This study develops a driving system for an electrowetting-on-dielectric (EWOD) device comprising a 9 V battery, an ATmega8535 microprocessor, a DC/DC converter, two regulator ICs and a switch circuit. The driving system greatly improves the portability of the EWOD device and is capable of generating a square wave with voltages ranging from 50~100 V(pp) and frequencies in the range 1~5 kHz. A series of experimental and numerical investigations are performed to investigate the effect of the conducting electrode geometry on the droplet velocity in the EWOD device. Three different electrode configurations are considered, namely a linear array of square electrodes, a series of interdigitated electrodes having either two or three fingers, and a series of interdigitated electrodes having five or six fingers. The experimental results show that the corresponding droplet velocities are 7.25 mm/s, 8.17 mm/s and 7.82 mm/s, respectively. The simulation results indicate that the pressure difference induced within the droplets actuated by the square, interdigitated (2323) and interdigitated (5656) electrodes has a value of 15.5 N/m², 262 N/m² and 141.1 N/m², respectively. The corresponding droplet velocities are 33.8 mm/s, 72.7 mm/s and 64.5 mm/s, respectively. Overall, the experimental and numerical results indicate that the interdigitated (2323) electrode optimizes the transportation of the droplets in the EWOD device. The improved droplet velocity obtained using this particular electrode configuration is attributed to an increased length of the contact line between the droplet and the actuating electrode, which in turn increases the driving force.

Electrowetting on dielectric (EWOD) moving fluid by surface tension effects offers some advantages, including simplicity of fabrication, control of minute volumes, rapid mixing, low cost and others. This work presents a numerical model using a commercial software, CFD-ACE+, and an EWOD system including a microfluidic device, a microprocessor, electric circuits, a LCD module, a keypad, a power supply and a power amplifier. The EWOD model based on a reduced form of the mass conservation and momentum equations is adopted to simulate the fluid dynamics of the droplets. The EWOD device consists of the 2 x 2 mm bottom electrodes (Au/Cr), a dielectric layer of 3,000 A nitride, 500 A Teflon and a piece of indium tin oxide (ITO)-coated glass as the top electrode. The complete EWOD phenomenon is elucidated by comparing simulation with the experimental data on droplet transportation, cutting and creation. In transportation testing, the speed of the droplet is 6 mm/s at 40 V(dc). In addition, the droplet division process takes 0.12 s at 60 V(dc) in the current case. Finally, a 347 nl droplet is successfully created from an on-chip reservoir at 60 V(dc).

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