Photoelectrowetting
Photoelectrowetting is a modification of the wetting properties of a surface using incident light.
Working principle
Whereas ordinary electrowetting is observed in surfaces consisting of a liquid/insulator/conductor stack, photoelectrowetting can be observed by replacing the conductor with a semiconductor 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 and charge-coupled devices. Replacing the conductor with a semiconductor results in asymmetrical electrowetting behavior, depending on the semiconductor doping type and density.Incident light above the semiconductor's band gap creates photo-induced carriers via electron-hole pair generation in the depletion region of the underlying semiconductor. This leads to a modification of the capacitance of the insulator/semiconductor stack, resulting in a modification of the contact angle 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-Lippmann equation.
The figure illustrates the principle of the photoelectrowetting effect. At zero bias the conducting droplet has a large contact angle if the insulator is hydrophobic. As the bias is increased the droplet spreads out – i.e. the contact angle decreases. In the presence of light the droplet spreads out more due to the reduction of the thickness of the space charge region at the insulator/semiconductor interface.