From approved patent 7699988
A sonoluminescence effect is observed due to acoustic cavitation as these ultrasonic waves propagate in the flow back water and catch the micro bubbles in the valley of the wave. Sonoluminescence occurs whenever a sound wave of sufficient intensity induces a gaseous cavity within a liquid to quickly collapse. This cavity may take the form of a pre-existing bubble, or may be generated through hydrodynamic and acoustic cavitation. Sonoluminescence can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. A standing acoustic wave is set up within a liquid by four acoustic transducers and the bubble will sit at a pressure anti node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained. The light flashes from the bubbles are extremely short, between 35 and few hundred picoseconds long, with peak intensities of the order of 1-10 mW. The bubbles are very small when they emit light, about 1 micrometer in diameter depending on the ambient fluid, such as water, and the gas content of the bubble. Single bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them. However, the stability analysis of the bubble shows that the bubble itself undergoes significant geometric instabilities, due to, for example, the Bjerknes forces and the Rayleigh-Taylor instabilities. The wavelength of emitted light is very short; the spectrum can reach into the ultraviolet. Light of shorter wavelength has higher energy, and the measured spectrum of emitted light seems to indicate a temperature in the bubble of at least 20,000 Kelvin, up to a possible temperature in excess of one mega Kelvin. The veracity of these estimates is hindered by the fact that water, for example, absorbs nearly all wavelengths below 200 nm. This has led to differing estimates on the temperature in the bubble, since they are extrapolated from the emission spectra taken during collapse, or estimated using a modified Rayleigh-Plesset equation. During bubble collapse, the inertia of the surrounding water causes high speed and high pressure, reaching around 10,000 K in the interior of the bubble, causing ionization of a small fraction of the noble gas present. The amount ionized is small enough for the bubble to remain transparent, allowing volume emission; surface emission would produce more intense light of longer duration, dependent on wavelength, contradicting experimental results. Electrons from ionized atoms interact mainly with neutral atoms causing thermal bremsstrahlung radiation. As the ultrasonic waves hit a low energy trough, the pressure drops, allowing electrons to recombine with atoms, and light emission to cease due to this lack of free electrons. This makes for a 160 picosecond light pulse for argon, as even a small drop in temperature causes a large drop in ionization, due to the large ionization energy relative to the photon energy.
