Electrowetting
Encyclopedia
Electrowetting is the modification of the wetting
properties of a surface (which is typically hydrophobic) with an applied electric field
.
and other liquid
s on variably charged surfaces was probably first explained by Gabriel Lippmann
in 1875 and was certainly observed much earlier. Froumkin used surface charge to change the shape of water
drops in 1936. The term electrowetting was first introduced in 1981 to describe an effect proposed for designing a new type of display device. The "fluid transistor" was first investigated by J. Brown in 1984-1988 under NSF Grants 8760730 & 8822197, employing insulating dielectric and hydrophobic layers, immiscible fluids, DC or RF power; and mass arrays of miniature interleaved electrodes with large or matching Indium tin oxide
(ITO)
electrodes to digitally relocate nano droplets and control fluid flow electronically or optically. Electrowetting using an insulating layer on top of the bare electrodes was 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, rather than along a line 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 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, and Digital PCR by Life Technologies
and Sequenom
. 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.
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 tend 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 Gibbs 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.
The simplest derivation of electrowetting behavior is given by considering its thermodynamic model. While it is possible to obtain a detailed numerical model of electrowetting by considering the precise shape of the electrical fringing field and how it affects the local droplet curvature, such solutions are mathematically and computationally complex. The thermodynamic derivation proceeds as follows. Defining the relevant surface tensions as:
- The total, electrical and chemical, surface tension between the electrolyte and the conductor
- The surface tension between the electrolyte and the conductor at zero electric field
- The surface tension between the conductor and the external ambient
- The surface tension between the electrolyte and the external ambient
- The macroscopic contact angle between the electrolyte and the dielectric
- The capacitance of the interface, єrє0/t, for a uniform dielectric of thickness t and permittivity єr
- The effective applied voltage, integral of the electric field from the electrolyte to the conductor
Relating the total surface tension to its chemical and electrical components gives:
The contact angle
is given by the Young-Dupre equation, with the only complication being that the total surface energy
is used:
Combining the two equations gives the dependence of θ on the effective applied voltage as:
An additional complication is that liquids also exhibit a saturation phenomena: after certain voltage, the saturation voltage, the further increase of voltage will not change the contact angle, and with extreme voltages the interface will only show instabilities.
However, surface charge is but one component of surface energy, and other components are certainly perturbed by induced charge. So, a complete explanation of electrowetting is unquantified, but it should not be surprising that these limits exist.
It was recently shown that contact angle saturation can be explained if electrowetting is observed as a global phenomena affected by the detailed geometry of the system. Within this framework it is predicted that reversed electrowetting is also possible (contact angle grows with the voltage).
, CYTOP is sold by Asahi Glass Co.
, and Teflon AF is sold by DuPont
.
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
properties of a surface (which is typically hydrophobic) with an applied electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
.
History
The electrowetting behavior of mercuryMercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
and other liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s on variably charged surfaces was probably first explained by Gabriel Lippmann
Gabriel Lippmann
Jonas Ferdinand Gabriel Lippmann was a Franco-Luxembourgish physicist and inventor, and Nobel laureate in physics for his method of reproducing colours photographically based on the phenomenon of interference....
in 1875 and was certainly observed much earlier. Froumkin used surface charge to change the shape of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
drops in 1936. The term electrowetting was first introduced in 1981 to describe an effect proposed for designing a new type of display device. The "fluid transistor" was first investigated by J. Brown in 1984-1988 under NSF Grants 8760730 & 8822197, employing insulating dielectric and hydrophobic layers, immiscible fluids, DC or RF power; and mass arrays of miniature interleaved electrodes with large or matching Indium tin oxide
Indium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
(ITO)
electrodes to digitally relocate nano droplets and control fluid flow electronically or optically. Electrowetting using an insulating layer on top of the bare electrodes was 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, rather than along a line 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 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
Cytonix
Cytonix is an American manufacturing company.The company was founded in 1984 as "Cyto Fluidics" at the University of Maryland's technology incubator to develop materials and applications for a "fluid transistor" concept based on electrowetting and digital microfluidics...
