An Electrohydrodynamic Phase-Field Model for Contact Angle Hysteresis in Electrowetting Pixels: Decoupling Physical...

Area of Science:

• Fluid dynamics
• Microfluidics
• Electrowetting

Background:

• Contact angle hysteresis (CAH) limits precise two-phase flow manipulation in micro-confined electrowetting pixels.
• Understanding the non-equilibrium processes driving CAH is crucial for advancing microfluidic devices.

Purpose of the Study:

• To establish a high-fidelity electrohydrodynamic (EHD) phase-field simulation framework to elucidate CAH.
• To investigate the physical mechanisms of CAH, including pinning and charge trapping.
• To evaluate the effectiveness of different driving strategies, such as AC and DC waveforms and overdrive pulses, in mitigating CAH.

Main Methods:

• Coupling Navier-Stokes equations with molecular kinetic theory (MKT) to model energy dissipation at the three-phase contact line (TCL).
• Integrating charge transport kinetics into the EHD phase-field simulation framework.
• Simulating droplet dynamics under various electrical driving conditions (DC, AC, overdrive pulses).

Main Results:

• CAH is driven by physical pinning and interfacial charge trapping, with charge trapping significantly impacting residual displacement.
• Alternating current (AC) waveforms reduce charge accumulation and hysteresis compared to direct current (DC) waveforms.
• An overdrive strategy using Maxwell stress pulses reduced the time to reach 90% of the steady-state target from 19.6 ms to 7.4 ms.

Conclusions:

• The developed EHD phase-field model provides a comprehensive theoretical basis for understanding and controlling CAH.
• AC waveforms and overdrive strategies offer effective methods for optimizing active driving in microfluidic systems.
• This research provides design criteria for enhancing droplet manipulation in optofluidic and digital microfluidic applications.