Sonoluminiscencia

Páginas: 12 (2919 palabras) Publicado: 19 de octubre de 2011
VOLUME 84, NUMBER 4

PHYSICAL REVIEW LETTERS

24 JANUARY 2000

Effect of Noble Gases on Sonoluminescence Temperatures during Multibubble Cavitation
Yuri T. Didenko, William B. McNamara III, and Kenneth S. Suslick*
School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801 (Received 12 July 1999) Sonoluminescence spectra werecollected from Cr CO 6 solutions in octanol and dodecane saturated with various noble gases. The emission from excited-state metal atoms serves as an internal thermometer of cavitation. The intensity and temperature of sonoluminescence increases from He to Xe. The intensity of the underlying continuum, however, grows faster with increasing temperature than the line emission. Dissociation of solventmolecules within the bubble consumes a significant fraction of the energy generated by the collapsing bubble, which can limit the final temperature inside the bubble.
PACS numbers: 78.60.Mq

Large oscillations of gas and vapor filled bubbles in liquids occur during acoustic cavitation [1,2]. This highly nonlinear bubble motion is accompanied by the emission of light—sonoluminescence (SL).Spectral analysis of SL from clouds of cavitating bubbles (multibubble sonoluminescence, MBSL) has shown that the implosive collapse of bubbles in argon saturated silicone oil creates local hot spots with extreme transient conditions: 5000 K and 500 atm [3,4]. The effective temperature decreases as the polytropic ratio of the bubble content decreases, consistent with compressional heating duringcavitation [4]. We have now been able to measure directly for the first time the effect of the thermal conductivity of the bubble contents on the observed emission temperature of cavitation. We reach two conclusions: first, bubble collapse is not completely adiabatic, and second, the final temperature is significantly affected by chemical reactions occurring within the bubble. MBSL intensity andsonochemical reaction rates are well known to be affected by the thermal conductivity of the dissolved gas in the liquid [5–9]. This has generally been attributed to a decrease in the effective maximum temperature reached in the collapsing bubbles due to increased thermal transport from the heated gas to the cold surrounding liquid [7,10]. It has been widely accepted that cavitation bubbles filled withhelium should be cooler than those filled with xenon, but to date there has been no direct experimental evidence for this. We have collected MBSL spectra from solutions of volatile metal carbonyls in different solvents saturated with various noble gases in order to definitively probe the effect of thermal conductivity on the temperature reached within a cavitating bubble. The observed emission isfrom excited state metal atoms and serves as an accurate spectroscopic thermometer for the cavitation event [4]. We have found that both the intensity and temperature of MBSL increase as the atomic weight of the noble gas increases, from He through Xe. This is consistent with the thermal conductivity of the bubble contents during collapse, which decreases roughly thirtyfold from He to Xe.0031-9007 00 84(4) 777(4)$15.00

MBSL spectra were collected [11] from dodecane and octanol solutions of chromium hexacarbonyl, Cr CO 6 . The use of long-chain hydrocarbons or alcohols gives high MBSL intensity due to their low volatility; in addition, these solvents have well-measured physical properties (e.g., vapor pressure, gas solubilities, etc.), which are often not available for the previouslyused silicone oils [3,12]. The experimental apparatus is shown in Fig. 1. Details of synthetic spectra calculations and the determination of metal emission temperatures are described elsewhere [4]. Briefly, synthetic spectra are generated using the wellunderstood theory of atomic emission [13]. This technique has been well established for flame and plasma diagnostics [14,15], and it is proving to...
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