Application Cases

Experiment Name: Dielectrophoretic Sorting Based on High-Voltage Amplifier

Research Direction: Droplet Sorting

Test Principle:
In a non-uniform electric field, the positive and negative charges induced on the surface of a dielectric experience different field strengths. When the resultant force acting on the dielectric exceeds a certain threshold, it is pulled toward the direction of the stronger electric field.

Testing Equipment: ATA-7030 High-Voltage Amplifier, High-Pressure Nitrogen Cylinder, Constant Pressure Microfluidic Pump, PC, FEP Tubing, Fluidic Resistor, Dielectrophoresis-Based Microfluidic Chip, Water Purification System, 15 mL Conical Tube.

Figure 1: Dielectrophoretic Droplet Sorting System

Experimental Procedure:

The structure of the dielectrophoresis-based droplet sorting system is shown above in Figure 1. Channels 1, 2, and 3 (yellow) are oil-phase channels, while channels 4, 5, and 6 (blue) are aqueous-phase channels. Droplets are generated using the flow-focusing method via FEP tubing and fluidic resistors. After generation, droplets naturally flow toward outlet 2, which has lower flow resistance. When a high-voltage square wave pulse is applied to the positive electrode, droplets are pulled toward outlet 1, which has higher flow resistance, under the influence of dielectrophoretic force.

For sorting using dielectrophoretic force, no additional fluidic resistors are added at the chip outlets; only the difference in channel length on the chip is utilized to create flow resistance variation. First, droplets are generated to flow exclusively toward outlet 2. The pressure values for the oil and aqueous phases are fixed to ensure uniform droplet diameter. Subsequently, a square wave voltage is applied. Initially, the voltage amplitude is kept constant while varying the frequency to observe whether droplets can be pulled into channel 1. Then, for each voltage value, multiple frequencies are adjusted to assess whether the dielectrophoretic force is effective.

Due to the high uniformity of droplets generated on the chip, they should all be sorted under the same dielectrophoretic force. The droplet sorting rate (ratio of droplets pulled into channel 1 to the total number of droplets) was first investigated. When the oil phase pressure was 150 mbar and the aqueous phase pressure was 45 mbar, the droplet diameter without applied voltage was approximately 63.7 μm, and the droplet generation rate was about 2.333 droplets per second. The variation of sorting rate with voltage frequency for applied voltage amplitudes of 500 V, 800 V, 1000 V, and 1500 V is shown below in Figure 2. It can be observed that droplets could not be pulled at very low frequencies.

Figure 2: Variation of Droplet Sorting Rate with Frequency for Different Voltage Values

Except for 500 V and 2500 V, the droplet sorting rate for other voltages increased with frequency, eventually reaching 100%. However, for 500 V, the sorting rate showed a decreasing trend with increasing frequency, indicating that this voltage is insufficient to generate adequate dielectrophoretic force. When 2500 V was applied, droplets fragmented completely at 80 Hz. During the experiment, it was observed that excessively high voltage or frequency caused droplets to break into smaller sub-droplets. This indicates that proper sorting requires selecting appropriate voltage amplitude and frequency; higher values are not necessarily better. For the 63.7 μm droplets mentioned above, the motion process of a sorted droplet under 800 V and 1000 Hz is shown in Figure 3 below, with the target droplet marked by a red circle. The two black triangles indicate the positions of the gold electrodes. In the Y-shaped structure, the upper channel has lower flow resistance and connects to a waste reservoir, while the lower channel has higher flow resistance and connects to a collection reservoir. Figure B shows the target droplet beginning to deflect under dielectrophoretic force.

Figure 3: Motion Process of a 63.7 μm Droplet at 800 V and 1000 Hz. Time interval: 0.2 s.

Within the pressure range of 0–450 mbar, three different droplet diameters (63.7 μm, 60 μm, and 53.33 μm) were selected by adjusting pressure values. The 53.33 μm droplets corresponded to an oil phase pressure of 450 mbar and an aqueous phase pressure of 35 mbar. Videos were recorded for each condition following the aforementioned method to investigate the approximate relationship between voltage and frequency when droplets first achieved full sorting. The results are shown in Figure 4 below. It can be seen that higher applied voltage amplitudes require lower frequencies to achieve full sorting. For the same voltage, larger droplets require higher frequencies to achieve full sorting.

Figure 4: Relationship Between Applied Voltage Amplitude and Frequency for Full Sorting at Different Droplet Diameters

Role of High-Voltage Amplifier ATA-7030 in This Experiment:
Provides a controllable voltage source applied to the microfluidic chip to analyze the droplet sorting rate under different voltages.

Figure: ATA-7030 High-Voltage Amplifier Specifications and Parameters