High-speed imaging unveils fluid mysteries

Compiled by Photonics Spectra staff

Tibetan singing bowls have been used in religious ceremonies to produce rich, beautiful sounds. Now physicists are filling them with water and using them to quantify how droplets are propelled from the water’s surface as the bowls are excited.

Generally composed from a bronze alloy containing nickel, tin, copper, zinc, iron, silver and gold, Tibetan singing bowls are a type of standing bell. They produce complex sounds when their rims are struck or rubbed with a leather-wrapped or wooden mallet – much like the sound that floats out of a wine glass when the edge is rubbed, and which inspired Benjamin Franklin to invent the glass harmonica. A Tibetan singing bowl is easier to excite than a wine glass because its resonant frequency is much smaller.

The singing properties of the bowl were used by scientists at Université de Liège and MIT to investigate how liquids interact with solid materials – a situation that arises in many engineering applications, including wind-loading of buildings and bridges.

When the fluid-filled Tibetan bowl is rubbed, the slight changes in its shape disturb the surface at the water’s edge, generating waves. When these changes are sufficiently large, the waves break, causing droplets of water to be ejected. These new findings could benefit processes where droplet generation plays a significant role, such as fuel injection and perfume sprays.

To quantify how the droplets were formed, ejected, accelerated and bounced on the surface of the fluid in the bowl, the researchers used a Vision Research Phantom high-speed video camera. They measured the droplets from the images captured with the camera. To generate the droplets, they set up a loudspeaker adjacent to the bowl, which then emitted sound at specific frequencies. Once the sound hit the resonant frequency of the bowl, it generated waves, allowing the scientists to capture images of ejected droplets.

The study was published in the July 1 issue of Nonlinearity (doi: 10.1088/0951-7715/24/8/R01).