Fundamentals of a liquid (soap) film tunnel

The continuously running liquid film tunnel (LFT) is a novel device suitable for the study of two-dimensional flows. In this innovation, the films start from a reservoir, run over a horizontal or non-horizontal wire frame and get pulled/washed by a water sheet or by gravity of liquid film. How-ever, despite the simple design and widespread application of LFT, its working mechanisms are not well understood. In the present work, an experimental effort for explaining these mechanisms is reported. The results show that both film velocities and film flow rates increase with water sheet velocity up to a saturation level. This behavior is described via a force balance between the shear force produced by the water sheet and the opposing pulling force of reservoir and boundary layer frictions. The results also show that the average film thickness depends on the surfactant concentration. This is as predicted by a model based on Langmuir’s adsorption theory, in which the liquid film contains two external monolayers of surfactant and a slab of surfactant solution in between. When a film is drawn from the reservoir to the water sheet, the surfactant molecules start migrating from the former to the latter. To restore the thermodynamic equilibrium, the dragged film pulls more surfactant due to Marangoni elasticity, and thus a flow is established. The film flow soon reaches an equilibrium rate as required by the force balance mentioned above

Visualization of Two-dimensional Flows by a Liquid (Soap) Film Tunnel

Experimentally produced two-dimensional flows have become possible in recent years due to the invention of Liquid Film Tunnel (LFT) in 1987 by Gharib and Derango. This simple, inexpensive, yet powerful device, which we have improved extensively over the last decade, can generate a variety of flows. Liquid (soap) films can be visualized through light interference effects produced by small variations in the film thickness. Flow-disturbing objects such as cylinders, wedges, and air bubbles create these variations. Monochromatic visualization of these thickness variations will render phenomenally accurate graphic information about the flow patterns thus produced. Under a polychromatic light, these interference effects can be spectacular, due to reflection of different colors by different isothickness regions