Working Principle
Droplets are formed using the surface tension properties of liquid. For example, water placed on a hydrophobic surface will lower its contact with the surface by creating drops whose contact angle with the substrate will increase as the hydrophobicity increases. However, in some cases it is possible to control the hydrophobicity of the substrate by using electrical fields. This is referred to as Electrowetting on dielectric or EWOD. In thin layers of Teflon AF, FluoroPel V-polymer or CYTOP, for example, while no field is applied the surface will be extremely hydrophobic and a droplet of water will try to 'stay away' from the surface, resulting in a droplet with steep walls. When a field is applied, a polarized hydrophilic surface is created, and the water droplet tries to 'get closer' to the surface, resulting in much more spread out droplet. By controlling the localization of this polarisation it is possible to control the displacement of the droplet.
Read more about this topic: Digital Microfluidics
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