Boundary-layer flow of air induced by a falling soap film*

In a soap-film channel, the soap film is embedded in the surrounding air. The falling film drags the air, inducing it to flow in a thin layer adjacent to the film. This flowing air, in turn, resists the motion of the falling film; thus, the film-air interface is a dynamic boundary. We measure the airflow velocity profile in this interface's boundary layer using super-resolution Particle Image Velocimetry. We find that the downstream evolution of the air velocity profile manifests self-similarity, which we analyze using the framework of the boundary-layer theory. Beyond air-film interaction, our findings may bear on a broader class of flows over dynamic boundaries, e.g. ocean-air interaction.