Tae-Jun Ko

Nanostructured self-cleaning lyocell fabrics with asymmetric wettability and moisture absorbency (part I)

A single-faced superhydrophobic lyocell fabric maintaining its inherent high moisture absorbing bulk property was produced by oxygen plasma-based nanostructuring and a subsequent coating with a low-surface-energy material. After 5 minutes of oxygen plasma etching, followed by 30 seconds of a plasma polymerized hexamethyldisiloxane coating, the treated surface of lyocell turned into a superhydrophobic surface with a static contact angle greater than 160° and a sliding angle less than 2°; however, the backside was hydrophilic, untreated lyocell fabric. As a result of oxygen plasma etching, dual hierarchical roughness was formed on the lyocell fabric as nano scale pillars or hairs were added onto the lyocell fabric surface with micro scale roughness. Extremely opposite wetting behavior was observed, when a water droplet was deposited on the face and backside of the plasma-treated lyocell fabric. A water droplet was immediately absorbed and spread out on the untreated backside, while it rolled off the treated surface, demonstrating a bouncing effect.

Thermal stability of superhydrophobic, nanostructured surfaces

The thermal stability of superhydrophobic, nanostructured surfaces after thermal annealing was explored. Flat surfaces coated with hydrophobic diamond-like carbon (DLC) via plasma polymerization of hexamethyldisiloxane (HMDSO) showed a gradual decrease in the water contact angle from 90(o) to 60(o) while nanostructured surfaces maintained superhydrophobicity with more than 150° for annealing temperatures between 25 and 300°C. It was also found that surfaces with nanostructures having an aspect ratio of more than 5.2 may maintain superhydrophobicity for annealing temperatures as high as 350°C; above this temperature, however, the hydrophobicity on surfaces with lower aspect ratio nanostructures gradually degraded. It was observed that regardless of the aspect ratios of the nanostructure, all superhydrophobic surfaces became superhydrophilic after annealing at temperatures higher than 500°C.

Adhesion behavior of mouse liver cancer cells on nanostructured superhydrophobic and superhydrophilic surfaces

The control of cancer cell adhesion behavior on certain surfaces has been widely studied in recent years to enhance cell adhesion, which is required for bio-sensing, implant biomaterials, or to prevent infections from bacteria or germs. In addition, it helps to preserve the original functions of medical devices such as implants, catheters, injection syringes, and vascular stents. In this study, we explored the behavior of mouse liver cancer cells on nanostructured surfaces in extreme wetting conditions of a superhydrophobic or superhydrophilic nature. Oxygen plasma treatment of polymeric surfaces induced the formation of nanostructures such as bumps or hairs with various aspect ratios, which is defined as the height to diameter ratio. A superhydrophobic surface with a contact angle (CA) of 161.1° was obtained through the hydrophobic coating of a nanostructured surface with a high aspect ratio of 25.8. On the other hand, an opposite extreme wetting surface with a superhydrophilic nature with a CA of 1.7° was obtained through the hydrophilic coating of the same structured surface. The mouse liver cancer cells significantly proliferated on a mild hydrophilic surface with a low aspect ratio nanostructure due to the mild roughness and improvements of mechanical anchoring. However, the superhydrophilic surface with a high aspect ratio nanostructure (i.e., hair shaped) suppressed the growth of the cancer cells due to the limited number of sites for focal adhesion, which restricted the adhesion of cancer cells and resulted in a decrease in the cell-covered area. The superhydrophobic nanostructured surface with a high aspect ratio further restricted the adhesion and growth of the cancer cells; the cell activity was extremely suppressed and the spherical shape of the cancer cells was maintained. Thus, this simple method for fabricating nanostructured surfaces with various wetting conditions might be useful for producing biomedical devices such as stents, implants, drug delivery devices, and detection and/or sensing devices for cancer cells.

Hierarchical structures of AlOOH nanoflakes nested on Si nanopillars with anti-reflectance and superhydrophobicity

A novel method to fabricate ultra-low reflective Si surfaces with nanoscale hierarchical structures is developed by the combination of AlOOH or boehmite nanoflakes nested on plasma-etched Si nanopillars. Using CF4 plasma etching, Si surfaces are nanostructured with pillar-like structures by selective etching with self-masking by fluorocarbon residues. AlOOH nanoflakes are formed by Al thin film coating with various thicknesses and subsequent immersion in boiling water, which induces the formation of nanoscale flakes through the hydrolysis reaction. AlOOH nanoflakes are formed on Si nanopillared surfaces for hierarchical structures, which are coated with a low-surface-energy material, resulting in a higher water wetting angle of over 150° and a very low contact angle hysteresis of less than 5°, and implying a self-cleaning surface. Reflectance reduced to 5.18% on average on hierarchical nanostructures in comparison to 9.63% on the Si nanopillar surfaces only. We found that Si nanopillars reduced reflection for wavelengths ranging from 200 to 1200 nm while AlOOH nanoflakes reduced reflection for wavelengths longer than 600 nm.

