Citation of our works in a Book, “Sonoluminescence”
4.6.2 The hot spot theory – Bremsstrahlung (p. 156)
Kwak’s group at Chung-Ang University, Seoul, Korea, have made a careful study of bremsstrahlung as the mechanism for the emission of SL (Kwak and Na (1996, 1997), Jeon et al. (2000)). They argue that in the ultra high temperature just before bubble collapse, the gas molecules may dissociate and/or ionize, so that elastic collisions can occur between ions and electrons. These collisions produce light. They conclude that the emission mechanism is the same as black body radiation with finite absorption which confirms that the principal mechanism of SBSL is bremsstrahlung with slight black body emission. The spectrum was measured (Jeon et al. (2000)). Experimental and calculated values yield common spectral behavior in the visible region.
4.7.5 Kwak and Yang (pp. 165-167)
Kwak and Yang (1995) solved the conservation equations analytically to obtain the density, pressure and temperature distribution of the gas in the bubble. The heat transfer in the thermal boundary layer adjacent to the bubble wall was treated by solving the liquid energy equation in this layer. And the time dependent radius and wall velocity of the oscillating bubble under sound was found by using the equation derived from the Keller and Miksis (1980) formulation. Lee et al. (1997) continued the above treatment using the Kirkwood-Bethe hypothesis for the outgoing wave from the SL collapse. The rise time and the magnitude of the shock pulse were in good agreement with the observed values, and may provide an approximate value of the gas pressure near the collapse point of the SL bubble. However, in 1999, Kwak et al. took up the fundamental question of whether a shock wave or a pressure wave is formed inside a SL bubble. They found that a pressure wave rather than a shock wave is launched into the bubble center just prior to the bubble collapse and that the wave is reflected near the center. The temperature rise associated with the pressure wave developed inside the bubble turned out to be insufficient for emitting light.
4.9 THE QUANTUM RADIATION THEORY (p. 178)
Quantum theory has been invoked to explain sonoluminescence by Schwinger (1992a,b, 1993a,b,c,d, 1994), Eberlein (1992a,b, 1993, 1996a,b,c), Barton (1999) and Barton and Eberlein (1993). Schwinger used the Casimir (1948) effect which says that parallel conducting plates attract each other due to zero-point fluctuations in the fields. In particular, Eberlein (199ba,b) argued that SL is closely related to the Unruh effect, the dynamic generalization of the Casimir effect. However, Kwak and Na (1997) point out that the Unruh temperature TUnruh=ћa/(2πkc), where a is the acceleration, k is Boltzmann’s constant and c is the velocity of light, is close to absolute zero with the maximum achievable bubble wall acceleration, a, of 1013 m/s2 (Hiller et al. (1992), Kwak and Na (1996)). Thus the radiation due to the Casimir effect is nothing but a quantum fluctuation at zero temperature.
4.15 KWAK’S CONTRIBUTION (pp. 190-191)
Kwak et al. (2001) point out that the light emission of SBSL occurs during the 0.5 ns before collapse (Kwak and Na (1996)). Hence the bubble dynamics theory in the last nanosecond before collapse is crucial to understanding SBSL. Since the Rayleigh-Plesset equation breaks down if the speed of the bubble wall is greater than the sound speed in the liquid, a proper numerical procedure for the bubble wall motion at the moment of collapse is needed. Kwak et al. (2001) overcame this problem by using two time scales, one for the driving force and one for the bubble motion. Radiustime curves are calculated for various parameters and one of these is shown in Fig. 4.25, which is for two air bubbles of equilibrium radii R0=4.5 μm and R0=6.5 μm with PA=1.35 atm and f=26.5 kHz. This nine-page paper is well worth studying