July 1, 2011 | 3
What do instruments used in religious ceremonies since the fifth century have to do with modern physics? When those instruments can create liquid fountains, wave patterns, and flying droplets—quite a lot.
For centuries, Tibetan singing bowls have produced sound to aid meditation. The meditator can either strike the bowl, usually made of a bronze alloy, to create a ringing tone or rub a mallet around the bowl’s rim to produce a continuous humming noise.
Rubbing the rim creates sound for the same reason that running a damp finger around the edge of a wine glass makes a high-pitched note: the motion of mallet or finger incites vibration in the instrument, producing a sound wave.
The reverse is also true: playing a note into a Tibetan singing bowl will stimulate vibration at the same frequency as the sound wave, a phenomenon that we can actually see when the bowl is filled with water. The vibration of the bowl pushes at the water molecules in direct contact with the sides of the bowl, which in turn jostle their neighbors, creating waves in the fluid. (This effect also occurs in Chinese singing bowls, broad-brimmed metal bowls with two handles, except that instead of rubbing the edge to stimulate vibrations, you dampen your hands and then rub the handles.)
At certain "resonant" frequencies, the bowl vibrates in fundamental patterns, or "modes," which creates interesting wave designs. For example, in this mode, the waves spread across the surface of the liquid as if they were emanating from the sides of a square rather than a circle.
When the bowl is vibrating hard enough, the waves are so energetic that the waves’ motion becomes chaotic, and droplets break free from the bulk of the water pool, flying out of the bowl. Researchers Denis Terwagne of the Belgian Université de Liège and John Bush of the Massachusetts Institute of Technology recorded the waves with a high-speed camera in order to investigate the fluid dynamics that led to flying droplets. They found that they could even make droplets skip across the surface of the fluid or bounce up and down like balls on a trampoline.
Terwagne and Bush compare the size of the droplets to those created when a liquid surface is shaken up and down. But how do you shake the surface of a liquid? In Terwagne’s previous research, published in pre-print blog arXiv.org in 2010, he dunked various shapes of diving bells into silicon oil to create controlled liquid surfaces, then jiggled the diving bells up and down to shake those surfaces. This work resulted in cascades of drops, seen in the following video.