Electrowetting

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With electrowetting ( English electrowetting ) is referred to the physical phenomenon in which an electric field by varying the surface tension of a liquid ( electrocapillarity leads) to the liquid wets a subject stronger. Gabriel Lippmann provided the physical explanation for this in 1875.

Construction of an electrowetting cell

Electrowetting effect (L - conductive liquid, I - hydrophobic insulator layer, S - substrate).

Basically, an electrowetting cell always consists of a type of capacitor in which the space between the electrodes is filled with a hydrophobic liquid (e.g. oil) and water, with one of the electrodes having a hydrophobic coating. Without a field, the oil forms a film over the coated electrode; with a field, the water displaces the oil film, as the applied field cancels the polarization of the dipoles in the water surface.

Contact angle of liquids

The contact angle decreases with increasing voltage U between the liquid drop and the electrode under the contact surface according to the Lippmann equation:

Φ : contact angle of electrowetting
Φ 0 : contact angle without electrical voltage
ε : permittivity of the dielectric between electrode and liquid
d : layer thickness of the dielectric
σ : surface tension of the liquid

application areas

Different electro-optical components can be realized through the design of the liquid space, the electrodes and the color of the oil:

In microfluidics, electrowetting can stabilize liquid surfaces that are otherwise dynamically unstable due to the capillary forces on the contact surface.

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