Lord Rayleigh discovered in 1917 that sound waves from a ship’s propeller can cause tiny gas bubbles in the water to explode and erode the propeller. The process is called cavitation, and today the same scrubbing bubbles are used for everything from cleaning semiconductors to carving the fat out of a human body. Now, physicists have taken the highest resolution photographs ever made of the destructive shock wave leaving the surface of a collapsing bubble. Reported in the 7 February PRL and the February issue of Physical Review E, the new images confirm previous doubts about theoretical estimates of the bubble temperature and move physicists one step closer to answering a long-standing question: Can nuclear fusion occur in a water bubble?
As sound waves ripple through water, the accompanying pressure oscillations stretch and compress gas bubbles. If the vibrations are violent enough, the viscous bubbles can’t keep up with the rapidly changing wave pressure. Contrary to their usual behavior, the bubbles begin to collapse when the water pressure is small and expand when the pressure is high. This arrangement is catastrophically unstable. “Inside the expanding bubble you have a near vacuum,” explains Seth Putterman of the University of California in Los Angeles, “and the positive outside pressure crushes the bubble.”
The gas in the collapsing bubble can become extraordinarily hot. Using current theories, researchers have set a lower limit of about 25,000 degrees Kelvin for the internal temperature of the completely collapsed bubble, although Putterman says it could be much higher. Some physicists have even suggested that the bubble temperature could reach the 15 million degrees needed for hydrogen atoms in the gas to fuse together by the same process that powers the sun. But these temperature estimates depend upon theories that are not well tested. The collapse happens in about 100 picoseconds, much too fast for most high-speed cameras.
Now, groups led by Putterman and by Rainer Pecha of the University of Stuttgart in Germany have turned the eye of a streak camera on individual bubbles levitated in water by a standing sound wave oscillating at 20 kHz . A faint flash of ultraviolet light from the crushed bubble accompanies the outgoing shock wave–a process called sonoluminescence–and the researchers used the flash to trigger the camera, which blinks once every 400 picoseconds and produces a time-lapse photo of the expanding shock wave. Contrary to the assumptions of current theories of sonoluminescence, both teams confirmed earlier lower resolution experiments indicating that the supersonic collapse of the bubble walls launches a shock wave at four times the speed of sound. Although it is not clear how this will change temperature estimates, says Putterman, none of the subsonic collapse theories can possibly be correct.
“This is really nice work,” says Larry Crum of the University of Washington in Seattle. But Crum doubts that these bubbles are hot enough to fuse hydrogen. “I think we will have to goose the bubbles to get fusion,” he says. Putterman is already designing an experiment to find out for sure. The new experiment will directly measure the internal temperature of the bubble by tracking the thermal motion of electrons.