Subject Categories: Physics
Published online 15 October 2008
Gas bubbles in liquids can emit bursts of light called sonoluminescence when they are 'popped' by ultrasound. Weizhong Chen and co-workers at Nanjing University have developed a new way of monitoring the wavelengths emitted in sonoluminescence, revealing the extreme conditions inside the bubble.
During sonoluminescence, the gas inside a collapsing bubble reaches pressures and temperatures so high that it ionizes to produce light. This has led to speculation that sonoluminescence could be used to achieve thermonuclear fusion, but the process is still poorly understood.
Chen and co-workers directed ultrasound onto bubbles of krypton gas dissolved in sulphuric acid. The emitted light was recorded on a spectrograph, then delivered to a streak camera, which deflects light to trace wavelengths over time. The streak images showed that the wavelength decreased from infrared to ultraviolet during the sonoluminescence pulse, which lasted around nine nanoseconds.
According to the laws of black-body radiation, the wavelength trend corresponds to an exponential increase in temperatures as high as 100,000 kelvins. However, this does not take into account the high pressures in the bubble, which could produce much higher peak temperatures on the scale of millions of kelvins.
The researchers were unable to record images fast enough to observe the cooling process. It remains a mystery exactly how the bubbles are able to cool down so rapidly.