Electrification of Sonoluminescing Single Bubble.

The charge of microbubbles in water is considered to be negative at the liquid-bubble interface. We investigated, for the first time, the charge of a single bubble sonoluminescence (SBSL bubble) that exhibits spatiotemporally stable light emission. When negative DC voltage was applied, the SBSL bubble was attracted to a hot electrode. Conversely, the SBSL bubble was repelled by the hot electrode when positive DC voltage was applied. The translation of the SBSL bubble under an electric field suggests that it is positively charged, and the bubble moved to an equilibrium position to balance the primary Bjerknes force and the electrostatic force. The amount of bubble translation under an electric field depended on the elapsed time of sonoluminescence, suggesting that the products generated inside the SBSL bubble affect the mechanism of bubble charging. Furthermore, we measured the electric field effects on bubble expansion and contraction by a light scattering technique. Applying a positive voltage decreased the maximum bubble diameter and also the intensity of the SBSL. Conversely, applying a negative voltage increased the maximum bubble diameter and also the intensity of the SBSL. The present study revealed that a SBSL bubble is positively charged.

Water-molecular emission from cavitation bubbles affected by electric fields.

Orange emission was observed during multibubble sonoluminescence at 1 MHz in water saturated with noble gas. The emission arose in the vicinity of the peeled ground electrode of a piezoceramic transducer exposed to water, suggesting that cavitation bubbles were affected by the electric fields that leaked from the transducer. The spectrum of the emission exhibited a broad component whose intensity increased towards the near-infrared region with peaks at 713 and 813 nm. The spectral shape was independent of the saturation gas of He, Ne, or Kr. The broad component was attributed to the superposition of lines due to vibration-rotation transitions of water molecules, each of which was broadened by the high pressure and electric fields at bubble collapse. An emission mechanism based on charge induction by electric fields and the charged droplet model is proposed.

Na emission and bubble instability in single-bubble sonoluminescence.

Na emission in single-bubble sonoluminescence (SBSL) was observed from 0.1mM sodium dodecyl sulfate (SDS) solution containing a dissolved noble gas at a low acoustic pressure, at which a continuous spectral component was negligible. High-speed shadowgraph movies were captured at a frame rate of 30,000fps, which indicated that bubble dancing is responsible for the Na emission. The measured bubble path length was well correlated with the Na intensity. The disintegration of a daughter bubble followed by immediate coalescence was frequently observed, which may have been the cause of the bubble dancing. A comparison of the Na spectra obtained in SBSL and multibubble SL showed that the conditions under which Na emission is generated are twofold. A narrow component was observed in the Na spectrum in SBSL, while narrow and broad components were observed in MBSL.

Two components of Na emission in sonoluminescence spectrum from surfactant aqueous solutions.

Sonoluminescence from sodium dodecyl sulfate (SDS) aqueous solutions exhibits Na emission. The spectrum of Na emission was measured as a function of sonication time for a total of 30 min at an ultrasonic frequency of 148 kHz. The spectral line profiles changed with the sonication time, suggesting that the Na emission consists of two components: broadened lines, which are shifted from the original D lines, and unshifted narrow lines. The intensity of the unshifted narrow lines decreased at a greater rate than that of the broadened lines with increasing sonication time. This effect was enhanced at a higher acoustic power. The shifted broadened lines remained after sonication for 30 min. We propose that these quenching effects are caused by the accumulation of gases decomposed from SDS molecules inside bubbles. The CO2 gas dependence of Na emission in NaCl aqueous solutions showed a similar change in the line profiles to that in SDS aqueous solutions, which supported this proposition. The unshifted narrow lines are easily affected by foreign gases. The results suggest that the two components originate from different environments around the emitting species, although both of them originate from the gas phase inside bubbles. The generation mechanisms of the two components are discussed.

Acoustic power dependences of sonoluminescence and bubble dynamics.

The decreasing effect of sonoluminescence (SL) in water at high acoustic powers was investigated in relation to bubble dynamics and acoustic emission spectra. The intensity of SL was measured in the power range of 1-18W at 83.8kHz for open-end (free liquid surface and film-covered surface) and fixed-end boundaries of sound fields. The power dependence of the SL intensity showed a maximum and then decrease to zero for all the boundaries. Similar results were obtained for sonochemiluminescence in luminol solution. The power dependence of the SL intensity was strongly correlated with the bubble dynamics captured by high-speed photography at 64kfps. In the low-power range where the SL intensity increases, bubble streamers were observed and the population of streaming bubbles increased with the power. At powers after SL maximum occurred, bubble clusters came into existence. Upon complete SL reduction, only bubble clusters were observed. The subharmonic in the acoustic emission spectra increased markedly in the region where bubble clusters were observed. Nonspherical oscillations of clustering bubbles may make a major contribution to the subharmonic.

