Low Voltage Electrowetting-on-Dielectric for Microfluidic Systems

Manipulating fluids in micro to nano-liter volumes poses significant challenges, due to the dominance of surface forces with increasingly large surface to volume ratios. Commonly, external pumps or precisely micromachined on-chip pumps were used to generate pressure gradients to induce fluid motion. Alternatively, the wettability of a surface can be manipulated by an applied electric field. This phenomenon, known as electrowetting, consists in the spread of a liquid drop placed on an electrode upon application of a voltage. Electrowetting based devices flourished upon the discovery that an added dielectric layer between the drop and a conductive electrode would tolerate the application of large voltages, allowing for significant contact angle change. Several current commercial devices exploiting this effect include electro-optic displays, electronic paper, and variable focus lenses. Additionally, electrowetting has achieved particular prominence in lab-on-a-chip applications, controlling drop transport across patterned electrode structures.
Initially, polymers were utilized as the dielectric layer preventing current flow between the fluid and electrode, while providing a hydrophobic surface to minimize the resistance of drop movement. However, these layers were typically in excess of one micron thickness. As a consequence, these layers required over 100 Volts to achieve appreciable contact angle change. This dissertation aims to reduce the voltage dependence for contact angle change by optimization of a dual layer structure utilizing a dielectric and hydrophobic layer. The dielectric layer uses aluminum and tantalum oxides (15-44 nm thick) formed by electrochemical anodization. The electronic and ionic conduction, breakdown characteristics, and dielectric properties of these films were studied in detail, achieving a comprehensive understanding of the charge transport and failure mechanisms. Three hydrophobic layer were investigated: a commercial fluoropolymer Cytop, and two self-assembled monolayers (SAMs), phosphonic acid and silane. Each layer was seen to form reproducible surfaces, with high initial water contact angles and limited hysteresis, favoring contact line movement during electrowetting testing.
The electrowetting response of these bilayer structures was characterized by concurrently measuring contact angle and leakage current during stepped voltage measurements up to failure, showing three electrowetting response regimes: ideal Lippmann-Young behavior, contact angle saturation, and dielectric breakdown. The onset of ionic conduction in the metal oxide layer and the resulting breakdown determine when the layer would ultimately fail, but the thickness of the hydrophobic layer determined the achievable contact angle vs. voltage characteristics. The study successfully showed ideally modeled electrowetting of greater than 25o under applied voltages of 12 V for Cytop, 6 V for phosphonic acid, and 5 V for silane. Cyclic wetting measurements using an “on” voltage above or below the voltage drop needed for polymer breakdown found that the decay rate of the contact angle decreases significantly over time only above this voltage threshold. The leakage current and charge injected in the polymer cannot comprehensively assess the stability of the system during operation. The contact potential difference measured with a Kelvin probe provides an alternative means to assess the extent of damage.