| The intense shock wave launched from collapsing bubbles during the sonication of slurries allows for the study of chemical and physical events that occur when a solid is stressed or fractured. One such event is mechanoluminescence (ML): light produced by any mechanical action on a solid. ML has been studied for over 400 years, but much is still not known about it because the emission is inherently weak. Sonicating slurries of mechanoluminescent crystals (such as sucrose, sodium chloride, resorcinol, m-aminophenol, or coumarin) in long chain alkanes has produced very bright ML, up to 1,000 fold more intense than from manual grinding. The large increase in intensity has revealed a number of new emitting species including C2, CH, CO, CO+, CO2+, H, and He+, many of which have not been reported from ML before. In addition, the emission products show that gas phase reactions are occurring within the plasma generated from the ML discharge. The intense ML induced by acoustic cavitation allowed the plasma to be characterized in terms of heavy atom temperature of ∼400 K, electron density of 1014 cm-1, and electron energy of ∼3.5 eV. These conditions are very similar to other highly reactive microdischarges.;To further extend the knowledge of the conditions generated within a cloud of cavitating bubbles, multi-bubble sonoluminescence (MBSL) of sulfuric acid has been studied. The MBSL spectrum from 95 wt % H2SO 4 consists of a broad continuum extending into the UV with SO and Ar emission lines also observed. The Ar lines were used to determine an effective emission temperature of ∼8,000 K, which is substantially greater than in other low vapor pressure systems (e.g., silicone oil, where MBSL emission temperature is only ∼5,000 K). The observation of Ar lines at this temperature also indicates that a hot plasma core is probably generated during multi-bubble cavitation in sulfuric acid. In addition, the effect of solution composition was studied by varying the acid concentration.;The effect of single-bubble sonoluminescence (SBSL) intensity and spectral profile from the addition of low concentrations of organics to sulfuric acid has been investigated. It was found that the addition of small quantities of organics to sulfuric acid greatly reduced the total SBSL intensity. The addition of small quantities of organics resulted in intense carbon-containing molecular emission lines in the SBSL spectrum. The spectral profile of the molecular emission lines was found to be dependent on concentration of the organic and the applied acoustic pressure. At low concentration, the molecular emission bands showed a rotational temperature of ∼280 K with high energy vibrational population, while at higher concentrations and higher acoustic pressure the molecular emission bands indicated thermodynamic equilibrium with rotational and vibrational temperatures of ∼5,800 K. By analyzing the spectral profile of the molecular emission bands it was determined that at very low concentration the emitting molecular species were excited by electron impact of the organic molecules within the interfacial layer of the bubble while at higher concentration and higher acoustic pressure thermal emission dominated. |
