Droplet Motion in Electrowetting On Dielectric (EWOD) Devices
Brief History of Electrowetting
In 1875, Gabriel Lippmann demonstrated, through rigorous theory and experiments, a relationship between electrical and surface tension phenomena  (see  for an English translation). This relationship allows for controlling the shape and motion of a liquid meniscus (i.e. liquid-gas interface) through the use of an applied voltage. The liquid surface changes shape when a voltage is applied in order to minimize the total energy of the system (i.e. the sum of the surface tension energy and electrical energy). In his seminal paper, he showed applications of this effect ranging from allowing sensitive voltage measurements to a working electro-capillary motor. Today, this effect is known as electrowetting and has seen a resurgence in modern applications in the area of Micro-Electro-Mechanical Systems (MEMS). Some of these applications include cell phone camera lenses , video speed electronic paper , and `lab-on-a-chip' devices .
Applications of EWOD
The ability to manipulate fluids at the micro-scale is an important tool in the area of bio-medical applications. Micro-fluidic devices often exploit surface tension forces to actuate or control liquids [9, 5, 6] by taking advantage of the large surface-to-volume ratios found at the micro-scale. This paper is concerned with developing a mixed finite element method to simulate droplet motion in a micro device driven by Electrowetting-On-Dielectric (EWOD) [4, 15, 1, 11], which consists of two closely spaced parallel plates with a droplet bridging the plates and a grid of square electrodes embedded in the bottom plate . Applying voltages to the grid allows the droplet to move, split, and rejoin within the narrow space of the plates. Applications range from mass spectrometry [21, 12], to `lab-on-a-chip' [14, 7], and particle separation/concentration control [3, 18].
Experiments and Simulations of EWOD
Here we show comparisons between our simulation method and experiments. The thick curve is the simulated droplet boundary, where grayed regions correspond to pinned parts of the boundary. See the references in [16, 17, 19, 18, 20] for more details on the modeling and numerical method.
- A water droplet splitting into two drops.
Experimental data courtesy of CJ Kim and Sung Kwon Cho at UCLA.
- A moving water droplet (MPG). Experimental data courtesy of CJ Kim and Jian Gong at UCLA.
- A moving glycerin droplet (MPG). Experimental data courtesy of CJ Kim and Jian Gong at UCLA.
- Two water droplets merging (MPG). Experimental data courtesy of CJ Kim and Jian Gong at UCLA.
- A glycerin droplet splitting into two drops (MPG). Experimental data courtesy of CJ Kim and Jian Gong at UCLA.
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