Hideto Mitome

Control of a Standing Wave Field Using a Line-Focused Transducer for Two-Dimensional Manipulation of Particles

Control of the positions of particles using acoustic radiation pressure in water was studied to develop a noncontact micromanipulation technique. In this paper a method to transport the particles two-dimensionally using an ultrasonic standing wave field between a line-focused transducer with multiple electrodes and a reflector placed at the focal line is described. When alumina suspension of mean diameter 16 µm was poured into the standing wave field of 2.1 MHz, the particles were trapped and agglomerated at sound pressure nodes existing at half wavelength on the sound beam axis near the reflector. Changing the frequency alters the wave-length and hence the interval of agglomeration. Therefore the trapped particles were transported along the sound beam axis. When the next electrodes were driven, the standing wave field shifted laterally and the trapped particles moved to the corresponding nodal points. Thus two-dimensional transportation was realized using the line-focused transducer. A sound field generated by the line-focused transducer was discussed based on numerical calculations to design an optimum shape of a transducer for manipulation.

The mechanism of generation of acoustic streaming

Acoustic streaming is a stationary fluid motion induced by intense ultrasound. This paper describes the mechanism of generation of acoustic streaming, theoretically and experimentally. A qualitative explanation in relation to acoustic radiation pressure is followed by a detailed discussion based on the formulation of the phenomenon. The spatial non-uniformity of the sound field and energy dissipation due to viscosity are requisite for the generation of streaming. In the development of streaming, two kinds of nonlinearity are important: one is fluid-dynamic nonlinearity, which accounts for effects of inertial force compared to viscous force in fluid motion, and the other is acoustic nonlinearity, which determines the driving force of streaming.

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.

Effects of Nonlinearity in Development of Acoustic Streaming

Development of acoustic streaming generated along the beam axis of ultrasound in unbounded medium is discussed using a numerical analysis. Acoustic nonlinearity changes the distribution of driving force of streaming. If fluid-dynamic nonlinearity is neglected, the streaming velocity essentially increases in proportion to the square of source amplitude. Incorporation of acoustic nonlinearity enhances the streaming velocity near the sound source and suppresses it at a farther distance compared to the acoustically linear case. When fluid-dynamic nonlinearity is taken into account, the fundamental trend in the increase of streaming velocity changes from the second power of the source amplitude toward the first power. Thus the fluid-dynamic nonlinearity delays the development of acoustic streaming.

Micro Bubble and Sonoluminescence

The author reviews the interaction of micro bubbles with ultrasound. First, the action of acoustic radiation pressure on bubbles is discussed in contrast with that on small particles noting the concept of Bjerknes force, resonant bubbles and nonlinear oscillation of bubbles. In the past decade, sonoluminescence, light emission from a single oscillating bubble, attracted attention of researchers because of its strange characteristics. A short history of sonoluminescence and its characteristics are summarized based on bubble motion in a sound field. Lastly, industrial and medical applications of extreme environment generated by collapsing micro bubbles are discussed as promising technology in the new century.

Non-contact micromanipulation using an ultrasonic standing wave field

Transportation of particles using acoustic radiation pressure in water is studied to develop non-contact micromanipulation technique. The radiation pressure traps particles suspended in water to form agglomeration every half wavelength in a standing wave field. Using two sets of standing wave fields crossing to each other, the shape of agglomeration was varied. Applying focused acoustic radiation pressure of traveling wave, a limited part of trapped particles was transported for long distance. Using a concave transducer to generate a standing wave, trapped particles were transported stably along the sound beam axis by changing the ultrasonic frequency. The resolution of transportation to the order of submicron is possible to achieve. Furthermore, the column of trapped particles was separated and transported oppositely by using an appropriate value of frequency increment. Although intense ultrasound generates acoustic streaming, it was possible to avoid its influence using a burst wave.

Control of position of a particle using a standing wave field generated by crossing sound beams

Non-contact control of position of a particle using an ultrasonic standing wave field in water has been studied experimentally. Although a standing wave field generated between a transducer and a reflector makes it possible to trap particles at nodes of the sound pressure distribution and to transport them by changing frequency, the method has several problems such as resonance of the sound field. The present paper describes a method to generate a standing wave field by two transducers whose sound beam axes are crossing to each other without using a reflector. Since the sound field does not resonate, it is stable for any frequency changes. When a particle was put in the region of the crossing sound beams, it was trapped at nodes of the sound pressure. By changing the phase difference between the transducers, the trapped position shifted. By assigning slightly different frequency to each transducer, transportation at constant speed was realized. It is possible to extend this scheme into two or three-dimensional manipulation by adding more transducers.

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.

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.

Measurement of Distribution of Acoustic Radiation Force Perpendicular to Sound Beam Axis

To clarify the range of force acting on a particle in the direction perpendicular to the sound beam axis of an ultrasonic standing wave for particle manipulation, the radiation force on a polystyrene sphere of submillimeter size was measured through observation of its movement in a 1.75 MHz ultrasonic standing wave. The radiation force was estimated at some points by balancing with the force from viscous drag proportional to the measured velocity of a particle and the gravitational force, and its distribution was determined. The measured distribution of the radiation force agreed qualitatively with the theoretical distribution derived from Nyborgs formula. The influence of gravitational force on a stable position was also investigated. It was shown that the shift from the stable position is marked for particles with large radius. This effect should be considered in particle manipulation.