The effect of soluble surfactants on liquid film flow

We investigate experimentally the modifications in the dynamics of liquid film flow, resulting from the addition in water of the soluble surfactants iso-propanol (IP) and Sodium Dodecyl Sulfate (SDS). Recent experiments indicate that, the primary instability of film flow in channels of finite width depends on surface tension. Using IP solutions of varying concentrations, we have shown that this dependence scales with Kapitza number. Based on the high solubility and diffusivity of iso-propanol in water, we argue that these solutions behave as pure liquids with reduced surface tension. Indeed, low-frequency inlet disturbances turn in the unstable regime into solitary humps preceded by capillary ripples, with the scaling predicted by theory for simple liquids. Aqueous solutions of SDS exhibit a remarkably different behavior, with more pronounced feature of strong damping of all inlet disturbances. The dominant structures for the entire range of inlet frequencies tested, even at surprisingly high Re, are sinusoidal traveling waves of very small amplitude.