Yongjoo Kwon

Design of Surface Hierarchy for Extreme Hydrophobicity

An extreme water-repellent surface is designed and fabricated with a hierarchical integration of nano- and microscale textures. We combined the two readily accessible etching techniques, a standard deep silicon etching, and a gas phase isotropic etching (XeF2) for the uniform formation of double roughness on a silicon surface. The fabricated synthetic surface shows the hallmarks of the Lotus effect: durable super water repellency (contact angle>173 degrees) and the sole existence of the Cassie state even with a very large spacing between roughness structures (>1:7.5). We directly demonstrate the absence of the Wenzels or wetted state through a series of experiments. When a water droplet is squeezed or dropped on the fabricated surface, the contact angle hardly changes and the released droplet instantly springs back without remaining wetted on the surface. We also show that a ball of water droplet keeps bouncing on the surface. Furthermore, the droplet shows very small contact angle hysteresis which can be further used in applications such as super-repellent coating and low-drag microfludics. These properties are attributed to the nano/micro surface texture designed to keep the nonwetting state energetically favorable.

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.

Electroless Gold Plating on Aluminum Patterned Chips for CMOS-Based Sensor Applications

We presented an approach for the activation of aluminum Al alloy using palladium Pd and the subsequent gold Au electroless plating ELP for complementary metal oxide semiconductor CMOS-based sensor applications. In this study, CMOS process compatible Al patterned chips were used as substrates for easy incorporation with existing CMOS circuits. To improve the contact resistance that arose from the Schottky barrier between the metal electrodes and the single-walled carbon nanotubes SWCNTs, electroless deposition of gold that has a higher work function than Al was adopted because the SWCNTs has p-type semiconductor properties. Each step of the Au ELP procedure was studied under various bath temperatures, immersion times, and chemical concentrations. Fine Pd particles were homogeneously distributed on the Al surface by the Pd activation process at room temperature. Au ELP allowed selective deposition of the Au film on the activated Al surface only. The SWCNT networks formed on the Au plated chip by a dip-coating method showed improved contact resistance and resistance variation between the Au electrode and SWCNTs. We also tried SWCNT decoration with the Au particle using the upper Au ELP method, which was expected to be applied in various areas including field-effect transistors and sensor devices.

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.

Measurement of the optical characteristics of electrowetting prism array for three-dimensional display

Recently liquid-based optical devices are emerging as attractive components in three-dimensional (3D) display for its compact structure and fast response time. Among them an electrowetting prism array is one of the promising 3D devices. It steers a beam, which enables to provide corresponding perspectives to observer. For high quality autostereoscopic 3D displays the important factors are the beam steering angle and the beam profile, the optical characteristics. In this paper, we propose a method to measure the optical characteristics of the liquid prism and show experimental results on our prototype electrowetting prism array, which consists of prisms with 200um by 200um size. A modified 4-f system is adopted for the proposed method. It provides two kinds of information of the optical characteristics of the liquid prism at the image plane and at the Fourier plane. First, the proposed measurement setup magnifies the image of the liquid micro prism array so that we can observe the status of the each prism array directly with bare eye and align a mask easily for selecting a prism to be examined at the image plane. Secondly, the steering angle can be calculated by measuring the displacement of the beam at the Fourier plane, where the angular profiles that have important information on the oilwater interface is observed precisely. The principle of the proposed method will be explained, and the measured optical characteristics from experimental results on the liquid prism we fabricated will be provided, which proves the validity of the measurement method.

Arrayed beam steering device for advanced 3D displays

An arrayed beam steering device enables much simplified system architectures for high quality multiview 3D displays by adapting time multiplexing and eye tracking scheme. An array device consisting of microscale liquid prisms is presented, where the prism surface between two immiscible liquids is electrically controlled to steer light beams by the principle of electrowetting. An array prototype with 280×280μm pixels was fabricated and demonstrated of its full optical performances. The maximum tilting angle of each prism was measured to be 22.5° in average, with a tracking resolution of less than 0.04°. In this paper, we report a design and fabrication of eletrowetting based prism array, opto-fluidic simulations, optical characterizations, as well as applications to achieve low fatigue 3D displays.

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.

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].