Principle of the photoelectrowetting effect

Photoelectrowetting is a modification of the

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. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...

properties of a surface (typically a

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

surface) using incident light.

Working principle

Whereas ordinary

electrowetting Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field. History The electrowetting of mercury and other liquids on variably charged surfaces was probably first ex ...

is observed in surfaces consisting of a

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

/ insulator/ conductor stack, photoelectrowetting can be observed by replacing the conductor with a

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

to form a liquid/insulator/semiconductor stack. This has electrical and optical properties similar to the metal/insulator/semiconductor stack used in metal-oxide-semiconductor field effect transistors (

MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

s) and

charge-coupled device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...

s (CCDs). Replacing the conductor with a semiconductor results in asymmetrical electrowetting behavior (in terms of

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

polarity), depending on the semiconductor doping type and density. Incident light above the semiconductor's

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...

creates photo-induced carriers via electron-hole pair generation in the

depletion region In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...

of the underlying semiconductor. This leads to a modification of the

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...

of the insulator/semiconductor stack, resulting in a modification of the

contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...

of a liquid droplet resting on the surface of the stack in a continuous way which can also be non-reversible. The photoelectrowetting effect can be interpreted by a modification of the

Young Young may refer to: * Offspring, the product of reproduction of a new organism produced by one or more parents * Youth, the time of life when one is young, often meaning the time between childhood and adulthood Music * The Young, an American roc ...

- Lippmann

equation In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...

. The figure illustrates the principle of the photoelectrowetting effect. At zero bias (0V) the conducting droplet has a large contact angle (left image) if the insulator is

. As the bias is increased (positive for a p-type semiconductor, negative for an n-type semiconductor) the droplet spreads out – i.e. the contact angle decreases (middle image). In the presence of light (having an energy superior to the

of the semiconductor) the droplet spreads out more due to the reduction of the thickness of the

space charge Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...

region at the insulator/semiconductor interface (right image).

Optical actuation of MEMS

Photoactuation of

microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

(MEMS) has been demonstrated using photoelectrowetting., A micro

cantilever A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a canti ...

is placed on top of the liquid-insulator-photoconductor junction. As light is shined on the junction, the capillary force from the droplet on the cantilever, due to the contact angle change, deflects the cantilever. This wireless actuation can be used as a substitute for complex circuit-based systems currently used for optical addressing and control of autonomous wireless sensors

Droplet transport

Photoelectrowetting can be used to circulate aqueous solution-based sessile droplets on a silicon wafer covered with

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

and

Teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemou ...

– the latter providing a

surface. Droplet transport is achieved by focusing a laser at the leading edge of the droplet. Droplet speeds of more than 10 mm/s can be achieved without the necessity of underlying patterned electrodes.C. Palma and R.D. Deegan "Droplet Translation Actuated by Photoelectrowetting" Langmuir 34, 3177 (2018). .

References

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External links

Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN) - Centre National de la Recherche Scientifique (CNRS) - University of Lille

The Deegan Group - University of Michigan
Fluid mechanics Microfluidics

Working principle

Optical actuation of MEMS

Droplet transport

See also

References

External links