Surfactant effects on waves in gravitational liquid sheets

An experimental investigation is performed to study the main features of stationary waves on thin sheets of liquid falling freely in still air under the effects of surface-active agents. Sinuous and varicose waves are produced by an obstacle, a thin rod, placed normally through the sheet. It is shown, in agreement with theoretical results, that the current acceleration along the vertical direction causes both the wavelength and the amplitude to decrease. The angle between the tangent to the curve of constant phase of sinuous waves at the obstacle and the vertical, predicted by theory and well recovered experimentally for pure water, is found to be parctically unchanged when surfactants are added to the water. On the contrary, the presence of surfactants gives rise to waves attenuation due to surface dissipation.