Flapping Flags

Image courtesy of Jun Zhang, NYU

A silk thread flapping in the "breeze" of a flowing soap film. Image courtesy of Jun Zhang, NYU

The symbolic beauty of a flag flying high in the wind is simple to understand. The physics behind that beauty is not. The flapping flag is an everyday example of the complicated interaction between a flexible object and the fluid flowing around it. This interaction is not yet fully understood, but a group of physicists in New York City recently studied the simplified system of a silk thread (the flag) in a flowing soap film (the breeze). Among their results is the surprising observation that the motion of the "flag" depends on the length of the flag itself. Below a critical length, the silk thread remains in a stretched-out state. The flapping only begins when the flag exceeds this length. This simple observation could be crucial to the understanding this complex--yet beautiful--phenomenon.