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
Liquavista
Liquavista is a Dutch company founded in 2006 as a spin-out from Philips. It develops colour e-paper video screens that can work with or without a backlight using electrowetting technology...
which was spun out of Philips Research, liquid lenses by Varioptic, and Digital PCR by Life Technologies
Life Technologies
Life Technologies is a global biotechnology company headquartered in Carlsbad, California. It possesses a portfolio of more than 9 million genetic research assays and custom solutions...
and Sequenom
Sequenom
Sequenom is a manufacturer of DNA massarrays, based in San Diego, California, United States. The MassARRAY platform is used for SNP genotyping, methylation detection and quantitative gene expression analysis. Sequenom also manufactures clinical tests, such as SEQureDx, a noninvasive prenatal test...
. 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.
Electrowetting theory
The electrowetting effect has been defined as "the change in solid-electrolyteElectrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...
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 tend 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 Gibbs free energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
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
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
formed between the conductor and the electrolyte.
The simplest derivation of electrowetting behavior is given by considering its thermodynamic model. While it is possible to obtain a detailed numerical model of electrowetting by considering the precise shape of the electrical fringing field and how it affects the local droplet curvature, such solutions are mathematically and computationally complex. The thermodynamic derivation proceeds as follows. Defining the relevant surface tensions as:
- The total, electrical and chemical, surface tension between the electrolyte and the conductor - The surface tension between the electrolyte and the conductor at zero electric field - The surface tension between the conductor and the external ambient - The surface tension between the electrolyte and the external ambient - The macroscopic contact angle between the electrolyte and the dielectric - The capacitance of the interface, єrє0/t, for a uniform dielectric of thickness t and permittivity єr - The effective applied voltage, integral of the electric field from the electrolyte to the conductorRelating the total surface tension to its chemical and electrical components gives:
The contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...
is given by the Young-Dupre equation, with the only complication being that the total surface energy
is used:Combining the two equations gives the dependence of θ on the effective applied voltage as:
An additional complication is that liquids also exhibit a saturation phenomena: after certain voltage, the saturation voltage, the further increase of voltage will not change the contact angle, and with extreme voltages the interface will only show instabilities.
However, surface charge is but one component of surface energy, and other components are certainly perturbed by induced charge. So, a complete explanation of electrowetting is unquantified, but it should not be surprising that these limits exist.
It was recently shown that contact angle saturation can be explained if electrowetting is observed as a global phenomena affected by the detailed geometry of the system. Within this framework it is predicted that reversed electrowetting is also possible (contact angle grows with the voltage).
Materials
For reasons that are still under investigation, only a limited set of surfaces exhibit the theoretically predicted electrowetting behavior. Amorphous fluoropolymers are by far the best electrowetting materials discovered so far, and it has been found that their behaviour can be enhanced by the appropriate patterning. Three types of such polymers are commercially available: FluoroPel hydrophobic and superhydrophobic V-series polymers are sold by CytonixCytonix
Cytonix is an American manufacturing company.The company was founded in 1984 as "Cyto Fluidics" at the University of Maryland's technology incubator to develop materials and applications for a "fluid transistor" concept based on electrowetting and digital microfluidics...
, CYTOP is sold by Asahi Glass Co.
Asahi Glass Co.
is a Japanese manufacturing company. It is one of the core Mitsubishi companies.Founded in 1907 by Toshiya Iwasaki, the second son of the second president of the original Mitsubishi Zaibatsu. It was the first Japanese producer of sheet glass. Asahi Glass Co...
, and Teflon AF is sold by DuPont
DuPont
E. I. du Pont de Nemours and Company , commonly referred to as DuPont, is an American chemical company that was founded in July 1802 as a gunpowder mill by Eleuthère Irénée du Pont. DuPont was the world's third largest chemical company based on market capitalization and ninth based on revenue in 2009...
.
Applications
Electrowetting is now used in a wide range of applications from modular to adjustable lenses, electronic displays (e-paper) and switches for optical fibers. Electrowetting has recently been evoked for manipulating Soft Matter particularly, suppressing coffee stain effect . Furthermore, filters with Electrowetting functionality has been suggested for cleaning oil spills and separating oil-water mixtures .External links
- Autofocus Liquid Lens working on electrowetting principle
- Fan-TASY Lab at National Chiao Tung University
- Wheeler Digital Microfluidics Group at the University of Toronto
- Electrowetting at the University of Cincinnati.
- Digital Microfluidics at Duke University
- Physics of Complex Fluids at University of Twente
- Diagram explaining electrowetting
- Progress with electrowetting displays
- Electrowetting flexible display at UC NanoLab, University of Cincinnati
- Liquidvista Low Frequency Electrowetting 6.2-inch Display