Electrowetting: a model for contact-angle saturation |
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Authors: | V Peykov A Quinn J Ralston |
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Institution: | (1) Ian Wark Research Institute University of South Australia The Levels, SA 5095, Australia e-mail: john.ralston@unisa.edu.au Tel.: +61-8-83023066 Fax: +61-8-83023683, AU |
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Abstract: | Electrowetting (EW) involves the application of an electric potential across a solid–liquid (SL) interface, which modifies
the wetting properties of that interface by reducing the SL surface energy and induces a contact-angle change without altering
either the bulk liquid or solid properties. Reversible contact angles are achieved at low potentials, while the application
of high potentials results in contact-angle saturation and system instabilities. In the present work, an EW system consisting
of a substrate (plate or rod) coated with a thin Au underlayer, a dielectric (parylene) layer of various thickness and a 100-nm
layer of fluoropolymer (Teflon AF1600) is studied both theoretically and experimentally. A theoretical consideration of the
EW effect, taking into account the detailed structure of the electrical double layer, and a model for the saturation of the
contact angle observed at high applied potentials are presented. The theoretical treatment presented here confirms that the
EW effect is, in the most part, due to polarization of the dielectric, as opposed to purely a rearrangement of the double
layer, and explains why no dependence of the EW effect on the electrolyte concentration has been observed. The theory is only
applicable over a limited potential range, and we have developed a model that predicts the potential at which this limit occurs.
This is the potential at which the SL surface energy becomes zero. The limiting potential is dependent only on the bulk liquid
and solid properties and is thus fixed for a given system. Experimentally, the limiting potential corresponds to the onset
of contact-angle saturation, although variations in the exact angle of saturation are not uncommon due to the kinetic effects
involved in a real system as opposed to a strict thermodynamic analysis. The model predicts that for an EW device in which
an aqueous droplet can be forced to completely wet a hydrophobic surface, a surface with basically the same surface energy
as the liquid is required.
Received: 2 January 1998 Accepted: 21 March 1998 |
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Keywords: | Electrowetting theory Contact-angle saturation Parylene dielectric Fluoropolymer AF1600 |
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