Research Of Wettability On Nanoarrays Surface

Liquid wetting behaviors at micro-nano scale have received continuous attention of both basic science and applied technology such as:microfilud, biomimic, surface coating, anti-fogging, fog-harvesting, ink-jet printing, biosensor and so on. For this study, an observing method with both high temporal resolution and high spatial resolution is necessary.The SEM and AFM have high spatial resolution, but the shape distortion caused by liquid evaporating will become serious in high vaccum SEM observing eviroment or long-time AFM imaging. Based on this background, this thesis presents a new method for observing UV-curable resist liquid morphologies on nanostructures at micro-nano scale by UV curing. The liquid acrylated materials were transferred onto the nanostructured surfaces by a double transfer method. Further, they were frozen (solidified) by UV-light exposure. The morphology of the solidified liquid reflects the original liquid-state profile because the UV-curing shrinkage of the acrylated material was only a few percent in volume so that it would not largely alter the liquid morphology. The cured samples were then examined with high resolution SEM or AFM measurement. This proposed method allows for the observation of liquid behavior down to sub-20nm scale with more subtle details such as nano-menisci between nanogrooves, and liquid bridges covering nanostructures.Based on these observations, we found that the wetting behavior of the liquid strongly depends on the surface properties and geometries of the underlying nanostructures:poor wettability liquid and low aspect ratio nanostructures are in favor of the liquid contracting into droplets. Experimental results agree with the prediction of a simple surface energy variation model. They indicate that macroscopic wetting behaviors of liquid are preserved as dimensions go down to100nm. Based on these reasearchs, the requirements for UV nanoimprint with no residual layer were deduced and nanorings array with ultra small line width was fabricated in this thesis.The UV curing method was used for research on micro-nano condensed water droplets on nanostructured surface. The water-insoluble acrylated materials were used to cover the water droplets, and then cured by UV light. The wetting morphologies of water droplets were frozen as holes in the cured acrylated materials after the UV-cured material were separated from the nanostructured surface and the embedded water evaporated. The frozen morphologies can be observed by SEM. Based on this method, wetting behavior of condensed water droplets on different nanoarrays and in different UV-curable liquids was studied in this thesis. The study shows that whether water droplets adopt Wenzel or Cassie-Baxter wetting type depends on the structure parameter and surface free energy of nanostructures, surface tension of UV resist liquid. The higher the surface tension of the UV-curable liquid is, the more wetting anisotropy water droplets exhibit on nanogrooves. While hydrophobic and high aspect-ratio nanoarrays were favor of water droplets adopting Cassie-Baxter wetting type. The experimental results verify the prediction of the free energy model. This study is helpful for designing high-performance superhydrophobic surface.In recent years, self-cleaning superhydrophobic surface has drawn a lot of attention.It has important application in anti-fogging, self-cleaning and oil-water separation. Given this, we fabricated superhydrophobic surface on silicon substrate and fluoropolymer surface respectively by micro-nano machining technology including nanoimprint based on the wetting research of condensed water droplets on nanoarrays. The research shows that static contact angle of water droplet on superhydrophobic surface of silicon substrate is158.3°and the rolling angle is less than5°. This is the result of the combination of high aspect ratio sharp-top nanogrooves and low free energy surface modified by fluorosilane.This sharp-top nanogrooves also have antireflective effect in wide spectrum that is of great significance in fabricating self-cleaning and high efficiency silicon solar cells. For fluoropolymer, we prepared superhydrophobic surface with151.2°contact angle of water droplet on fluoropolymer film by one-step hot nanoimprint through reasonable structure design. This method has many advantages, such as low-cost, high efficiency, large area, so this method has bright application prospect in preparation of polymer superhydrophobic surface.Electrowetting is also called electric hydrophilic effect. It means the phenomenon of contact angle decreasing when conductive liquid is in the current state. Electrowetting effect is widely used in microfluidic device, liquid microlens, and color display etc. However, study about electrowetting on nanoarray is little.On the other hand, nanoimprint lithography has been widely used to pattern nanostructures on surface since it is rapid, low-cost and high resolution, and has capability of large area nanostructure preparation. Under this background, this one-step hot nanoimprint method was also used for electrowetting research on fluoropolymer nanoarrays. Using this method, electrowetting properties on different nanoarrays were studied in the thesis. The experiments show that the nanoarray surfaces have larger contact angle change than the smooth fluoropolymer surface under the same voltage while superhydrophobic nanoposts array with high aspect ratio has the largest contact angle change. We adopt high dielectric constant material silicon nitride to solve the electric breakdown problem in electrowetting and obtain good electrowetting restoration and repeatability. Combination of elerctrowetting and UV curing liquid freezing technology, the electrowetting phenomenon of micro UV resist droplets in the water was observed. This research work is significant for preparating durable small size electrowetting device with low driving voltage and high responsiveness.