By Peter Weiss
For more than 120 years, the English physicist Lord Rayleigh has had the last word on why flags flap in the breeze. A keen observer who figured out how the scattering of light makes the sky blue, Rayleigh attributed flag flutter to the interplay between deformations of a flag’s surface and subtle gusting of the wind. Because the tiniest ripples in the flag and puffs of wind end up amplifying each other, even a smooth, or so-called laminar, wind inevitably causes the larger motions of flags, he reasoned.
Now, an experiment involving a thread buffeted by a flowing film of soapy water suggests that the time-honored view was wrong. Furthermore, the new research may provide fluid-dynamics specialists with a new model for studying questions as disparate as how blood flows in vessels and how insectlike fluttering might best propel aerial microrobots.