It bothered Jun Zhang, a physicist at New York University, that nobody could answer a question a child might ask at a parade: Why do flags flap in the wind? "Some people had believed that a flagpole causes the air to tumble, and the tumbling drives the flapping," he says, but nobody knew for sure. So he and his colleagues placed a silk thread in a dish of watered-down detergent and let the liquid flow through a channel, creating a two-dimensional model of a flag in the wind.
At slow flow rates, the thread extended straight downstream with nary a wiggle. In a faster current, the thread began to undulate in a regular pattern (right). "We've shown that a flag doesn't need a pole to flap. The flapping is caused only by the interaction of the flag with the air," says Zhang. The source of the motion actually lies in ...
