Measurements of air drag on two-dimensional soap-film flow

The gravity-driven soap film is a well-established experimental setup for investigating two-dimensional (2D) flows. As with any 2D geophysical flow, the soap film interacts with the 3D environment in which it is embedded. Even in numerical simulations of 2D turbulence, this interaction is modeled by adding a drag term to the governing equations, which also serves as an energy sink at large scales. In a soap-film flow, the interaction with the surrounding air engenders drag, which has never before been directly measured. We measure the movement of the air around the film by seeding the air with atomized droplets, whose motion we track using Particle Tracking Velocimetry (PTV). This yields the velocity profile in the air, with which we compute the drag on the soap film. Besides addressing a fundamental question in laboratory 2D flows, our work also has implications for understanding the interaction between the atmosphere and the ocean surface currents.