Cavitation Thermometry Using Molecular and Continuum Sonoluminescence

Abstract

The use of molecular and continuum emission spectra from multiple bubble (MB) and single bubble (SB) sonoluminescence (SL) is explored as a probe of bubble temperature during cavitational collapse. It is proposed that molecular and continuum SL arise from different chemical pathways, which occur during discrete intervals along the cavitational collapse time line, thus yielding different cavitation temperatures. A coupled bubble dynamics/chemical kinetic model of cavitational collapse is developed and used to explore a variety of proposed molecular SL mechanisms for the C2(d→a), CN(B→X), and OH(A→X) emissions. Molecular SL is shown to arise from chemiluminescent reactions of seed molecules (e.g., hydrocarbons, N2, H2O) and their dissociation products, and occurs during the early and middle stages of cavitational collapse. This emission is broadly characterized as originating from reactions involving singly or multiply bonded molecular precursors with corresponding effective emission temperature ranges of approximately 3000−8000 and 8000−25 000 K, respectively. An analysis of an experimentally observed CN(B→X) MBSL spectrum is reported which is consistent with CN emission occurring over a broad distribution of cavitation temperatures ranging from approximately 5000 to 15 000 K. Continuum SL is attributed to transitions of electrons produced by high-temperature ionization and confined to voids in the dense fluid formed during the latter stages of cavitational collapse. The continuum is similar for both SBSL and MBSL, and is characterized by a temperature range of ≈20 000−100 000 K. The observation of significant molecular emission for MBSL, and not for SBSL, is attributed to the broad distribution of initial bubble sizes for MBSL. In SBSL, a single bubble is repetitively cycled through collapse and reexpansion, and its collapse is driven well into the continuum emission regime. In MBSL, only a small fraction of the bubbles will be driven to this level of collapse, while a much larger fraction will attain only the single or multiple bond chemistry regimes. Thus in MBSL the bubble size distribution averaged emission will tend to enhance the molecular relative to the continuum emission. It is concluded that both SBSL and MBSL are consistent with an adiabatic compressional heating description of bubble collapse.