Nanostructured superhydrophobic silk fabric fabricated using the ion beam method

Superhydrophobic silk fabric surfaces with high-aspect-ratio nanostructures were fabricated using ion beam treatment. The ion beam irradiated silk fabrics were characterized for wettability, as well as other physical properties unique to silk. The nanostructures were produced in various configurations, ranging from columnar to hairy shapes, on the silk fibers through anisotropic etching using oxygen ion beam treatment. Subsequent hydrophobic coating on the nanostructured, superhydrophobic silk fiber surfaces was achieved with an increase of the static contact angle from 0° for the pristine hydrophilic silk fabric to 170° for the superhydrophobic silk fabric and with a decrease of shedding angle by less than 5°, which is sufficient roll-off a water droplet to the silk fabric surface. Because the ion beam-treated side of silk fabric becomes superhydrophobic, while the opposite side, or body contacting side, remains pristine or superhydrophilic, an extreme asymmetric wettability can be achieved in the silk fabric, which improves its breathability by improving moisture transmittance through the fabric from the body to the outer surface. The luster and color of silk fabric before and after ion beam irradiation were found to exhibit no significant degradation. The breaking load of the silk fabric after the ion beam treatment was assessed to be mechanically durable in comparison to that of the pristine fabric. Therefore, after introducing the superhydrophobic property via the ion beam treatment, the silk fabric maintained its primary advantages, as a result, the range of its applications can expand into breathable self-cleaning clothing textiles, such as neckties, blouses and dresses.

젖음성 차이와 무전해도금을 이용한 연성 구리 회로패턴 형성

Cu circuits were successfully fabricated on flexible PET(polyethylene terephthalate) substrates using wettability difference and electroless plating without an etching process. The wettability of Cu plating solution on PET was controlled by oxygen plasma treatment and SiOx-DLC(silicon oxide containing diamond like carbon) coating by HMDSO(hexamethyldisiloxane) plasma. With an increase of the height of the nanostructures on the PET surface with the oxygen plasma treatment time, the wettability difference between the hydrophilicity and hydrophobicity increased, which allowed the etchless formation of a Cu pattern with high peel strength by selective Cu plating. When the height of the nanostructure was more than 1400 nm (60 min oxygen plasma treatment), the reduction of the critical impalement pressure with the decreasing density of the nanostructure caused the precipitation of copper in the hydrophobic region.

Water Wetting Observation on a Superhydrophobic Hairy Plant Leaf Using Environmental Scanning Electron Microscopy

Mankind has been continuously learning from nature via careful observation in attempt to improve their own lives and to further overcome the environmental issues. In the last 10 years, researchers have focused their attention on the surface functions of parts of living organisms, such as gecko’s feet, water strider’s legs, and lotus leaves for various applications (e.g., self-cleaning surfaces, nano-micro robotics, and water harvesting) (Bhushan & Her, 2010; Cheng et al., 2005; Cho & Choi, 2008; Gao & Jiang, 2004; Hansen & Autumn, 2005; Neinhuis & Barthlott, 1997). In particular, functional plant surfaces such as lotus or acacia leaves have been studied owing to their excellent characteristics of superhydrophobicity and self-cleaning, which are a result of their structures and surface materials (Cha et al., 2010; Cheng et al., 2005; Neinhuis & Barthlott, 1997). For characterizing superhydrophobicity, one can use simple water droplet measurements on target surfaces or measure the squeezing pressure at the micro-scale. Measurement of water condensation with water vapors is also a key indicator of robust superhydrophobicity at the nanoscale (Ko et al., 2012, 2015; Quéré, 2008; Shin et al., 2012; Varanasi et al., 2009). When a water droplet is placed on a superhydrophobic plant leaf possessing nanoor micro-scale roughness like hair or bump-shapes (Fig. 1A), the droplet forms a nearly perfect spherical shape, rolls off, and cleans the leaf surface as shown in Fig. 1B. This self-cleaning and water-repellent behavior is typically attributed to the roughness of the surface and lowsurface-energy coatings such as the hydrophobic epicuticular wax crystalloid coating of lotus leaves. Superhydrophobicity has been commonly characterized by higher water contact angles (i.e., higher than 160) and wetting angle hysteresis of drop rolling. However, because this method uses relatively large water droplets with millimeter-scale sizes, additional information on the quality of superhydrophobicity has been recently obtained. A robust superhydrophobic surface should be able to sustain high contact angles against condensation