Seungyul Choi

Is the Cassie-Baxter formula relevant?

The utility of the Cassie-Baxter formula to predict the apparent contact angle of a drop on rough hydrophobic surfaces has been questioned recently. To resolve this issue, experimental and numerical data for advancing and receding contact angles are reported. In all cases considered it is seen that contact angles follow the overall trend of the Cassie-Baxter formula, except for the advancing front on pillar type roughness. It is shown that deviations from the Cassie-Baxter angle have a one-to-one correlation with microscopic distortions of the contact line with respect to its configuration in the Cassie-Baxter state.

Improvement in the breakdown properties of electrowetting using polyelectrolyte ionic solution.

We report the improvement in the breakdown properties of electrowetting using a mixture of poly(acrylic acid) (PAA) polyelectrolyte and a surfactant (Tween 80, TW80). Onset of breakdown was initially determined via visual observation and further verified by investigating impedance phase shift. Breakdown characteristics of the large-molecule ionic solution were compared with those of conventional electrolytes (Na(2)SO(4)) that produce small molecules. Experiments with various conductivities and hydrophobic coatings on a thin silicon nitride dielectric layer (∼500 Å) showed that the breakdown voltage of the PAA-TW80 system was at least two times higher than that of the Na(2)SO(4)-TW80 system. Our results demonstrate that defects in the dielectric and hydrophobic layers are less vulnerable to larger ionic molecules.

Open-structure electrowetting display with capacitive sensing feedback system

We report an open-structure electrowetting-based reflective display with capacitive sensing feedback that enables an effective self-dosing of ink, high contrast, and the precise control of color level. EWOD-based reflective display has been considered as a promising display technology due to high speed and contrast for e-paper application, but needs more improvements in effective packaging, enhanced contrast, and operation reliability. Here we introduce an EWOD display that can achieve such improvements via an open structure design and a capacitive feedback system. Our report includes the demonstration of quick and effective ink dosing process, off color area being ~ 8% of viewable area, and precision control of color area even under a large variation of interfacial tension.

Development of micro variable optics array

This research is on the development of a micro variable optics array which employs electrowetting as the working principle. The single pixel of the array has four separated electrodes and each of them is controlled independently giving the device multi-degree of freedom. The separated electrodes are fabricated using a thick photoresist and electroplating. Several formulas showing the relation among the radius of curvature, the prism angle, and electrowetting parameters are provided. The prism angles are measured to be ±30° and compared to the calculated values. The measurement of the radius of curvature is also presented showing that the various radiuses of curvature are achievable from concave to convex.

Electrowetting-based measurement of interfacial tension

We report all-electrical monitoring of interfacial tension, enabled by the electrowetting on dielectric (EWOD) technique. The contact angle of a sessile oil droplet in water was obtained via an in-situ capacitive measurement of the EWOD area on a single chip. Contact angle data measured at different operating voltage values were used to calculate the interfacial tension using the Young-Lippmann equation. The results were in good agreement with the results of an image-based analysis, which was performed by using an electrolyte liquid (Na2SO4)/oil (n-decane) system with a surfactant (Tween® 80) to alter the interfacial tension. The maximum deviation of the contact angle and interfacial tension values determined using EWOD from the values determined using the image analysis were 2.8° and less than 1 mN/m, respectively.

Electrowetting interfacial tension measurement system

It is reported that electrical monitoring of interfacial tension by the electrowetting on dielectric (EWOD). The contact angle of a sessile oil droplet in conducting liquid was calculated with an capacitance measurement on a single chip. Contact angle can be measured at different operational voltage and it is used to calculate the interfacial tension by the Young-Lippmann equation. The results were in agreement with an optical analysis. It was performed by a conducting liquid (Na2SO4) / oil (n-decane) system with a non-ionic surfactant (Tween 80) to change the interfacial tension. The deviation of the contact angle and interfacial tension from the values determined by the optical analysis were 2.8° and less than 1 mN/m, respectively.

An open-structure electrowetting-based reflective display with a feedback system

Electrowetting-based reflective displays have long been considered a promising display choice for electronic paper applications due to their fast operating speeds and high color contrasts. However, they still require several improvements in terms of their packaging process, color contrast, and reliability. This study investigates such enhancements via an open-structure design and a capacitive feedback system. It has a femto-farad level resolution and demonstrates a high operating speed (approximately 10 ms), effective ink dosing, and large color area change (approximately 92%). The feedback system for the precise control of the color area is verified by testing under a wide range of interfacial tension. The system reduces the deviation of color contrast by 85%. Working with high fidelity with large disturbances, which are represented by interfacial variations, the system shows robust performance against other disturbances, such as temperature variation and contact angle hysteresis.

Surface tension monitoring in a “soft” interface using electrowetting on dielectric

We report the first demonstration of interfacial tension monitoring across two immiscible liquids using electrowetting on dielectric (EWOD). Impedance measurement during EWOD reveals the variation of surfactant concentration at the liquid-liquid interface in real time. The discrepancy with optical method was within 3.5 mN/m and the noise level was under 0.5 mN/m. We also show such approach can be used for label-free monitoring of DNA hybridization. Our approach opens a new horizon of EWOD used as a molecular sensing mechanism across a “soft” interface between liquids.

Reflective display using electrowetting with self-dosing and large viewable area ratio

Electrowetting reflective display has been widely studied since EWOD [1] was introduced. The electrowetting display has been suggested using two immiscible liquids such as colored oil and conductive liquid, normally the mixture of deionized water and some electrolytes [2]. In this case hydrophilic grid walls prevent the colored oil from spreading over the neighboring cell, and render the conducting liquid easy to separate from the corner of the cell. This method, however, has some disadvantages such challenges in oil dosing process and relatively small viewable area ratio (VAR). Previous research has addressed these issues by introducing a self dosing method, and enhanced the VAR through the use of capillary forces[3].