The research objective of this project is to investigate the interactions during electrowetting between liquid electrolytes and dielectric surfaces. These interactions result in a cessation of spreading with increasing voltage and ultimately irreversible failure of the insulating properties of the dielectric layer. Electrowetting is a phenomenon where the application of an electric potential causes a conducting liquid to spread upon an insulated electrode. This effort seeks to develop a fundamental understanding of limiting phenomena in electrowetting and its dependence on the properties of the ions, solvent, and dielectric layer. Furthermore, protocols will be developed for the fabrication of oxide dielectrics and self-assembled monolayer topcoats so as to achieve significant, reversible, and reliable electrowetting behavior. The research effort is motivated by the application of capillary force actuators. These actuators for microelectromechanical systems can deliver forces up to 100 times greater than similarly sized electrostatic actuators. Of particular interest is the optimization of the surface/liquid system so as to enhance the performance of these microactuators.
Understanding limiting phenomena in electrowetting will lead to improved electrowetting performance in a variety of applications including microdevice actuation, hand-held medical testing equipment, electronic paper and displays, and focusable liquid lenses for cameras. Moreover, this effort will provide both undergraduate and graduate students a highly interdisciplinary research opportunity. As undergraduates from underrepresented groups will be part of the research team, this effort will directly impact the training of a diverse engineering workforce and the preparation of underrepresented students for graduate study.
This research project investigates fundamental phenomena occurring during the electrowetting of liquids at dielectric surfaces, aiming to develop better dielectric layers for capillary force actuators. Electrowetting on a dielectric (EWOD) is a phenomenon whereby the application of a voltage to a conducting liquid placed on an insulating electrode causes the liquid to spread over the surface, varying the contact angle. This phenomenon enables manipulation of liquid motion using electric fields and is currently utilized for laboratory-on-a-chip diagnostics, electrically controlled microlenses and reflective displays for electronic paper. A better understanding of the process would likely result in improved and entirely devices. Currently, electrowetting devices utilize thick (~ 1 micron) insulating polymeric layers, which require large voltages (> 100 V) in order to provide significant changes in contact angle. This project seeks to develop and understand an alternative design, consisting of a bilayer system where the first layer would be a metal oxide dielectric to provide the necessary insulating behavior and high breakdown field, while the second layer would tailor the surface tension to enable large initial contact angle. The first aim of this project was to develop high quality dielectric films with low thickness and high dielectric strength. Al, Ta and Zr oxide have been selected as candidates due to their high dielectric constant and large band gap; so far, Al and Ta oxides have been synthesized by electrochemical anodization and their electrical conduction properties studied. Electronic conduction in Al oxide is asymmetric, resulting in large currents at low negative potentials, and low currents at positive potential. This behavior was traced back to the formation of two doped layers (one n-type, the other p-type) separated by an undoped layer, acting all together as a non-ideal diode. Thermal annealing of Al oxide films has been shown to passivate a significant fraction of the dopants, resulting in a much lower electrical conductivity. In preliminary work on Ta oxides, it has been verified that conduction in contrast is much more symmetric, enabling in principle bipolar operation. The polymers used to control the surface tension of the insulating electrode are most commonly fluoropolymers, among which Cytop and Fluoropel are the most used in the scientific community. The electrowetting performance of bilayers consisting of Al oxide and a fluoropolymer has been determined, showing ideal characteristics contact angle vs. applied voltage, only up to a saturation angle which is achieved at a voltage much lower than that necessary to induce breakdown. The phenomenon of contact angle saturation is not yet understood. Conventional fluoropolymers are usually spin coated on the electrode with a thickness of ~ 1 micron. Attempts to produce thinner filmes (10-100 nm) yield defected layers, with high porosity and numerous pinholes, which lead to early electrolyte penetration and breakdown. In order to avoid these shortcomings, self-assembled monolayers (SAMs) have been recently grown on metal oxide surfaces and tested for their electrowetting performance. The quality of the SAM depends strongly on the time of adsorption and temperature; optimized parameters allow to form continuous monolayers. These structures produce ideal characteristics only up to 8-10 V, indicating that failure occurs already at low voltages. In parallel, novel methods have been developed to measure the advancing and receding contact angle by using a Wilhelmy plate; the contact angle hysteresis can thus be measured, and this has been found to be the lowest when the SAMs present a close to ideal structure. The measurement of the contact angle can be achieved only under elaborate experimental conditions, which require visual access to the liquid drop and an imaging software to calculate the angle. A purely electrical method has been developed in this project, whereby the interface capacitance at the dielectric/electrolyte interface is separated from other capacitance contribution in the device, and a mathematical method has been developed to determine the drop shape as a function of contact angle and then coded to derive the contact angle directly from the measured capacitance. The project is currently ongoing even if unfunded, in order to complete the current investigations.
