Abstract
Micro-fluid manipulation has been used extensively in biomedical chips and micro-electro-cooling, so the research in fluid transporting has become more and more important. Based on electro-wetting on dielectric (EWOD) effect, we designed and manufactured a coplanar digital micro-fluidic manipulation system. Using this platform, the nature frequency of droplet and the effect of ionic concentration on the contact angle were tested and explained in detail. When weak electrolyte is added in the water, Our result shows that increasing of ions enhances the contact angle variation in the same driving potential. A more obvious electric double layer(EDL) compared with water formed near the solid-liquid interface makes a significant contact angle variation and delays the contact angle saturation, which is strongly influenced by the surface charge density. When the weak electrolyte is replaced by strong electrolyte, the stronger charge accumulation shields the driving field, causing the contact angle to saturate earlier. As the result, when electrolytes are used, we have to take concentration and dissociation rate into consideration as far as the contact angle is concerned. We also investigate the natural frequency shift with droplet size. It becomes higher as the droplet size decreases. When driving frequency couples with the natural frequency of droplet, the irregular and strong vortex due to the violently periodic vibration could be used in fluid mixing.