Effects of rare gases on sonoluminescence spectrum of the K atom.

Multibubble-sonoluminescence spectra from Ar-saturated KCl aqueous solutions were measured in the temperature range of 15-40 °C at frequencies of 48 kHz, 148 kHz, and 1 MHz. The effects of dissolved rare gases and the ultrasonic frequency on the shape of the K atom emission spectrum were examined. The line width of the K doublet was independent of the solution temperature, whereas the K line intensity decreased with increasing temperature. The spectra from Xe- and Ar-saturated solutions at 148 kHz exhibited a red-shifted and asymmetrically broadened doublet of K lines. The spectrum from a He-saturated solution, on the other hand, exhibited symmetrically broadened K lines, which were slightly blue-shifted. The observed effects of rare gases are in good agreement with those obtained by gas-phase spectroscopy. These results strongly indicate that the excited K atoms are perturbed by rare gases inside bubbles. The spectra from Xe- and Ar-saturated solutions also indicated that the K doublet is composed of two types of peaks, shifted broadened lines and unshifted narrow lines. These two types of peaks were clearly separated at a frequency of 48 kHz with a high acoustic power in the case of Ar-saturated solutions. The intensity of the broadened lines relative to that of the narrow lines markedly decreased at 1 MHz. Although the narrow lines as well as the broadened lines may have originated from inside the bubbles, the exact mechanism is still unclear.

Multibubble sonoluminescence pulses from Na atoms in viscous liquid.

Multibubble sonoluminescence pulses of Na and continuum emissions were measured from NaCl-ethylene glycol solution saturated with Xe at 28 kHz. The Na emission consisted of multiple-peak pulses and single pulses. The intrinsic pulse width estimated from single pulses was 0.37 ns, which differs from 10-165 ns obtained by previous work. High-speed shadowgraphs of bubble dynamics and high-speed movies (32000 fps) of sonoluminescence were observed. The observations suggest that the multiple-peak pulse is due to the superposition of single peaks resulting from bubbles fragmented from a characteristic bubble which repeats the fragmentation and coalescence. This phenomenon may be specific to viscous liquids.

Sonoluminescence of Na atom from NaCl solutions doped with ethanol.

Sonoluminescence spectra from argon-saturated NaCl solution were measured in the concentration range of 0.5-4 M at the frequency of 138 kHz. The line broadening of sodium atom emission was observed at various acoustic powers in the range from 1.8 to 16.2 W. The sodium D line showed a maximum intensity at a NaCl concentration of 2 M, which corresponded to the maximum production of OH radicals estimated by KI dosimetry. The effects of the addition of a small amount of ethanol on the line width and intensity were closely investigated at various acoustic powers. The sodium line width increases with ethanol concentration and also with power, whereas the line intensity is strongly quenched with increasing ethanol concentration. The results conclusively show that the sodium emission occurs in the gas phase within bubbles. The line broadening is due to interactions with high-pressure argon, and the maximum relative density of gas at bubble collapse was estimated to be 59.5 from the comparison with spectroscopic data. Further line broadening and quenching upon the addition of ethanol arise from collisions with gaseous products obtained from the decomposition of ethanol. The mechanism of sodium excitation is inferred to be as follows. Sodium ions enter bubbles as droplets, and salts are formed because of the high temperature within bubbles. Sodium atoms are generated by the dissociation of salts and then undergo electronic excitation by OH and H radicals.

Effects of alcohols on multi-bubble sonoluminescence spectra.

Spectra of multi-bubble sonoluminescence (MBSL) were measured in argon-saturated water and ethanol solutions in the concentration range 2-100 mM at the frequencies of 116 kHz and 1.0 MHz. The spectral peaks from OH-radical emission near 310 nm were observed at relatively low ultrasonic power at both frequencies. The sub-peaks at 290 and 340 nm were also observed, which are attributed to OH-radical vibronic transition. The MBSL spectra from ethanol solutions indicated that the quenching of OH-radical emission was more efficient than that of underlying continuous spectrum. The continuous spectrum suggested the decrease in temperature with increasing ethanol concentration. The concentration dependence of spectrum quenching showed no frequency dependence. A new peak was observed at 385 nm only at small ethanol concentrations and at the frequency of 1.0 MHz, which attributed to CH or CN molecule emission.