P. Ciuti

Enhancement of high-frequency acoustic cavitation effects by a low-frequency stimulation

The cavitation effects given by a high-frequency pulsed ultrasound field are studied with and without the stimulation of a low-frequency field. Sonoluminescence intensity and subharmonic one-half intensity of the high-frequency field are measured. The stimulation gives a sharp rise of both subharmonic and sonoluminescence intensities.

Cavitation activity stimulation by low frequency field pulses

The influence of a short-time action of a low-frequency ultrasound on the sonoluminescence generation by a high frequency pulsed field has been studied. This action remarkably lowers the cavitation thresholds and increases the sonoluminescence intensity. The stimulating effect of the low-frequency field action depends on its duration and on the intensities of both fields. Possible mechanisms of this effect are discussed.

Study into mechanisms of the enhancement of multibubble sonoluminescence emission in interacting fields of different frequencies.

The main factor of the enhancement of sonoluminescence (SL) emission by the interaction of two fields of highly different frequencies is the generation of new cavitation nuclei upon collapse of bubbles driven by the low-frequency (LF) field. The factors connected with the direct interaction of the two fields play a significant role in the enhancement of SL emission only in the case when intensities of the fields are less or not much higher than the corresponding thresholds of SL emission. The phenomena of afteraction of the LF field on cavitation generated by the high-frequency field is explained also by the generation of new nuclei upon collapse of bubbles driven by the LF fields.

Enhancement of sonoluminescence emission from a multibubble cavitation zone.

Investigations have been performed on various methods of increasing cavitation activity measured by the intensity of sonoluminescence. It is shown that the effect of the combined action of (a) pulsed modulation of an acoustic field, (b) liquid degassing and cooling and (c) increasing the static pressure considerably exceeds the sum of the effects achieved by each of these methods individually. A more than 250-fold increase of the sonoluminescence intensity has been attained compared with continuous irradiation under normal conditions (room temperature, atmospheric pressure, gas-saturated liquid). An interpretation of the results obtained is proposed.

Cavitation phenomena in pulse modulated ultrasound fields

Sonoluminescence (SL), subharmonic generation (SH) and ultrasound absorption in a cavitation zone generated by pulse modulated fields were studied. It has been shown that SL intensity may be increased due to pulse modulation of an ultrasound field by more than two orders of magnitude as compared to the SL intensity obtained by continuous irradiation. The achieved increase in the erosion rate is smaller with pulse modulation. The ultrasound absorption decreases with the inverse pulse duty ratio (or off/on factor), i.e. clarification effect is found to exist in pulsed fields. The thresholds of studied phenomena increase with inverse pulse duty ratio. The intensity of the subharmonic and of other low-frequency components of the cavitation noise decreases; this allows us to conclude that the decreased bubble volume concentration is mainly caused by a decrease of the large bubbles that have low energy concentration efficiency and make little contribution to the cavitation effects.

Optical visualization of non-linear acoustic propagation in cavitating liquids

Abstract Non-linearity effects on sound propagation induced by cavitation bubbles are investigated. The convergence of an acoustic wave due to the interaction with the microbubbles produced in the cavitation zone is shown experimentally. In these conditions the theoretical analysis shows that the self-focusing primarily depends on the effective microbubble volume fraction. This fraction turns out to be about 10−6 with a corresponding self-focusing distance of about 9 cm in the Fraunhofer region of a plane circular transducer.

Cavitation threshold dependence on the rate of the transducer voltage variation

The cavitation thresholds of subharmonic emission and of sonoluminescence in water have been measured in a focused ultrasound beam by two different methods: (1) by increasing the transducer voltage at different rates; (2) by keeping a chosen constant transducer voltage and waiting for the appearance of the subharmonic or of the sonoluminescence. The thresholds increase with increasing the rate of voltage increase. The waiting time diminishes with the increase of the chosen threshold voltage.

Equivalent temperature of sonoluminescence at incipient and desinent thresholds of light emission

Abstract The emission of light in air-water has been studied by the pulse technique during different stages of acoustic cavitation, in particular at the incipient and desinent thresholds of strong cavitation. It has been observed that the equivalent temperature of the light emitted at the incipient cavitation threshold is higher than that corresponding to the light emitted during the last stages of desinent cavitation. With respect to the irradiation time, the equivalent temperature of the light pulses increases in the beginning of the incipient phase, then decreases by a further increase of irradiation time. The equivalent temperature values of the light emitted during the stable cavitation present immediately before the onset of the bubble chain multiplication phase are near to the values found at the desinent emission. Both stable and transient cavitation contribute to sonoluminescence by different ratios at different development stages of the cavitation zone. Equivalent temperatures of about 4000 K for the desinent light and of about 10 000 K for the incipient light are found.

Two Tresholds Of Multibubble Cavitation

It has been shown that cavitation zone generated by the high intensity focused ultrasound (HIFU) passes through different stages of evolution with either increasing pulse duration, decreasing pulse period or increasing driving voltage of pulsed ultrasound. In the first stage sonoluminescence (SL) is absent. Here sound absorption is weak in the cavitation zone and the hydrophone output is nearly constant if the transducer voltage is constant. The second stage corresponds to the onset of sonoluminescence and the smooth increase of its intensity. In the third stage, the sonoluminescence intensity increases in a sudden manner, accompanied by a synchronous rise of the ultrasound absorption in the cavitation zone. The sonoluminescence intensity reaches a maximum value and then decreases while ultrasound absorption increases smoothly upon further cavitation zone evolution. Two cavitation thresholds separate these stages: first is related to the SL appearance and the second—to the sudden increase of the SL intensity.

Multibubble sonoluminescence in interacting fields of different frequencies

Multibubble sonoluminescence has been studied in conditions of low frequency field (LF), high-frequency field (HF) and combined (HF+LF) sonification. It is shown that combined action of the two fields can exceed the sum of the effects of each of them acting separately, i.e. there is considerably nonadditive summation of the effects. The highest effect has been reached in the case when the HF field was pulse modulated and the LF field was switched on for a short time (0.1–60 sec.) After deenergizing the LF field, the intensity of sonoluminescence (SL) generated by the HF field increases at first in a stepwise manner. After this jump the SL intensity slowly decreases tending to some limiting value, which usually is higher than the SL intensity before impact by the LF field.