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Temperature inside collapsing
bubble four times that of sun
Kloeppel, Physical Sciences Editor
photo to enlarge
by D. Flannigan and K. S. Suslick.
cinematographic sequence of photos of the growth and
implosive collapse of a single bubble (shown in blue)
in sulfuric acid irradiated with high intensity ultrasound.
The images are shown in false color.
Ill. — Using a technique employed by astronomers to determine
stellar surface temperatures, chemists at the University of Illinois
at Urbana-Champaign have measured the temperature inside a single, acoustically
driven collapsing bubble.
Their results seem out of this world.
“When bubbles in a liquid get compressed, the insides get hot
– very hot,” said Ken Suslick, the Marvin T. Schmidt Professor
of Chemistry at Illinois
and a researcher at the Beckman
Institute for Advanced Science and Technology. “Nobody has
been able to measure the temperature inside a single collapsing bubble
before. The temperature we measured – about 20,000 degrees Kelvin
– is four times hotter than the surface of our sun.”
This result, reported in the March 3 issue of the journal Nature by
Suslick and graduate student David Flannigan, already has raised eyebrows.
Their work is funded by The National Science Foundation and the Defense
Advanced Research Projects Agency.
Sonoluminescence arises from acoustic cavitation – the formation,
growth and implosion of small gas bubbles in a liquid blasted with sound
waves above 18,000 cycles per second. The collapse of these bubbles
generates intense local heating. By looking at the spectra of light
emitted from these hot spots, scientists can determine the temperature
in the same manner that astronomers measure the temperatures of stars.
By substituting concentrated sulfuric acid for the water used in previous
measurements, Suslick and Flannigan boosted the brilliance of the spectra
nearly 3,000 times. The bubble can be seen glowing even in a brightly
lit room. This allowed the researchers to measure the otherwise faint
emission from a single bubble.
“It is not surprising that the temperature within a single bubble
exceeds that found within a bubble trapped in a cloud,” Suslick
said. “In a cloud, the bubbles interact, so the collapse isn’t
as efficient as in an isolated bubble.”
What is surprising, however, is the extremely high temperature the scientists
measured. “At 20,000 degrees Kelvin, this emission originates
from the plasma formed by collisions of atoms and molecules with high-energy
particles,” Suslick said. “And, just as you can’t
see inside a star, we’re only seeing emission from the surface
of the optically opaque plasma.” Plasmas are the ionized gases
formed only at truly high energies.
The core of the collapsing bubble must be even hotter than the surface.
In fact, the extreme conditions present during single-bubble compression
have been predicted by others to produce neutrons from inertial confinement
“We used to talk about the bubble forming a hot spot in an otherwise
cold liquid,” Suslick said. “What we know now is that inside
the bubble there is an even hotter spot, and outside of that core we
are seeing emission from a plasma.”
Editor’s note: To reach Ken Suslick, call 217-333-2794; e-mail: firstname.lastname@example.org.