Shin-ichi Hatanaka

Influence of bubble clustering on multibubble sonoluminescence

Influence of clustering of cavitation bubbles on multibubble sonoluminescence (MBSL) in standing wave fields is studied through measurement of MBSL intensity with a photomultiplier tube and observation of corresponding bubble behavior with a high-speed video camera and an intensified charge-coupled device one. It is clarified that, when the SL is quenched suddenly at excessive ultrasonic power, the behavior of bubbles clearly changes; the bubbles which form dendritic branches of filaments change into clusters due to the secondary Bjerknes force. The cluster is composed of several bubbles surrounded by many tiny bubbles, in which bubbles repeatedly coalesce and fragment, and run away from pressure antinodes. When the clusters are broken up by forced fluid motion, the quenching of MBSL is suppressed.

Difference in Threshold between Sono- and Sonochemical Luminescence

The difference in threshold between sonoluminescence (SL) and sonochemical luminescence (SCL) has been investigated. The intensity of both SL from distilled water and SCL from a luminol solution in a rectangular glass cell was measured while changing the driving frequency and voltage applied to the transducer. The second hamonic component of the sound-pressure waveform was also measured simultaneously. The results show that the thresholds in sound pressure become higher for cavitation, SCL and SL in this order. As the dissolved gas in a liquid decreases, the SCL intensity decreases but the SL intensity increases. For a constant quantity of dissolved gas, as the liquid temperature becomes higher, the SCL intensity increases but the SL intensity decreases. In the case of air-saturated liquids, the difference in threshold between SCL and SL becomes larger as the liquid temperature increases. The dependence of the ratio of SCL to SL on temperature is similar to that of vapor pressure.

Quenching Mechanism of Multibubble Sonoluminescence at Excessive Sound Pressure

Decrease in luminous intensity of multibubble sonoluminescence (MBSL) at excessive ultrasonic intensity is studied in connection with the behavior of cavitation bubbles. The intensity of MBSL from aerated distilled water in a rectangular vessel with six transducers, in which a standing wave field was established, was measured while the voltage applied to the transducers was increased. The corresponding change in the distribution of cavitation bubbles in the field was observed. The distributions of bubbles were compared with those of MBSL using an intesified charge-coupled device (ICCD) camera, and the bubble motion was observed with a high-speed camera. It is clarified that the expulsion of bubbles from the pressure antinode in the field is responsible for the decrease in MBSL intensity at high ultrasonic intensity. The expulsion is caused by the primary Bjerknes force which changes from an attractive force to a repulsive one depending on pressure amplitude and bubble radius, assisted by coalescence of bubbles due to the secondary Bjerknes force.

Orientation of Fibers in Liquid by Ultrasonic Standing Waves

The orientation of fibers in a liquid irradiated with ultrasound is studied to realize the noncontact directional control of reinforcing fibers in the molten matrix of a composite material. The equations of translational and rotational motions of a fiber in a standing wave field are derived. The numerical solutions show the movement of the fibers at various initial positions and their stable positions and directions. Experiments are performed using polystyrene fibers of various lengths suspended in an aqueous sugar solution. Both numerical and experimental results indicate that polystyrene fibers shorter than one-fourth of the wavelength are constrained at the pressure node and are oriented in the direction perpendicular to that of wave propagation. On the other hand, fibers ranging from one-fourth to one-half of the wavelength have orientation either parallel to the direction of wave propagation at the pressure loop or perpendicular to that at the pressure node depending on their initial positions and directions.

Simultaneous Observation of Motion and Size of a Sonoluminescing Bubble

This paper describes a method of simultaneously executing both observations using a charge-coupled device (CCD) camera with stroboscopic light and measurement of bubble radius with light scattering. This optical system consists of a beam splitter inserted between a lens and the CCD camera, and a photomultiplier tube (PMT) mounted at the beam splitter. It is confirmed that this system is useful for the simultaneous observation of the bubble size, shape and translational motion.

Observation of a Sonoluminescing Bubble Using a Stroboscope

A method to observe sonoluminescing bubble motion has been studied. By a single flash of a stroboscope much shorter than the acoustic cycle, a charge coupled device (CCD) camera captures an instantaneous image of the bubble, which includes the dancing condition. Changing the flash timing of the stroboscope slowly made it possible to observe periodical expansion and contraction of the bubble. It is clarified that the bubble size and the phase at the time the bubble collapses changes according to the amplitude of sound pressure.

Study of an Acoustic Field in a Microchannel

Using a standing-wave field, it is possible to trap small objects at nodes of a sound pressure distribution. In the present study, a sound wave was generated by a transducer outside of a microchannel, and propagated into a microchannel on a glass plate, where it generated a standing wave field. When water containing alumina particles was injected into the microchannel, several layers of particles were formed in the sound field. Moreover, when the ultrasound driving frequency was swept, it was possible to control the direction of the particle flow. The sound field was numerically calculated and the experimental results are discussed.

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 is studied theoretically and experimentally. The model is a rectangular glass cell set in a water-cooling bath. Ultrasound is irradiated from the bottom of bath into the cell. Theoretical analysis of transmitted acoustic energy assuming a plane acoustic 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 is examined through intensity measurement of SL emitted in the cell using a photomultiplier tube. Dependence of SL intensity on the above parameters agreed with the theory very well. Distributions of SL are also observed using a high sensitivity CCD camera and are compared with optically visualized sound fields to show the relationship between the sound field and the SL.

Manipulation of Particles in a Microchannel with Various Geometric Spaces Using Ultrasound

A noncontact micromanipulation technique is required in micromachine technology, biotechnology, and other fields. In this study, a standing wave field was generated in a microchannel with a geometric space. It was possible to trap small objects at nodes of the sound pressure distribution in the medium. A microchannel of 1×50×1 mm3 was formed at the center of a glass plate of 50×50×5 mm3. In the experiment, when the liquid water containing particles was injected into the microchannel on the glass plate irradiated by ultrasound, the particles flowed along several layers. It was shown that the traveling wave was transmitted into the microchannel and the standing wave field was formed in the microchannel. The micromanipulation technique enabled the control of the direction of the particle flow by changing the ultrasound frequency in branched channels with a geometric region at the junction. Moreover, in a triangular region, the particles moved towards the top from the base of the triangle with a fixed frequency.

Influence of dissolved oxygen content on multibubble sonoluminescence with ambient-pressure reduction

The efficiency of chemical reactions in the presence of ultrasound at reduced pressures has been monitored using the influence of dissolved oxygen (DO) content on a luminol solution undergoing multibubble sonoluminescence. From these measurements under the condition of constant ultrasonic frequency and constant amplitude of sound pressure, it is shown that the intensity of sonoluminescence is higher at subatmospheric ambient pressure than at atmospheric pressure under the same degree of saturation. Also, it is found that there is an appropriate content of DO to produce the highest intensity of the luminescence and its value varies with ambient pressure.