Electrical Control of Liquids at Interfaces: Electrowetting, Dielectrowetting, and Dewetting
Date Issued
2015
Date
2015
Author(s)
Hsieh, Wan-Lin
Abstract
Electrically based control of the geometry or transport of a fluid meniscus has been a technological trend used in microfluidic applications in the last decade. Aside from experiments, numerical models could provide an effective tool for testing the feasibility of device applications. Here, we present a numerical simulation technique to calculate the deformation of fluids at interfaces by coupling the electrohydrodynamic (EHD) theory considering the assumption of leaky dielectric model with the Phase Field Method (PFM). With the assumption, the proposed model could simulate the interaction of a fluid-fluid interface with an electric field, using conductive liquids as well as dielectric liquids. The fluid dynamic behavior within a pixel of an electrowetting display (EWD) is studied. The complete switch processes, including the break-up and the electrowetting stages in the switch-on process (with voltage) and the oil spreading in the switch-off process (without voltage), are successfully simulated. By considering the factor of the change in the apparent contact angle at the contact line, the electro-optic performance obtained from the simulation is found to agree well with its corresponding experiment. In addition, the proposed modeling is used to parametrically predict the effect of interfacial (e.g., contact angle of grid) and geometric (e.g., oil thickness and pixel size) properties on the defects of an EWD, such as oil dewetting patterns, oil overflow, and oil non-recovery. With the help of the defect analysis, a highly stable EWD is both experimentally realized and numerically analyzed. Dielectrowetting effects of surface wrinkling, droplet size, isotropic vs. anisotropic spreading, electrode geometry, and deterministic dewetting, are presented both experimentally and by the developed model. The dynamic behavior of the two phase system has been accurately characterized on both the macro and microscopic level. These results can be used to further optimize dielectrowetting optical shutter and to provide a deeper theoretical insight into the operating physics of dielectrowetting effect. Existing techniques for electronic control of the interface between two immiscible fluids are typically limited to simple periodic geometries (symmetric waves) or spherical geometries (only two principle radii of curvature). Presented here, is a new technique with much more sophisticated electronic control of fluid meniscus geometry. Two distinct approaches are demonstrated: (1) application of voltages, electrical capacitance sensing of meniscus geometry, followed by further feedback control of the applied voltages based on the sensed electrical capacitance; (2) use of multiple periodic voltage waveforms and wave propagation across the meniscus to build up complex meniscus geometries by Fourier construction. The results can be achieved using conventional materials, and the fluids respond with speeds that are adequately slow (ms-µs) such that even conventional control electronics (µs-ns) are more than adequate. Furthermore, because the conducting fluid never dewets the oil film from the solid surface, dielectric degradation issues are likely eliminated.
Subjects
electrohydrodynamic theory
Phase Field Method
leaky dielectric model
electrowetting display
dielectrowetting optical shutter
feedback control
Fourier construction
Type
thesis
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