Principle of the photoelectrowetting effect

Photoelectrowetting is a modification of the

wetting Wetting is the ability of a liquid to displace gas to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. These interactions occur in the presence of either a gaseous phase or ...

properties of a surface (typically a

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

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 Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

/ insulator/

conductor Conductor or conduction may refer to: Biology and medicine * Bone conduction, the conduction of sound to the inner ear * Conduction aphasia, a language disorder Mathematics * Conductor (ring theory) * Conductor of an abelian variety * Cond ...

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

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

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 upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...

field effect transistors (

MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...

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 (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

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

band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

creates photo-induced carriers via

electron-hole pair In solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are fund ...

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 mobil ...

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

capacitance Capacitance is the ability of an object 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 two closely related ...

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

contact angle The contact angle (symbol ) is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interfac ...

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's age is low, often meaning the time between childhood and adulthood Music * The Young, an America ...

Lippmann Lippmann is a German surname. Notable people with the surname include: * Alexandre Lippmann (1881–1960), French Olympic champion fencer * Bernard Lippmann, American physicist, known for the Lippmann–Schwinger equation * Edmund Oscar von Lippm ...

equation In mathematics, an equation is a mathematical 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 ...

. 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 MEMS (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices ...

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

cantilever A cantilever is a rigid structural element that extends horizontally and is unsupported at 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 cantilev ...

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 , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

and

Teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off from ...

– 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