Toru Tuziuti

Relationship between a Standing-Wave Field and a Sonoluminescing Field

The relationship between a sound field and a multibubble sonoluminescing field has been studied experimentally and analytically to clarify the sonochemical reaction field. The variation of each field in a rectangular glass cell filled with distilled water or a luminol solution was investigated while changing experimental conditions such as driving frequency, applied voltage to the transducer, and thickness of the cell bottom. As for the sonoluminescing field, the intensity of sonoluminescence (SL) was measured and its distribution was observed. As for the sound field, sound energy density was analyzed theoretically and sound pressure distribution was observed optically. Comparing them, it became clear that SL occurred at pressure antinodes of a standing-wave field in the cell. There are upper and lower thresholds of the sound pressure for SL to occur. This explains the amplitude dependence and spatial nonuniformity of SL in a standing-wave field.

Measurement of the establishment process of acoustic streaming using laser Doppler velocimetry

Acoustic streaming generated by 5.05 MHz CW ultrasound in water has been measured using a laser Doppler velocimeter to clarify the establishment process of the streaming. When the ultrasound is switched on, the streaming velocity along the beam axis increases, monotonously approaching a steady value as long as the applied voltage to the transducer is low. At a farther distance from the transducer, it takes more time to settle down and some cases show overshooting in the velocity changes. At higher voltages, the streaming velocity increases rapidly and there appears anomalous behavior in the establishment process showing plateaus and humps.

Influence of the Sound Field on the Intensity of Sonoluminescence.

Cavitation-field sonoluminescence intensity was measured in correlation with sound pressure and its harmonic and subharmonic component signals generated from a well-defined cavitation field in a distilled-water filled rectangular cell by changing the acoustic frequency and the voltage applied to the transducer. Optics was designed to ensure that all the sonoluminescing positions in the cell were detectable by a photomultiplier tube. The results show that it is the second harmonic component of the sound pressure waveform, rather than the subharmonic one, which gives a reliable indication of the threshold of the cavitation and the onset of the cavitation-field sonoluminescence. The sonoluminescence signal exhibits a linear relationship with the simultaneously measured second harmonic component signal and is likely to originate mainly from the subset of stable cavitation bubbles in the cavitation field.

Quasi acoustic streaming induced by generation of cavitation bubbles

Bulk fluid movement induced by the generation of acoustic cavitation bubbles is discussed using experimental results from laser Doppler velocimetry. In contrast to the ordinary acoustic streaming generated through absorption of ultrasound, which is apparent at higher frequencies, bubble-induced acoustic streaming comes from the direct action of acoustic radiation pressure on cavitation bubbles and is observed even at lower frequencies. The fluid motion is intermittent, corresponding to the generation of bubbles followed by an exponential decay in velocity. Since the generation mechanism is quite different from the ordinary one, this should be termed quasi acoustic streaming.

Observation of spatial nonuniformity in a sonochemical reaction field

A sonochemical reaction field in a rectangular glass cell driven by two kinds of ultrasonic transducer was visualized optically and compared with a distribution of multi-bubble sonoluminescence. Fluid motion in the cell was measured using a laser Doppler velocimeter. In a standing-wave type reactor driven at 140 kHz, the reaction field formed layers and was not uniform accompanying fluid motion at high intensity. In a horn-type reactor of 19.5 kHz, the luminescing region was confined in very close vicinity to the horn tip in contrast with broad region of fast fluid motion.

Dependence of sonoluminescence intensity on the geometrical configuration of a reactor cell

Dependence of sonoluminescence (SL) intensity on the geometrical configuration of a reactor cell has been studied theoretically and experimentally. The model was a rectangular glass cell settled in a water-cooling bath. Ultrasound was irradiated from the bottom of bath into the cell. Theoretical analysis of transmitted acoustic energy assuming a plane sound wave clarified the influences of distance between the transducer and the cell bottom, thickness of the cell bottom, and water depth in the cell. The theory was examined through intensity measurement of SL emitted in the cell using a photomultiplier tube. Dependence of SL intensity on the above-mentioned parameters agreed with the theory very well. Distributions of SL were also observed using a high-sensitive CCD camera and compared with optically visualized sound fields to show the relationship between the sound field and the SL.

Generation and aggregation of BaTiO3 nanoparticles under ultrasound

Numerical simulations of generation and aggregation of BaTiO3 nanoparticles have been performed under the experimental condition of Dang et al. [Jpn. J. Appl. Phys. 48, 09KC02 (2009)] in order to study the mechanism of mesocrystal formation. It is shown that electric dipole-dipole interaction model sufficiently reproduces the experimental results of the oriented aggregation of BaTiO3 nanocrystals. It is suggested that 5 nm BaTiO3 nanocrystals synthesized under ultrasound have spontaneous polarization.