Electrowetting-on-dielectric characteristics of ZnO nanorods

Abstract

Herein, we report the electrowetting-on-dielectric (eWoD) characteristics of Zno nanorods (nRs) prepared via the hydrothermal method with different initial Zn 2+ concentrations (0.03, 0.07, and 0.1 M). Diameter of the resultant ZnO NRs were 50, 70 and 85 nm, respectively. Contact angle (CA) measurements showed that the Teflon-coated ZnO NRs with diameters of 85 nm prepared from the 0.1 M solution had the highest CA (137°). During the EWOD studies, on the application of a voltage of 250 V, the water CA decreased to 78°, which demonstrates the potential application of this material in EWOD electronics. Furthermore, we explained the relationship between the applied voltage and CA based on the substrate nanostructures and our newly developed NR-on-film wetting model. In addition, we further validated our model by introducing the homo-composite dielectric structure, which is a composite of thin layered ZnO/Teflon and nano-roded ZnO/Teflon. Electrowetting (EW) is defined as the decrease in contact angle (CA) when a sufficiently large driving voltage is applied to the interface of solid/liquid. In direct EW, a voltage is used between a liquid and an electrode. Charges and dipoles redistribute at the interface between the liquid and solid, which changes the surface tension, leading to a decrease in the CA of the liquid droplets. However, direct EW can electrolyze the liquid before any change in the water contact angle (WCA), which limits its application 1. To overcome this issue, electrowetting-on-dielectric (EWOD) can be used. In this technique, a dielectric material is sandwiched between the liquid of interest and the underlying electrode (Fig. 1). EWOD is very effective for significantly changing the CA, because the dielectric layer effectively blocks the process of charge transfer at the liquid/electrode interface, which eliminates unwanted electrolysis 2. Therefore, EWOD is used to facilitate EW on solids, for practical applications. Although EWOD devices permit a large CA change, they generally need a high driving voltage. The dielectric layer blocks the transfer of electrons, while sustaining high voltage at the interface, which results in an electric double layer (EDL) formation in the presence of the applied voltage. When a hydrophobic dielectric material is used, the large initial WCA can result in a large change in the WCA 3. The advantages of EWOD include low power consumption, large WCA change, and fast response. It is therefore promising for diverse applications, including lab-on-chips 4 , liquid lenses 5 , optical waveguides 6 , electronic displays 7, 8 , microprism arrays 9 , and smart microbatteries 10. SiO 2 is a highly popular material for EWOD studies 11 because of its excellent compatibility with the microelectronic industry. In our previous study 12 , we investigated the EWOD properties of electrospray-deposited SiO 2 layers and found that the WCA of the optimal SiO 2 layer underwent a hydrophobic-to-hydrophilic transition under a driving voltage of 150 V in air. A novel EWOD mechanism, wherein the ...

3.1: Invited Paper: Electrowetting display: Towards full‐color video reflective display

“…Inorganic dielectric materials were attempted to use in electrowetting systems due to the high dielectric constant, such as ZnO and Al2O3 [14][15][16][17][18] . Yet, producing an inorganic layer was complicated and not comparable to the mass production of EWD.…”

67‐3: Invited Paper: Progress in Electronic Paper Displays: Color Video Electrofluidic Display Technology

“…The I-V test showed the IPP also improved the isolating property of the composite dielectric layer and decreased the leakage current after aging tests. Besides, related researchers have reported EFD properties of some potential dielectric materials and multilayer insulators, such as teflon-coated electrosprayed silica layers [14] , ZnO nanorods [15] , carbon nanotubes, SU8 photoresist, Parylene C, and so on [5] .…”

Application of Micro/Nanoporous Fluoropolymers with Reduced Bioadhesion in Digital Microfluidics

“…It was shown that structured superhydrophobic surfaces exhibit a higher droplet movement speed [31] and higher cleaning efficiency than unstructured hydrophobic surfaces for different (bio-)particles with EWOD, demonstrating that it is advantageous to use these structured surfaces in DMF [32]. Various surface structures on the nanoscale [32][33][34][35][36][37][38][39][40][41], the microscale [42][43][44][45] and on both nanoscale and microscale [46,47] were examined in EWOD. These surfaces consisted of an insulating layer and a repellent low surface energy one.…”

“…These surfaces consisted of an insulating layer and a repellent low surface energy one. Materials used for insulation include silicon dioxide [33,35,37,44,47], photoresists, such as, for example, SU-8 [42] and zirconium oxide [39,45], while commonly used hydrophobic materials are fluoropolymers, such as C 4 F 8 [33,35,37,44,47], Teflon [39,42], Cytop ® [32], or alkyl-/fluorosilanes [32,38,45] and carbon [36]. There are also systems that consist of one single layer that simultaneously serves as an insulating and repellent layer, examples of which include coatings with carbon nanotubes [34], graphene [46] and polypyrrole [48].…”