Scientists Bubble with Excitement

Kathleen G. Tatterson

If you've ever looked deep into a glass of champagne or beer you've probably noticed the strings of bubbles floating around in the liquid and have swallowed them without any great harm. But would you keep drinking if you knew that bubbles can start to glow when set into expanding and contracting motion, a phenomenon called sonoluminescence?
A group of researchers at Göttingen University has set out to study this occurrence with bubbles made with nothing but light. They use a Q-switched Nd:YAG laser from the former JK Laser, now Lumonics -- a laser that already served to prove the existence of stimulated recombination of highly charged ions and electrons in a storage ring.
The traditional way of investigating bubbles is by sound instead of light. "There is a limit of [sound wave] energy you can put into a liquid before it breaks up," explained Werner Lauterborn, professor of physics at the university. "That's called acoustic cavitation."
Not so with the bubbles made by light. The Nd:YAG device delivers 8-ns pulses at 1064 nm through an aberration-minimized focusing lens into an 85 3 85 3 75-mm cuvette filled with distilled water. A gas- and vapor-filled bubble forms at the focus of the laser pulse when it reaches the dielectric breakdown threshold, forming a rapidly expanding plasma, a shock wave, a bright white light flash and, finally, a bubble. Bubble size can range from 0.8 to 1.5 mm in radius. When the bubble pops (collapses) after having made an excursion to a larger radius, it dissipates energy through emission of acoustic transients and emits a short light flash, called bubble luminescence. A Photometrics 1317 3 1013 pixel-intensified charge-coupled device camera captures the light emission. The team has just begun to reap the benefits of the technique.
Although the team was mum about possible commercial applications, one could imagine Whammo buying the technology for the next generation of high-tech bubble wands.