Intense orange Na* emission was observed in different spatial locations from blue emission during multibubble sonoluminescence in sulfuric acid. The color change from blue to orange observed along the streamer in the filamentous structure of a bubble cloud. By stroboscopic observation, the Na* emission seemed to occur when a large bubble ejected tiny bubbles toward a pressure node after bubble coalescence around a pressure antinode. Comparing a high resolution Na* spectrum of sulfuric acid with water, the full width at half maximum of the spectra were almost the same, where the estimations of the temperature and pressure inside the bubbles were 3300 K and 390 atm. The intensity of Na* emission in sulfuric acid increased at lower frequency in contrast to the water case.
Sonoluminescence (SL) is light emission under high-temperature and high-pressure conditions of a cavitating bubble under intense ultrasound in liquid. In this review, the fundamentals of the interactions between the sound field and the bubble, and between bubbles are explained. Experimental results on high-speed shadowgraphy of bubble dynamics and multibubble SL are shown, demonstrating that the SL intensity is closely related to the bubble dynamics. SL studies of alkalimetal atom (Na and K) emission are summarized. The spectral measurements in solutions with different noble-gas dissolutions and in surfactant solutions, and the results of spatiotemporal separation of SL distribution strongly suggested that the site of alkali-metal atom emission is the gas phase inside bubbles. The spectral studies indicated that alkali-metal atom lines are composed of two kinds of lines: a component that is broadened and shifted from the original D lines arises from van der Waals molecules formed between alkali-metal atoms and noble-gas atoms under extreme conditions at bubble collapse. The other spectral component exhibiting no broadening and no shift was suggested to originate from higher temperature bubbles than those producing the broadened component.
To understand behavior of surface waves on sols and gels, the dispersion relation of surface modes on isotropic materials with both surface tension and elasticity is studied. The dispersion relation consists of two separate branches: one corresponds to the truly localized Rayleigh-type modes which can exist only at low frequencies, while the other, which is connected continuously to the capillary modes, gives pseudo-surface waves that are damped, radiating elastic transverse waves into the bulk of the medium just like the ones known in surface waves on elastically anisotropic materials. The result is compared with recent experiments on sols and gels.
A new light scattering system was developed to investigate the propagation of liquid surface waves over a wide wave-number range 4 x 1 O'-3 x 10" m-' (or 10 kHz-6 MHz in frequency). Thermal fluctuation of lliquid surface displacement excites a surface tension wave called a "ripplon" whose behavior yields information on the dynamics of the liquid surface. The transmission diffraction method and some optical and electrical improvements are successfully applied to the light scattering system and the upper limit of the frequency measured was extended about two orders higher than the conventional one. The wave-number dependence of the ripplon frequency and damping constant was measured for several kinds of liquids and the results agreed well with the classical hydrodynamic theory,
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