Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Mechanical Engineering

Major Professor

Nathan B. Crane

Co-Major Professor

Alex A. Volinsky

Keywords

Cytop, Droplet, Electrochemical Diode, Electrowetting, Silicon

Abstract

Electrowetting is an electromechanical response that can be used to change the equilibrium

shape of droplets on a surface through the application of an electric potential. By applying this potential asymmetrically to a droplet, the droplet can be moved. Typical electrowetting devices use an electrode covered by a dielectric to reduce electrochemical interactions. Successful electrowetting requires electrodes and dielectric layers that can resist damage through many cycles of voltage.

Continuous Electrowetting (CEW) is performed on high resistivity silicon wafers. In this process, when an electric potential difference is applied between the substrate ends, the droplet on the substrate moves towards the side with positive voltage. The diode behavior of consecutive metallic spots, placed in the oxide layer, is the root of the droplet movement. This thesis investigates electrode, dielectric, and electrolyte material combinations that can achieve long stable performance with a particular emphasis on continuous electrowetting.

Incorporation of diodes can also improve standard EW conditions to achieve lower voltage operation. In passivating systems, a reverse biased electrode becomes electrochemically passive. This way we have performed low voltage and reliable Electrowetting on Dielectric (EWOD) for 5000 test cycles. This is while, in non-passivating systems, EWOD degrades significantly from the first cycles. In CEW devices, SiO2 can also serve as a steady dielectric. It is observed that, with larger electrolytes, contact angle change would remain consistent for 10000 cycles with less than 19% degradation, while would be as high as 47% with small electrolytes.

In CEW device, consistent and ideal behavior of electrochemical diodes is expected. Even though diode pairs reduces current flow and the extend of electrochemical reactions, the diode behavior can degrade over test cycles due to electrochemical reactions. To evaluate the diode behavior of different electrodes, a coefficient (referred to as actuation coefficient) is introduced which varies between zero (the least favorable diode behavior) and one (the best diode behavior) It is shown that, with the use of titanium as the electrode, the diodes behave more ideally and they behave consistently over 2000 test cycles. The best diode performance was observed with Na2SO4 electrolyte solution, where actuation coefficient remains at around 0.8 for 10000 test cycles. Aluminum can perform well in the beginning of the test cycles, but its performance degrades significantly over the first cycles.

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