Application Cases

Experiment Name: Experiment on Droplet Directional Driving Characteristics Based on Piezoelectric-Acoustic Flow Effect

Research Direction: Interdisciplinary fields of thermal fluid science and engineering, and piezoelectric acoustics

Experimental Objective:
To verify the feasibility of droplet directional driving technology based on the piezoelectric-acoustic flow effect, clarify whether this technology can effectively drive condensed droplets at low voltages to provide a new method for frost suppression. Analyze the influence of key parameters (driving voltage, droplet volume) on droplet movement speed, determine the effective operating range of the technology (e.g., minimum driving voltage, optimal droplet volume range). Determine the optimal excitation frequency of the experimental setup to maximize the acoustic energy absorbed by droplets and improve driving efficiency. Through energy analysis, compare theoretical derivations with experimental results to reveal the energy absorption mechanism of droplet directional motion, providing theoretical support for technology optimization and engineering applications.

Testing Equipment: Signal generator, ATA-2022H, impedance analyzer, glass plate, piezoelectric ceramic, pipette, CCD camera, etc.

The schematic diagram of the experimental principle is shown in Figure 1.

Figure 1: (a) Schematic diagram of the experimental principle of droplet directional driving based on the piezoelectric-acoustic flow effect; (b) Experimental setup diagram; (c) Acoustic flow effect inside the droplet

Experimental Procedure:

1. Experimental Setup Construction

• Substrate treatment: Apply a hydrophobic coating (glass waterproofing agent) to the surface of the glass plate to ensure the droplet contact angle is close to 90°. Use ultraviolet light-curing adhesive (adhesive layer thickness < 0.1 mm) to attach the PZT-5H piezoelectric ceramic to one side of the glass plate. Adopt a flanged electrode design to ensure positive and negative electrode connections are on the same side, facilitating circuit connections.

• Circuit connection: Connect the circuit in the order "signal generator → power amplifier → piezoelectric ceramic". Simultaneously, connect the impedance analyzer to both ends of the piezoelectric ceramic to detect impedance characteristics. Position the CCD camera to face the droplet placement area on the glass plate to ensure clear capture of droplet movement. The actual experimental photo is shown in Figure 2.

  
  

Figure 2: Actual experimental photo

2. Determination of Optimal Excitation Frequency

• Use numerical simulation software to construct an experimental setup model, perform mesh division and impedance-frequency characteristic simulation, and analyze the impedance values and amplitude distribution at different frequencies. Based on simulation results, determine 650 kHz as the optimal excitation frequency (at this frequency, the impedance is only 114 Ω, output power is maximized under the same voltage, the amplitude of the plate's ultrasonic waves is strongest, and droplet energy absorption is highest). The determination of the optimal vibration frequency based on numerical simulation results is shown in Figure 3.

  
  

Figure 3: Determination of optimal vibration frequency based on numerical simulation results

3. Droplet Movement Characteristics Testing

• Driving voltage impact test: Fix the droplet volume at 100 µl, set the frequency to 650 kHz via the signal generator, and adjust the output voltage (peak 30–100 V) using the power amplifier, with measurements taken at intervals of 5–10 V. Place the droplet at the center of the glass plate using a pipette, start the equipment, and record the droplet movement process with the CCD camera. Calculate the average movement speed at each voltage using ImageJ software, with each condition repeated three times to reduce errors (speed measurement deviation < 5%).

• Droplet volume impact test: Fix the driving voltage at 50 V and the frequency at 650 kHz. Prepare droplets with volumes of 0–250 µl using a pipette (measurements taken at 50 µl intervals). Repeat the "placement-recording-calculation" steps and analyze the relationship between droplet volume and movement speed. The influence of driving voltage and droplet volume on droplet movement characteristics is shown in Figure 4.

  
  

Figure 4: Influence of driving voltage and droplet volume on droplet movement characteristics

4. Energy Analysis Verification

• Derive the formula for dimensionless acoustic energy absorbed by droplets based on perturbation theory and calculate the energy density at different droplet radii. Compare theoretical calculation results (after polynomial fitting) with experimental "droplet volume-movement speed" data to verify the correlation between energy absorption and droplet movement (e.g., theoretically, energy density is highest at a droplet radius of 10 mm, corresponding to the highest speed for droplets of 140–160 µl in experiments).

Experimental Results:

1. Feasibility Verification: The droplet directional driving technology based on the piezoelectric-acoustic flow effect is effective, capable of directionally driving droplets with volumes > 50 µl at low voltages above 35 V. At a driving voltage of 75 V, the maximum movement speed of a 100 µl droplet reaches 88 mm/s, with the droplet moving directionally along the propagation direction of the ultrasonic waves without significant deviation.

2. Driving Voltage Influence: The droplet movement speed increases linearly with the driving voltage. When the voltage is < 35 V, the acoustic energy absorbed by the droplet is insufficient to overcome surface pinning forces, preventing movement. The driving effect is optimal at voltages of 35–70 V, with stable speed growth and no droplet splashing.

3. Droplet Volume Influence: The droplet movement speed initially increases and then decreases with increasing volume. When the volume is < 50 µl, pinning forces dominate, making droplets difficult to drive. When the volume is 50–150 µl, the acoustic energy density absorbed by droplets gradually increases, with speed peaking at 53.5 mm/s (around 150 µl). When the volume is > 150 µl, the increase in droplet mass leads to a decrease in energy density, gradually reducing speed.

4. Optimal Frequency and Energy Matching: 650 kHz is the optimal excitation frequency, with an impedance of 114 Ω and maximum amplitude at this frequency. Energy analysis shows that the dimensionless energy density absorbed by droplets initially increases and then decreases with increasing radius, peaking at a radius of 10 mm (corresponding to volumes of 140–160 µl). This matches the experimental result of "maximum speed for 150 µl droplets", verifying the correctness of the theoretical model.

Recommended Voltage Amplifier: ATA-2022B

Figure: ATA-2022B High-Voltage Amplifier Specifications

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