G. Iernetti

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.

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.

Measurement of the Intensity of Sonoluminescence, Subharmonic Generation and Sound Emission Using Pulsed Ultrasonic Technique

The measurement of the intensity of sonoluminescence, subharmonic generation and sound emission with different pulse parameters is reported using a new method of stimulating the acoustic cavitation effect at high frequency (700 kHz) with a low-frequency (20 kHz) ultrasonic field. It is found that stimulation enhances the intensity of sonoluminescence and subharmonic generation at reduced threshold transducer voltage and inverse pulse duty ratio, while sound emission is oppositely affected. The bi-frequency effect arises due to space–time interaction. This work contributes to the understanding of the mechanism of light emission and nonlinear behavior of bubble dynamics.

Water Surface Tension in the Temperature Range from 288 K to 304 K

Abstract The surface tension of water has been measured by a new technique of relative sensitivity 0.005 based on surface standing waves in a cylindrical vessel of temperature 288 K to 304 K. The observed values of the surface tension are compared with those calculated by the significant structure theory under the assumption of a minelayer boundary, and using the Lennard-Jones potential. A plateau was found in the observed surface tension versus temperature plot at around 294 K, with slopes higher than the mean around 290 K and around 297 K. The calculated surface tension shows similar discontinuities at the said temperatures. This is a good indication for higher order transitions in water, desumed from the surface tension temperature dependence.

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.

ELECTRO-CHEMICAL POTENTIAL DIFFERENCE VARIATION BY PULSED ULTRASOUND

An experimental study using the pulsed ultrasonic technique to investigate the effect of the cavitation on zinc probes is reported. The difference in potential between two zinc electrodes one in the focal zone and the other one out of it, the intensity of sonoluminescence and that of subharmonic emission, have been measured at 0.7 MHz by varying the voltage of a converging transducer and the other two pulse parameters. A correlation between these data under the different stages of cavitation is observed, which may lead to the understanding of the mechanisms of the sonoluminescence and of the erosion.