Electrowetting

Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field.

The electrowetting behavior of mercury and other liquids on variably charged surfaces was probably first explained by Gabriel Lippmann in 1875 and was certainly observed much earlier. A.N. Frumkin used surface charge to change the shape of water drops in 1936. The term electrowetting was first introduced in 1981 by G.Beni and S.Hackwood to describe an effect proposed for designing a new type of display device for which they received a patent. The use of a "fluid transistor" in microfluidic circuits for manipulating chemical and biological fluids was first investigated by J. Brown in 1980 and later funded in 1984-1988 under NSF Grants 8760730 & 8822197, employing insulating dielectric and hydrophobic layer(s) (EWOD), immiscible fluids, DC or RF power; and mass arrays of miniature interleaved (saw tooth) electrodes with large or matching Indium tin oxide (ITO) electrodes to digitally relocate nano droplets in linear, circular and directed paths, pump or mix fluids, fill reservoirs and control fluid flow electronically or optically. Later, in collaboration with J. Silver at the NIH, EWOD-based electrowetting was disclosed for single and immiscible fluids to move, separate, hold and seal arrays of digital PCR sub-samples.

Electrowetting using an insulating layer on top of the bare electrodes was later studied by Bruno Berge in 1993. Electrowetting on this dielectric-coated surface is called electrowetting-on-dielectric (EWOD) to distinguish it from the conventional electrowetting on the bare electrode. Microfluidic manipulation of liquids by electrowetting was demonstrated first with mercury droplets in water and later with water in air and water in oil. Manipulation of droplets on a two-dimensional path was demonstrated later. If the liquid is discretized and programmably manipulated, the approach is called "Digital Microfluidic Circuits" or "Digital Microfluidics". Discretization by electrowetting-on-dielectric (EWOD) was first demonstrated by Cho, Moon and Kim, completing the four basic digital microfluidic functions of creating, transporting, dividing and merging droplets on chip by electrowetting

Since then, a large number of applications based on electrowetting have been demonstrated. Currently five companies are at the forefront in commercializing electrowetting-based applications based on Cytonix and Berge's later research: Clinical diagnostics by Advanced Liquid Logic which was spun out of Duke University, electronic paper by both Gamma Dynamics, which was spun out of the University of Cincinnati, and Liquavista which was spun out of Philips Research, liquid lenses by Varioptic,Digital PCR by Life Technologies and Sequenom and reflective outdoor displays by Etulipa. In some of these applications, electrowetting allows large numbers of droplets to be independently manipulated under direct electrical control without the use of external pumps, valves or even fixed channels. In e-paper and liquid lenses, droplets are manipulated in-place whereas in clinical diagnostics applications, droplets are moved around on the platform.

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