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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 [10] (see [13] 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 [2], video speed electronic paper [8], and `lab-on-a-chip' devices [7].
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 [19]. 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.

References

[1] B. Berge. ’Electrocapillarit’e et mouillage de films isolants par l'eau (including an english translation). Comptes Rendus de l'Acad’emie des Sciences de Paris, S’erie II, 317:157--163, 1993.

[2] B. Berge and J. Peseux. Variable focal lens controlled by an external voltage: An application of electrowetting. European Physical Journal E, 3(2):159--163, 2000.

[3] S. K. Cho and C.­J. Kim. Particle separation and concentration control for digital microfluidic systems. In The 16th Annual IEEE International Conference on MEMS, pages 686--689, Kyoto, Japan, Jan. 2003.

[4] S. K. Cho, H. Moon, and C.­J. Kim. Creating, transporting, cutting, and merging liquid droplets by electrowetting­based actuation for digital microfluidic circuits. Journal of Microelectromechanical Systems, 12(1):70--80, 2003.

[5] A. A. Darhuber, J. M. Davis, S. M. Troian, and W. W. Reisner. Thermocapillary actuation of liquid flow on chemically patterned surfaces. Physics of Fluids, 15(5):10150--10153, 2003.

[6] N. Fortner, B. Shapiro, and A. Hightower. Modeling and passive control of channel filling for micro­ fluidic networks with thousands of channels. In American Institute of Aeronautics and Astronautics (AIAA) 33rd AIAA Fluid Dynamics Conference and Exhibit, Orlando, Florida, June 23--26 2003.

[7] J. Gong, S. K. Fan, and C. J. Kim. Portable digital microfluidics platform with active but disposable lab­on­chip. In Proc. IEEE Conf MEMS, pages 355--358, Maastricht, The Netherlands, Jan. 2004.

[8] R. A. Hayes and B. J. Feenstra. Video­speed electronic paper based on electrowetting. Nature, 425(6956):383--385, 2003.

[9] B. He and J. Lee. Dynamic wettability switching by surface roughness effect. In The 16th Annual IEEE International Conference on MEMS, pages 120--123, Kyoto, Japan, Jan. 2003.

[10] G. Lippmann. Relations entre les ph’enom‘enes ’electriques et capillaires. Ann. Chim. Phys., 5:494-- 549, 1875.

[11] L. Minnema, H. A. Barneveld, and P. D. Rinkel. An investigation into the mechanism of water treeing in polyethylene high voltage cables. IEEE Transactions on Electrical Insulators, 15:461-- 472, 1980.

[12] H. Moon, A. R. Wheeler, R. L. Garrell, J. A. Loo, and C.­J. Kim. On­chip sample preparation by electrowetting­on­dielectric digital microfluidcs for matrix assisted laser desorption/ionization mass spectrometry. In Proceedings of IEEE MEMS, pages 859--862, Miami, Florida, Feb 2005.

[13] F. Mugele and J.­C. Baret. Electrowetting: from basics to applications. Journal of Physics: Con­ densed Matter, 17:R705--R774, 2005.

[14] W. Satoh, M. Loughran, and H. Suzuki. Microfluidic transport based on direct electrowetting. Journal of Applied Physics, 96(1):835--841, 2004.

[15] B. Shapiro, H. Moon, R. Garrell, and C.­J. Kim. Equilibrium behavior of sessile drops under surface tension, applied external fields, and material variations. Journal of Applied Physics, 93, 2003.

[16] S. W. Walker. Modeling, Simulating, and Controlling the Fluid Dynamics of Electro­Wetting On Dielectric. PhD thesis, University of Maryland, College Park, August 2007.

[17] S. W. Walker, A. Bonito, and R. H. Nochetto. Mixed finite element method for electrowetting on dielectric with contact line pinning. accepted to Interfaces and Free Boundaries, 2009.

[18] S. W. Walker and B. Shapiro. A control method for steering individual particles inside liquid droplets actuated by electrowetting. Lab on a Chip, 5:1404--1407, October 2005.

[19] S. W. Walker and B. Shapiro. Modeling the fluid dynamics of electrowetting on dielectric (ewod). Journal of Microelectromechanical Systems, 15(4):986--1000, August 2006.

[20] S. W. Walker, B. Shapiro, and R. H. Nochetto. Electrowetting with contact line pinning: Compu­ tational modeling and comparisons with experiments. Physics of Fluids, 21(10), Oct 2009.

[21] A. R. Wheeler, H. Moon, C.­J. Kim, J. A. Loo, and R. L. Garrell. Electrowetting­based microfluidics for analysis of peptides and proteins by matrix­assisted laser desorption/ionization mass spectrom­ etry. Analytical Chemistry, 76:4833--4838, 2004.