Page 42 - Summer 2006

The newsletter of
The Acoustical Society of America
Volume 16, Number 3 Summer 2006
Role of Acoustics in Energy Focusing Phenomena
Seth J. Putterman
Physics Department, University of California, Los Angeles Los Angeles, California 90024
Continued on page 42
Fluids and solids that are driven off equilibrium do not relax smoothly to equilibrium. Although the march to equilibrium is characterized by an ever increasing entropy, continuous media can nevertheless pass through configurations which display a wide range of phenomena which concentrate the energy density.
Sonoluminescence (SL) is the paradigm of energy
1
focusing phenomena. Here, a standing sound wave causes
a trapped bubble to pulsate so violently [acceleration of the bubble wall can reach 1012g] that acoustic energy is trans- duced into flashes of ultraviolet light whose duration can be much shorter than a nanosecond. When water is used as the fluid, SL can be observed at sound fields with an amplitude of about 1 atmosphere. At this amplitude each water molecule is vibrating with an energy of about 1.5x10- 23erg, whereas a photon, which we assume originates from the region of a single atom, has been observed to have an energy as high as 6 eV. A comparison of these numbers indicates that SL involves a concentration of the energy
11
density by at least a factor of 4x10 . The SL spectrum
might extend to even higher energies but observations have been limited by the extinction coefficient for light traveling through water.
When a 30-kHz sound wave acts on a bubble contain- ing helium atoms the observed spectrum is accurately fit by Planck’s blackbody law2 with a temperature of 20,000 K, four times hotter than the surface of the sun. A condition for blackbody radiation is that the size of the hot spot must be greater than the photon matter interaction distance. But this SL hot spot has a radius that is smaller than 1/2 micron and it is difficult to understand how it can be opaque. If the hot spot is smaller than the interaction length it would be transparent and the spectrum would look like Bremsstrahlung radiation from a plasma. A plasma forms because the contents of the imploding bubble get so hot
that the atoms ionize. The free electrons will then zip around with a velocity determined by the temperature of the plasma. When the electrons collide with much heavier ions they accelerate and radiate light [Bremsstrahlung]. If the light escapes without absorption the plasma is trans- parent, if it is absorbed the plasma is a blackbody. For the alternative model of the hot spot, the transparent plasma scenario, the best fit [helium] spectrum would have a tem- perature of 50,000 K [as compared to 20,000 K for the blackbody model which displays a better overall fit to the data]. At acoustic frequencies of 1MHz, the bubble is smaller and the observed spectrum is now best fit by the transparent plasma model, and a temperature of about one
3
vided by application of plasma diagnostics to argon bub-
4
bles in sulfuric acid. At drive levels so low that these bub-
bles are dimmer than bubbles in water, lines from excited states of argon ion [Ar+] that are 37eV above the ground state can be observed. The width of emission from neutral argon lines yields an average pressure of 1,500 atm, which also matches the average pressure determined by light scattering measurements of the radius of these bubbles. At higher sound fields the lines are broadened and unresolv- able but light scattering yields pressures over 3,500 atm suggesting that the high drive bubbles are substantially hotter than the weakly driven case where the Ar+ lines were first discovered.
The densities and temperatures which can be accessed with modest sound fields are extraordinary. One is tempt- ed to wonder if bubble acoustics can be used to reach con- ditions for nuclear fusion. Although this is an exciting and worthwhile direction of research claims of such a success5
6
have met with strong skepticism. Some optimism that
million degrees.
Evidence that SL originates in a plasma has been pro-
40 Acoustics Today, July 2006