfactant concentration. The system is nonlocal and arises by appropri- ately matching solutions of the linearised Navier–Stokes equations in the thicker layer to the solution in the thin layer. The scaled models are used to study different physical mechanisms by varying the Reynolds number, the viscosity ratio between the two layers, the total amount of surfactant present initially and a scaled P ́eclet number measuring diffusion of surfactant along the interface. The linear stability of the underlying flow to two– and three–dimensional disturbances is investigated and a Squire’s type theorem is found to hold when inertia is absent. When inertia is present, three–dimensional distur- bances can be more unstable than two–dimensional ones and so Squire’s theorem does not hold. The linear instabilities are followed into the nonlinear regime by solving the evo- lution equations numerically; this is achieved by implementing highly accurate linearly implicit schemes in time with spectral discretisations in space. Numerical experiments for finite Reynolds numbers indicate that for two–dimensional flows the solutions are mostly nonlinear travelling waves of permanent form, even though these can lose stabil- ity via Hopf bifurcations to time–periodic travelling waves. As the length of the system (that is the wavelength of periodic waves) increases, the dynamics become more complex and include time–periodic, quasi–periodic as well as chaotic fluctuations. It is also found that one–dimensional interfacial travelling waves of permanent form can become unstable to spanwise perturbations for a wide range of parameters, producing three–dimensional flows with interfacial profiles that are two–dimensional and travel in the direction of the underlying shear. Nonlinear flows are also computed for parameters which predict linear instability to three–dimensional disturbances but not two–dimensional ones. These are found to have a one–dimensional interface in a rotated frame with respect to the direction of the underlying shear and travel obliquely without changing form.