충전된 액적의 전기영동 현상에 대한 수치해석 기반의 전기역학적 연구

Abstract

The droplet contact charging phenomenon(ECD) has been studied as novel microfluidics research subject. Especially, ECD has been verified as new digital microfluidic technology to overcome drawbacks of EWOD(electrowetting on dielectric) and DEP(dielectrophoresis). We found two novel phenomena causing unsteady behavior of droplet in the contact charge electrophoresis(CCEP) of a water droplet. Those are the positive and negative charging difference and the overcharge of a droplet. In this work, we tried to study the origin of that through numerous experiments and numerical analysis for stable droplet movement control in ECD based digital microchip. For comparison between experimental and numerical results, it was necessary to develop a numerical model for the electrostatic analysis by a commercial numerical software COMSOL Multiphysics using appropriate boundary conditions and numerical analysis was conducted for simulating charging difference and overcharge using that model. Through systematic experiments and numerical analysis, we found that the positive and negative charging difference in CCEP of a water droplet is mainly originated from two electrostatic sources. Those are electric field concentration effect by one power supply setting and surface charge of cuvette. The former was excluded by using two power supplies setup and the latter could be reduced by cleaning surfaces of cuvette with IPA(isopropyl alcohol). As a result, we diminished successfully the charging difference. Furthermore, We observed the overcharge of deformed droplet under strong electric field and found two physical effects from deformed droplet. One is surface-increase effect by changing sphere droplet to deformed droplet and the other is electric field concentration effect as lightning rod effect. Through simulations about deformed droplet, we expected the overcharge amount from two physical effects. By providing the phase diagram using dimensionless number, CaE and Oh, we can estimate the overcharge from deformation in any experimental condition without regard to a scale of system. Therefore, the present findings well explain previous experimental results and also provide design guidelines for consistent droplet movement control in contact charge electrophoresis based digital microfluidic systems. Finally, further discussions on the obtained results, its implications, and future work are discussed.