FUNDAMENTALS OF A LIQUID (SOAP) FILM TUNNEL

The continuously running liquid film tunnel (LFT) is a novel device su itable fur the study of two-dimensional hows. 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 liq uid film. However, despite the simple design and widespread applicatio n of LFT, its working mechanisms are not well understood. In the prese nt work, an experimental effort for explaining these mechanisms is rep orted. The results show that both film velocities and film flow rates increase with wafer sheet velocity up to a saturation level. This beha vior is described via a force balance between the shear force produced by the water sheet and the opposing pulling force of reservoir and bo undary layer frictions. The results also show that the average film th ickness depends on the surfactant concentration This is as predicted b y a model based on Langmuir's adsorption theory, in which the liquid f ilm contains two external monolayers of surfactant and a slab of surfa ctant solution in between, When a Nm is drawn from the reservoir to th e water sheet, the surfactant molecules start migrating from the forme r to the latter. To restore the thermodynamic equilibrium, the dragged him pulls more surfactant due to Marangoni elasticity, and thus a flo w is established. The film flow soon reaches an equilibrium rate as re quired by the force balance mentioned above.