Application Cases

Experiment Name: Application of High-Voltage Power Amplifier in Underwater Particle Manipulation Using Ultrasonic Standing Wave Field

Research Direction: Ultrasonic Control

Test Objective:
Acoustic radiation force generated by ultrasound can achieve manipulation of micro-scale  objects. Addressing the manipulation of micron-scale particles in a liquid environment, based on the theory of acoustic radiation force in viscous media, an underwater particle manipulation model driven by double-concave spherical focused ultrasonic transducers was established. The acoustic field, acoustic streaming field, and the dynamic process of particle manipulation were simulated using COMSOL software. Finally, the simulation results were verified through underwater particle manipulation experiments. The study found that the underwater manipulation process of particles is subject to the combined action of acoustic radiation force and acoustic streaming drag force. The local standing wave field formed by acoustic wave interference primarily relies on acoustic radiation force to aggregate particles at the nodal positions. However, as particle size decreases, particles can no longer be confined, and the manipulation mechanism transitions from being dominated by acoustic radiation force to being dominated by acoustic streaming drag force. Furthermore, an increase in acoustic field intensity enhances the  disturbance resistance) capability of particle manipulation.

Testing Equipment: (1) Mechanical clamping device for adjusting the distance and angle of the transducers; (2) Concave spherical focused ultrasonic transducers; (3) Multi-function signal generator; (4) High-voltage power amplifier (ATA-4014); (5) Digital oscilloscope; (6) Transparent plexiglass water tank.

Experimental Procedure:
Polystyrene particles were used as the manipulation targets, with four particle radii selected: 150, 75, 37.5, and 7.5 μm. Since the density of polystyrene particles is slightly higher than water, they were poured into a small beaker with water and settled to the bottom upon standing. Before the experiment, the particles needed to be stirred to keep them suspended before being drawn into a dropper. Prior to the experiment, the water tank was filled with distilled water to immerse the setup. The distance between the aperture planes of the two transducers was adjusted to 1.5 times the focal length (30 mm). After connecting the equipment, the signal generator was controlled to emit a continuous sinusoidal wave signal with a frequency of 1 MHz. After passing through the power amplifier, the oscilloscope displayed an input voltage peak-to-peak value of 13.6 V. After activating the transducers, a dropper was used to the aqueous solution containing particles and repeatedly add  them into the acoustic radiation region. Once the confinement state of the particles stabilized, experimental recordings were made. The underwater aggregation states of particles of different sizes are shown in the figure below. Particles with radii of 150 and 75 μm were confined in the central axis region of the transducers, corresponding to the local standing wave field area. Some particles were located on the central axis, while others were slightly  deviated) 1–2 mm from the axis. As particle size decreased, the degree of aggregation weakened, and particles became  within the  radiation) acoustic field range. When the particle radius decreased to 7.5 μm, the particles could no longer be confined; the added particles slowly moved towards both sides with the acoustic streaming. Simultaneously, the stability of particle manipulation also weakened as the radius decreased. Manipulation was most stable for the 150 μm particles, which could slowly move along with the clamping device. However, when the particle size decreased to 37.5 μm, even  micro-disturbances  in the water could cause some particles to leave the radiation region; the smaller the particle size, the worse the stability.

Figure: Underwater Aggregation States for Different Particle Sizes

Experimental Results:

1. The simulation only considered the acoustic radiation force and acoustic streaming drag force acting on the particles during the underwater manipulation process. Particles in a standing wave field aggregate and are confined at the acoustic wave nodal positions, which correspond to the minimum of the average potential energy field. During the longitudinal aggregation process of particles, the longitudinal acoustic radiation force of the local standing wave field is the dominant force, with a magnitude much larger than the acoustic streaming drag force in the same direction. Lateral aggregation depends on the relative magnitudes of the lateral acoustic radiation force and the drag force. As the particle scale decreases, the acoustic streaming drag force gradually becomes the dominant force, and manipulation relying on acoustic radiation force alone becomes impossible.

2. In the simulation, under the premise that the flow field distribution remained unchanged, changing the acoustic field intensity did not alter the relative magnitudes of the two forces at any position within the acoustic field; the direction of the resultant force remained unchanged, and the particle manipulation mode remained the same. The magnitude of the resultant force was proportional to the square of the acoustic pressure value. However, in  actual situations , considering the effects of gravity, buoyancy, and environmental disturbances on the particles, increasing the acoustic field intensity can enhance the manipulation and  disturbance resistance) capability of the particles.

3. The results of the underwater particle manipulation experiment verified the first conclusion. The local standing wave field formed by the transducer radiation can stably aggregate particles relying on acoustic radiation force. However, as the particle size decreases, the particle distribution gradually disperses, and acoustic radiation force no longer acts as the dominant force. To aggregate particles of this size, other methods such as acoustic micro-vortices need to be employed to achieve the  objective .

   
   

Figure: ATA-4014C High-Voltage Power Amplifier Specifications and Parameters