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Abstract

Free surface control is inherently involved in various fields of science and engineering. In the present thesis, static and dynamic responses of the liquid-gas interface to the applied electric field are analyzed in both ways of experiments and numerical simulation. Furthermore, based on those understanding, new dispensing method is suggested and demonstrated successfully. In part I, firstly, the system of the electrowetting by a time-periodic electric field is analyzed. We numerically analyze the AC electric field around a droplet placed on an insulator-covered electrode. The time-averaged effective electrical wetting tension, which is a function of AC frequency, is computed by integrating the Maxwell stress. The computed wetting tension is compared with the experimental result converted from the separately obtained contact-angle data, showing a good agreement. Interestingly, the numerical results show that the electric-field strength is decreased remarkably in the insulating layer near the TCL as the AC frequency is increased. This decrease may account for the delay of the dielectric breakdown of an insulating layer in the AC case, which could be related to the contact-angle saturation phenomenon. Secondly, liquid bridge (LB) formation by the applied electric field is analyzed numerically. Numerical simulation captures the temporal behavior of liquid surface during the LB formation between a top plate and a bottom nozzle. Numerical results show the three stages of LB formation, including the effect of the applied voltage pulse. Non-linear behavior such as bubble trapping at the impact of liquid to plate is also captured and explained qualitatively. The wetting criterion for LB formation is suggested and explained in terms of capillary pressure. The linear decrease of the final contact radius with the top plate contact angle is shown from the numerical results. In addition, the effects of the liquid properties on the dynamics are briefly discussed. In part II, a new method is suggested and demonstrated for dispensing a droplet on the top plate with an inverted geometry by using electric field. The process of dispensing droplets consists of two stages: (i) formation of liquid bridge by moving up the charged fluid mass using the electrostatic force between the charges on the fluid mass and the induced charges on the substrate, and (ii) its break-up by the motion of the top plate. Use of capillarity with an inverted geometry removes the need of external pumps or elaborate control for constant flow feed. The droplet diameter has been characterized as a function of the nozzle-to-plate distance and the plate moving velocity. The robustness of the present method is shown in terms of nozzle length and applied voltage. Finally, its practical applicability is confirmed by rendering a 19 by 24 array of highly uniform droplets with only 1.8 % size variation without use of any active feedback control.