Application Cases

Experiment Name: Experimental Study on the Concentration of Suspended Particles Based on Surface Acoustic Standing Wave Technology

Research Direction: Biomedical Applications

Testing Equipment:

• PDMS microfluidic channel chip, piezoelectric substrate

• Observation device: Keyence VHX-2000 three-dimensional ultra-depth-of-field microscope

• Excitation equipment: RF signal generator, ATA-8202 RF power amplifier

• Materials used: fine filter paper, waste engine oil, test tubes, alcohol, connectors, infusion capillary tubes, syringes, or peristaltic pumps, etc.

Experimental Procedure:

Figure: Experimental Setup for Concentrating Suspended Particles Using Surface Acoustic Standing Waves

First, based on the purchased template, two sets of identical interdigital transducer (IDT) electrodes are printed on the piezoelectric substrate using the screen-printing method. To compare experimental outcomes, the IDT electrode pairs printed in this experiment are 12 and 24 pairs, respectively. The printed electrodes are dried and polarized to ensure good electrical conductivity. A microfluidic channel is fabricated between the two sets of IDT electrodes using a cutting machine or laser cutting. To form a standing surface acoustic wave (SAW) field with only one node in the microfluidic channel, the width of the channel should be half the wavelength, i.e., 500 μm. Subsequently, waste engine oil containing impurity particles is injected into the channel from point A using a pressure transmission device or a peristaltic pump. Before applying signals to the IDTs (i.e., without exciting SAWs), the particles are uniformly distributed in the channel. When the same RF signal is applied to both IDTs, two SAWs with the same amplitude and frequency but propagating in opposite directions are generated. These two SAWs superimpose within the microfluidic channel region, forming a SAW standing wave field. Under the influence of acoustic radiation forces in the standing wave field, the particles converge toward the node, eventually forming a straight, ribbon-like line at the center of the channel. The SAW device with the cut microfluidic channel is fixed onto pre-prepared plastic foam to facilitate placement under the three-dimensional ultra-depth-of-field microscope. The final fabricated SAW device and the cut microfluidic channel are shown in Figure 4.7.

Figure: SAW Device and Fabricated Microfluidic Channel

The driving/sensing device is fabricated by screen-printing electrodes on a piezoelectric substrate. When the same RF signal is applied to a pair of IDTs on the sensor, two SAWs with identical amplitude, frequency, and opposite propagation directions are generated. Their superposition forms an ultrasonic standing wave field with only one acoustic pressure node. Since particles in the standing wave field are subjected to acoustic radiation forces, and the force decreases as the distance between the particle and the acoustic pressure node decreases, the particles tend to gather toward the node and eventually stabilize near it, meeting the requirements for particle separation. To achieve particle separation, a standing wave field must be generated between the two IDTs.

Figure: SAW Center Frequency Test Diagram

During the experiment, it is first necessary to generate a stable standing wave field by varying the frequency of the IDTs. To verify the formation of the standing wave field, this study employs the method of analyzing strain data recorded by a static strain gauge. In the standing wave verification experiment, a signal generator is used to simultaneously excite continuous sine waves on both ends of the SAW transducer. Strain gauges are attached before cutting the microfluidic channel, and a static strain gauge is used to receive the strain values. An oscilloscope records the excitation signals from both channels. In this experiment, a frequency of 5 MHz is selected as the standing wave excitation frequency to achieve optimal excitation effects. Before cutting the microfluidic channel, strain gauges are attached between the two SAW transducers. SAWs are simultaneously excited at both ends, inducing standing wave vibrations. When the frequency of the externally applied excitation signal matches the center frequency of the transducer, the waves excited by each pair of interdigital electrodes add in phase, resulting in the strongest excitation of the IDT. Therefore, it is necessary to detect the center frequency of the IDT. Figure 4.9 shows the SAW center frequency test diagram. One end is excited by a signal generator, and the other end collects signals using an oscilloscope. As the excitation frequency varies, the corresponding received voltage amplitude changes. At a frequency of 5 MHz, the amplitude reaches its maximum value of 412 mV, indicating the applicable excitation frequency.

Experimental Results:

Table 4.2 presents the strain test results with and without standing waves applied. From the table, it can be observed that when the same excitation signal is applied to both ends, the strain difference shows alternating variations, indicating the generation of standing waves. The corresponding strain changes are caused by the variations in wave peaks and troughs induced by the standing waves.

Figure: Strain Test Results Induced by Surface Acoustic Standing Waves

Aigtek ATA-8000 Series Radio Frequency Power Amplifier:

Figure: ATA-8000 Series Radio Frequency Power Amplifier Specifications and Parameters