Electrowetting

Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field. 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 a bare electrode 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. Electrowetting can be demonstrated by replacing the metal electrode in the EWOD system by a semiconductor. Electrowetting is also observed when a reverse bias is applied to a conducting droplet (e.g. mercury) which has been placed directly onto a semiconductor surface (e.g. silicon) to form a Schottky contact in a Schottky diode electrical circuit configuration – this effect has been termed ‘Schottky electrowetting’. In this case, the space charge region at the droplet-semiconductor interface plays the role of the insulator in the EWOD. 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. The electrowetting effect has been defined as 'the change in solid-electrolyte contact angle due to an applied potential difference between the solid and the electrolyte'. The phenomenon of electrowetting can be understood in terms of the forces that result from the applied electric field. The fringing field at the corners of the electrolyte droplet tends to pull the droplet down onto the electrode, lowering the macroscopic contact angle and increasing the droplet contact area. Alternatively, electrowetting can be viewed from a thermodynamic perspective. Since the surface tension of an interface is defined as the Helmholtz free energy required to create a certain area of that surface, it contains both chemical and electrical components, and charge becomes a significant term in that equation. The chemical component is just the natural surface tension of the solid/electrolyte interface with no electric field. The electrical component is the energy stored in the capacitor formed between the conductor and the electrolyte.