Abstract
The manipulation and applications of digital electric and continuous magneticfluids are investigated in this thesis. For the digital electric fluids, the droplet resonance is excited by both single-plate and two-plates EWOD methods. By usingthe frequency scanning system, amplitude spectrum of an oscillating droplet can be measured, and the resonant frequencies of a droplet then can be figured out. Comparison of the resonant frequencies from theoretical predictions and experimental data is also performed. After modifying the effective curvature radius of droplet free surface, the theoretical predictions can agree with experimental data very well in the single-plane EWOD configuration. While concerning the cases that the droplet is constrained between two plates, factors as the frictional damping comes from the contact of liquid and solid and droplet aspect ratio are needed to be taken into consideration as well as the free surface curvature to complete the theoretic model. In addition, including the phase spectrum of an oscillating droplet and the beat phenomenon are also observed by experiment. It is the first time that these phenomenons be quantitatively presented. Besides, the ability of droplet resonance for enhancing the in-droplet mixing is verified experimentally. If the droplet resonance is excited, the mixing time required to reach 95 % mixing is about only 6.7 % of the mixing time required for the droplet without EWOD actuation. More, when a merged droplet is driven by two alternating driving frequencies, especially alternating resonant frequencies, a well-selected switching time interval of these two frequencies further helps the mixing. The affections of droplet resonance on in-droplet mixing are the first time be verified by this theses. In addition, the behaviors of a charged water droplet between two DC electrodes are also investigated experimentally. The droplet behaviors can be briefly classified into four categories: low field motion, stable back and forth, unstable bridge, and stable bridge. It is notable that the corresponding electric signals are successfully interpreted in this thesis. Not only the contact charging mechanism can be further understood, a liquid phase electrical converter as a novel applied concept is also associated through these signals. The charged droplet is also used as the cell carrier to transport cells between electrodes, and no obvious influence of the contact charging on cells viability can be found after hundreds times droplet-electrode contact. For the manipulation of continuous magnetic fluids, a magnetic micromixer and a microseparator integrating patterned magnetic thin films are firstly proposed. Magnetic field switches, which are composed of magnetic films with different aspect ratio, can produce open/closed magnetic flux by controlling the magnetization direction of films. The applications of magnetic switches on micro magnetic fluid mixing and microparticle separation are verified numerically.