Application Cases

Experiment Name: Application of Voltage Amplifier in Functional Research of Droplet Microfluidic Chips

Research Direction: Microfluidic Biochips

Test Objective:
Droplet microfluidic technology enables droplet generation within microchannels, allowing precise control over droplet size and generation frequency. By integrating chip structural design with external control conditions, droplets can be manipulated in diverse ways to meet the requirements of various research and application scenarios. In the biomedical field, droplets hold significant application value as independent microreactors with nanoscale volumes, facilitating high-throughput operations and making them highly suitable for biochemical detection and analysis in large sample volumes.

Testing Equipment: ATA-2042 Voltage Amplifier, Signal Generator, Chip, Syringe Pump, Polarizing Microscope, Camera, etc.

Experimental Procedure:
First, liquid crystal droplets were generated using a droplet preparation chip. These droplets were then transported via flow and captured within the array structure of a liquid crystal droplet trapping chip. Subsequently, electrical control studies were conducted based on this liquid crystal droplet array structure chip.

Figure: Experimental System (a) Flow-focusing chip; (b) Liquid crystal droplet array electrical control chip; (c) Assembled experimental system; (d) Array structure within the channel

The liquid crystal droplet preparation chip was fabricated by bonding a PDMS structural layer to a clean glass slide, as shown in Figure (a) above. For the array trapping chip, after processing the PDMS chip structure and ITO electrodes, the two parts needed to be bonded into a complete chip. Both parts were treated with plasma cleaning for 3 minutes, aligned according to marked reference points, ensuring the PDMS array trapping structure area  was completely positioned between the two electrodes on the underlying ITO glass substrate (as shown in Figure (d) above). Due to the presence of the ITO layer on the glass substrate, which affected bonding strength, the chip was placed on a hot plate for heating and subjected to heavy pressure for 1–2 hours after bonding to enhance bond  robustness . After bonding, tubing  were inserted into the chip's ports and sealed with PDMS mixed  adhesive) . Conductive tape was attached to the designated contact points on the electrodes. The physical chip is shown in Figure (b) above. A syringe fixed on the syringe pump was connected to the chip's tubing via a needle. The chip was placed under the microscope's field of view. The output port of the signal generator was connected via wires to the input port of the voltage amplifier. The output port of the amplifier was connected via wire clips to the conductive tape attached to the chip electrodes. Tape was used to secure the wire clips and the chip to the stage to prevent movement during the experiment that could interfere with observation and recording. Finally, the camera was connected to the external port  of the microscope. The completed system is shown in Figure (c) above.

Following hydrophilic modification treatment of the liquid crystal droplet preparation chip, the resulting droplets are shown in Figures (a) and (b) below. Trapping of liquid crystal droplets was driven by negative pressure, with the syringe pump set to withdrawal mode at a flow rate of 200 μL/h. After the droplets were trapped and filled the entire capture array (Figure (c) below), the remaining liquid crystal droplet suspension in the reservoir was aspirated and removed using a pipette, rinsing 3–5 times with SDS solution during the process. After removing excess droplets, the flow rate was reduced to 3 μL/h, and PBS buffer was introduced,  continuously  withdrawing at a low flow rate for 10 minutes to stabilize the conformational state of the liquid crystal droplets within the trapping chamber (Figure (d) below).

Figure: Liquid Crystal Droplet Preparation

After the conformation of the liquid crystal droplets in the array stabilized, electrical signals were applied to the chip electrodes. The output waveform from the signal generator was a sine wave. The voltage amplifier's amplification factor was set to 50. By adjusting the amplitude and frequency on the signal generator (electrical signal amplitude starting from 1 V...), the conformational changes of the droplets were observed and recorded to investigate the relationship between liquid crystal droplet conformation and electrical signal parameters.

Experimental Results:

Figure: (Liquid Crystal Droplet States) Without an applied electric field, the liquid crystal droplet exhibits  radially symmetric center  (a), and its conformation under a polarizing microscope appears as a cross structure (c). When the electric field strength is 0.25 V/μm (50 kHz), the liquid crystal droplet becomes axially symmetric , with the defect shifting along the symmetry axis (b), and the corresponding polarizing microscope image is shown in (d).

Liquid crystal droplets undergo conformational transitions under the influence of an electric field. After droplet trapping, the flow rate was reduced, diminishing the influence of flow, and the droplets returned to the cross conformation with a central defect under the influence of anchoring energy. Turning on the signal generator and voltage amplifier established an electric field within the channel along the direction of the main flow channel. At initial field E=0, the surfactant SDS adsorbed on the droplet surface, creating a certain anchoring energy. At this point, the liquid crystal molecules were arranged radially within the droplet (schematic shown in Figure (a) above), with the defect located at the geometric center, and the droplets exhibited a cross conformation under polarized light (Figure (c) above).

Upon applying an electric field, the free energy of the liquid crystal droplet comprises surface anchoring energy, elastic free energy, and electric field energy. When the voltage amplitude is too low, the effect of electric field energy is insufficient to overcome the free energy dominated by surface anchoring and elastic energy, so the droplet maintains its previous unpowered state, with essentially no conformational change. As the voltage amplitude increases, the electric field begins to influence the droplet's free energy, but it still must overcome surface anchoring and elastic energy, resulting in a shift of the central defect. Figure (b) above shows the condition when the signal generator voltage amplitude is 9 Vp-p, corresponding to an in-channel electric field strength of 0.25 V/μm (calculated as: voltage amplitude 9 V × 50 amplifier gain, divided by electrode spacing 1800 μm). Here, the droplet defect shifts from the center towards one end of the conformational symmetry axis, forming an escaped radial configuration. The polarizing micrograph of the liquid crystal droplet is shown in Figure (d) above.

ITO electrodes in contact with the solution within the channel are prone to electrolysis under low-frequency conditions. Therefore, the electrical signal frequency used in this study was chosen to be ≥ 1 kHz. The voltage amplitude was 5 Vp-p, applied for 20 s. The electrical signal frequency was set to 1 kHz, 10 kHz, 50 kHz, 100 kHz, 500 kHz, and 1 MHz. The liquid crystal droplets responded differently at different frequencies. The orientation shift of the liquid crystal droplets increased with increasing frequency (Figures (a~f) below). The corresponding relationship between the shift distance and the voltage frequency is shown in Figure (g) below.

Figure: (a~f) Changes in liquid crystal droplets at different frequencies. Scale bar in the figure is 50 μm. (g) Variation of the percentage ratio of defect shift distance  from the center to the droplet radius with voltage frequency

According to the Debye equation, the frequency of the electric field indirectly affects the dielectric constant components of the liquid crystal molecules within the field. Changes in the dielectric constant primarily reflect changes in the orientational order and symmetry of the director within the liquid crystal. The frequency variation ultimately manifests as the shift of the internal defect within the liquid crystal droplet under the electric field. Furthermore, according to the leaky dielectric model (a model describing droplet behavior in immiscible media under an electric field), a finite charge density exists at the interface between the droplet and the medium. Under the applied electric field, the interaction between these surface charges and the field generates electrokinetic effects, ultimately leading to fluid motion parallel to the droplet surface. The two media (droplet and immiscible solution) have different conductivities, viscosities, and dielectric constants, causing the flow direction around the droplet to be either outward or inward, resulting in directional differences. The frequency influences the deflection angle of the liquid crystal molecules, thereby inducing the deflection of the liquid crystal droplet.

Figure: ATA-2042 High-Voltage Amplifier Specifications and Parameters