Robust electrowetting system for applications in display and parameter tracking

Abstract

The main topic of this thesis can be divided into two parts. First, we developed robust electrowetting reflective display and micro prism array with feedback system. Second, we found a novel platform to measure the interfacial tension between two immiscible liquids with an electrical method. The interfacial measurement system is based on the same principle used in the feedback system. In the first part, we researched on two main issues which are essential in electrowetting systems: electrical breakdown and contact angle hysteresis. Electrical breakdown occurs in the dielectric layer which is placed between conducting liquid and electrode in electrowetting systems. The reason is that small ions (Na+, K+, Cl-) generated from electrolyte in the conducting liquid can easily penetrate the defects on the dielectric layer such as pin holes or cracks, allowing electric current to pass through the dielectric layer. Here, we used polyelectrolyte (polyacrylic acid, PAA) and non-ionic surfactant (tween® 80) to delay electrical breakdown. They dont produce small ions like conventional electrolytes (NaCl, Na2SO4, KCl, etc) and surfactant (Sodium dodecyl sulfate, SDS) do. We increased breakdown voltage with our approach. In addition, we can quantified electrical breakdown by measuring impedance of the dielectric layer. There is a phase angle term in impedance which indicates the ratio between capacitance and resistance factor. If the dielectric layer is not damaged, there is only capacitance factor and phase angle is -90°. However, the phase angle starts to approach 0° once electrical breakdown occurs. We improved breakdown property in electrowetting systems with polyelectrolyte and non-ionic surfactant and introduced the method to monitor the degree of the breakdown and to find the optimized condition with impedance measurement. Contact angle hysteresis results from the roughness of surface, the chemistry of liquid, and other factors. We focused on the type of liquids used for electrowetting. Polyelectrolytes and non-ionic surfactants were used for contact angle hysteresis experiment, and we tried to find electrolyte-surfactant combination which had low contact angle hysteresis. The results showed that conducting liquid with polyethylene glycerol (PEO) and Triton X-100 had the lowest contact angle hysteresis. We designed and fabricated electrowetting reflective display based on the results of breakdown and contact angle hysteresis experiments. Our design has an open structure unlike the conventional electrowetting display which has closed structure. Open structure has several advantages such as high contrast ratio (high white area ratio, ~90%), easy liquid dosing into pixel, and natural gray scale expression as well as high operational speed (~10 ms). In addition, we developed microprism array, where each prism is surrounded by four separated electrode walls. We can control the contact angle on each wall individually. However, such electrowetting systems are vulnerable to external environment such as temperature changes, humidity, and contamination. These effects reduce the reliability of the systems. An additional system is required to achieve the high reliability of electrowetting systems. We suggested a feedback system and integrated it with electrowetting optical device. The principle is to use capacitance measurement between conducting liquid and the electrode. The hydrophobic and dielectric layers between the conducting liquid and the electrode produce capacitance. We measured the capacitance which corresponded with the contact area between the conducting liquid and the electrode. The contact area defines color contrast (white area ratio) in electrowetting reflective display and the contact angle on each electrode walls in micro prism. The contact area can be affected by the external conditions as I mentioned, which reduces the reliability of electrowetting device. Our feedback system measures the contact area using capacitance and compares it with the electrical signal which has the information on target contact area. If there is a difference between two values, feedback system calibrates operational voltage to make the two values the same. As a result, electrowetting display device with the feedback system has 85% lower deviation than the system without the feedback system. With the addition of feedback system and use of polyelectrolyte solution to reduce hysteresis and electrical breakdown, we could achieve robust electrowetting optical device performance. Recently, we found another application for the feedback system. We developed an interfacial tension measurement system with the same method used in the feedback system. The contact angle of oil inside conducting liquid can be calculated once the contact area between the conducting liquid and electrode is measured by capacitance measurement. Interfacial tension information can be achieved form the contact angle data at several operational voltages and electrowetting equation. We did real-time interfacial tension measurement by applying varying surfactant concentration. Results are well-matched with optical measurement. The discrepancy between our method and optical measurement was under 1 mN/m.