Physicists in the US claim to have broken the record for the brightness of light generated by “sonoluminescence”, the imploding of a bubble when it is blasted with sound waves. With a peak power of 100 W, the light is 100 times as bright as seen in previous sonoluminescence experiments, and may help scientists understand how the strange phenomenon works.

Sonoluminesence was discovered in the first half of the 20th century but it was only in the 1990s that physicists began to investigate the phenomenon seriously. Although no-one is sure how it works, the basic idea is that sound waves are fed into a vessel containing one or more bubbles inside a liquid. The sound causes the bubbles to expand momentarily before water pressure takes over, imploding the bubbles in bursts of heat and light.

In many sonoluminescence experiments, the power of the generated light flash is just a few milliwatts. However, in 2004 Alan Walton and other physicists at the University of Cambridge subjected bubbles in a liquid column to vertical vibrations and produced flashes of light that peaked at 1 W. But now, a group led by Seth Putterman at the University of California, Los Angeles, has devised a new variation on the method to break that record and generate light that is 100 times as bright.

Shocking technique

In the California group’s experiment, the researchers fill a steel cylinder with phosphoric acid and position it almost a centimetre above a steel base. Using a needle at the bottom they inject a 1 mm-sized bubble of xenon into the tube and let it float towards the top. When the bubble reaches a height of 11 cm, the researchers let the cylinder drop and the resultant shock collapses the bubble in a brief flash. Analysis shows that this flash has a peak intensity of 100 W and a temperature of 10,200 K.

According to Putterman, this “one shot” method is more controllable than previous methods and should therefore offer a way of producing even higher-temperature and power-bubble collapses. It might also be a route to understanding sonoluminescence.

“Why does a diffuse sound field focus its energy density by such large factors to create sonoluminescence? In some set-ups this factor can reach one trillion,” asks Putterman, who believes that nonlinear processes are responsible. “We want to learn about these nonlinear processes and see if they can be generalized to other cases.”

No signs of fusion

In the past, sonoluminescence has proved a controversial subject. In 2002 Rusi Taleyarkhan, then at the Oak Ridge National Laboratory in Tennessee, US, and colleagues claimed to find evidence for nuclear fusion occurring alongside sonoluminescence. Although a few other research groups have since made similar claims, most nuclear scientists believe them to be misguided. Taleyarkhan has since moved to Purdue University, where in 2008 he was reprimanded by the university for “research misconduct” related to a paper on fusion.

Yet Putterman does not rule out the possibility of so-called bubble fusion. “This experiment is an important step in upscaling sonoluminescence with controlled bubble contents, but it has not yet yielded any sign of fusion,” he says. “The interior of the bubble would need to reach solid densities and temperatures greater than 10 million kelvin.”

Walton at the Cavendish Lab thinks that there is “little real prospect” for fusion in Putterman and colleagues’ experiment, but he does praise the general advance. “It will undoubtedly be of real use in making fundamental studies of the nature of sonoluminescence,” he says.

The research will be published soon in Physical Review E.