Instability of an interface, with large viscosity-contrast, under tangentially oscillatory motion

We shall present here an instability which initiates a pattern formation on the interface between two viscous fluids, with very strong viscosity contrast, subjected to tangential oscillatory motion at a moderate frequency. We carried out experiments and theoretical studies. Experimental results showed that the first selected wavelength, which is far from the capillary one, is controlled by the oscillation amplitude. A quasi static model is elaborated, predicting the instability threshold and the wavelength dependence on the amplitude, in agreement with the experiments. A detailed analysis reveals that the origin of the instability is not a simple Kelvin Helmholtz type, because the pressure perturbation distribution is different. This difference lies in the contribution of the streaming effect.