Washington DC
BUBBLES may not be the magic key to cheap energy from nuclear fusion after
all. A researcher in the US has come up with a new theory to explain how sound
waves in a fluid containing bubbles can make the bubbles produce light鈥攐r
鈥渟onoluminescence鈥濃�攁nd also suggests that the process would not be a
practical source of energy.
Sonoluminescence was discovered in 1934, but researchers have not agreed what
causes it. One of the leading theories is that when a sound wave strikes a
bubble, the bubble implodes. This creates shock waves that converge at the
bubble鈥檚 centre, then explode outward, releasing light and restoring the bubble
to its former condition. If that were true, the process could create
temperatures of more than 1 million 掳C for a split second. Some fusion
enthusiasts believe that this heat could be harnessed to trigger nuclear fusion
reactions.
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But Andrea Prosperetti of Johns Hopkins University in Baltimore proposes a
new explanation of sonoluminescence in the latest issue of the Journal of
the Acoustical Society of America (vol 101, p 2003). He argues that the
bubble would not remain spherical as it pulsated. Rather, the part of the bubble
that was struck directly by the sound wave would implode fastest, creating a
finger-like 鈥渏et鈥� that would pierce the opposite side of the bubble.
The jet would be moving at such a high speed鈥攑erhaps 6500 kilometres
per second鈥攖hat its impact would split the bubble sharply like a crystal
rather than force the water molecules to simply flow apart, Prosperetti says.
The bubbles would then emit light in the same way that some solid materials,
such as ice, emit light when fractured.
快猫短视频s don鈥檛 understand fractoluminescence, as this phenomenon is called,
any more than they do sonoluminescence. Prosperetti says that one explanation
may be that fracturing may heat some of the crystal material, which would then
emit some of the energy as light. But regardless of the mechanism that produces
fractoluminescence, Prosperetti says that splitting bubbles would create
temperatures lower than 6000 掳C, far too low to be of any use for
fusion.
鈥淎ndrea is one of the brightest fluid mechanics experts in the world, yet I
don鈥檛 think this is the right mechanism,鈥� says Lawrence Crum of the University
of Washington, one of the pioneers of sonoluminescence research. 鈥淚t鈥檚 too
complex.鈥� But Prosperetti says that the conventional idea is even harder to
believe. 鈥淚 am a bubble man,鈥� he says. 鈥淢y scientific guts refuse to believe
颈迟.鈥