Electrostatic Origins of the Positive and Negative Charging Difference in the Contact Charge Electrophoresis of a Water Droplet

Abstract

The positive and negative charging difference in the contact charge electrophoresis of a water droplet suspended in oil was investigated to find out the origin of this charging difference. Through numerous experiments and numerical analysis, the charging difference has been found to be mainly originated from electrostatic sources. Two electrostatic sources were found in the present experimental setup, and by excluding those two sources the charging difference was successfully diminished. 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.

Effect of Deformation on Droplet Contact Charge Electrophoresis

“…An experimental setup similar to the previous one was used. 36 A transparent square polystyrene cuvette (12 × 12 × 45 mm 3 ) containing silicone oil (Shin-Etsu KF-96 100 cSt) and an electrode assembly was placed between the light emitting diode light source and the observation digital camera (Canon EOS 100D) with a macro lens (Canon EF-S 60 mm f/2.8 USM Macro Lens). Two identical rectangular copper electrodes (9 × 60 mm 2 with 0.5 mm thickness) and an acrylic electrode holder (20 × 20 × 5 mm 3 ) make up the electrode assembly.…”

“…A series of experiments were conducted to investigate the effect of deformation by changing the size of the droplet (0.1−3 μL) and electric field strength (1.4−7.1 kV/cm). The video files (1280 × 780 pixels, 50 frames/s) recorded by the observation digital camera were processed in the same procedure as in the previous work 36 to extract the horizontal velocity and size of a moving droplet.…”

“…42 To rule out the effects of surrounding static electricity, positive and negative droplet charge averages were used for the droplet charge data. 36 The droplet charge was estimated based on the force balance between Coulomb force F e = QE 0 (droplet charge Q under uniform electric field E 0 ) and Stokes drag F d = −6πμaU 0 (the droplet of radius a moving in the suspending medium of viscosity μ at constant velocity U 0 ). 49 From the force balance (F e = −F d ), the droplet charge Q can be estimated as Q = 6πμaU 0 /E 0 .…”

“…To take into account the actual electrostatic environment of the experimental system, the same full 3D model as a previous work was used (consult the reference for numerical details). 36 The procedure for calculating deformed droplet charge is illustrated in Figure 1b. The droplet photo at the moment of contact was selected for each experimental condition.…”

“…3,34 Later, the contact charge was analyzed by normalizing it to the theoretical maximum charge derived by the perfect conductor theory called Maxwell charge. 1,18,35,36 For accurate charge measurement, a rigorous study has been conducted on the drag of a moving droplet 37 and high-resolution electrometer was used to directly measure the droplet charge. 35,38 In many reports, the charge acquired by aqueous droplets was less than the Maxwell charge.…”