Copper Oxide Layers Obtained Via Anodization for Electrowetting on Dielectrics

Electrowetting is a technique which allows to modify the wettability of a surface through the action of an electric field. At the time of its discovery the phenomenon was believed to have no practical use due to the electrochemical reactions occurring on the surface, which in fact limit the overall effect obtainable. In the last few decades the interest toward this technique arose and the first applications appeared. Fundamental to allow such renewed interest was the use of a dielectric hydrophobic layer interposed between the metallic surface and the electrolyte: it is known in this case as Electrowetting on Dielectric (EWOD). Possible applications of this technique include variable focal lenses, microfluidic systems and displays. The latter in particular can find possible wide industrial diffusion due to the development of the Liquavista technology by Samsung. The challenge of the next future is the transfer of the electrowetting technology from rigid and planar displays to flexible and curved ones. Flexible electronics are expected to be the technology of the future and applications include wearable devices, flexible screens and smart labels for packaging.

The present work investigates the possibility of creating compact layers of copper oxide via copper anodization for the EWOD technology. Copper was chosen instead of metals that can be more efficiently anodized due to its ease of deposition by mean of wet techniques. In fact copper can be easily electroless deposited on flexible substrates, while metals like aluminium or titanium need PVD deposition or thermal coupling techniques. Other advantages of copper include high conductivity and good mechanical properties. The first part of the study involves the fundamental study of electrowetting on copper oxide coated with a fluoropolymer. Hysteresis and saturation of contact angle are analysed, as well as the adherence of the behaviour to the Young-Lippman equation. In the second part a flexible substrate is tested for electrowetting on a curved surface. Such specimen is manufactured by electroless plating of copper on PET, anodizing the resulting layer and coating the oxide layer thus obtained with a fluoropolymer. Finally the result is compared with an equivalent sample obtained coupling an aluminium layer with a polymeric substrate and thermally bonding them. The result is then anodized to obtain a compact oxide layer and coated with the fluoropolymer.