Myoung-Woon Moon

Wrinkled hard skins on polymers created by focused ion beam

A stiff skin forms on surface areas of a flat polydimethylsiloxane (PDMS) upon exposure to focused ion beam (FIB) leading to ordered surface wrinkles. By controlling the FIB fluence and area of exposure of the PDMS, one can create a variety of patterns in the wavelengths in the micrometer to submicrometer range, from simple one-dimensional wrinkles to peculiar and complex hierarchical nested wrinkles. Examination of the chemical composition of the exposed PDMS reveals that the stiff skin resembles amorphous silica. Moreover, upon formation, the stiff skin tends to expand in the direction perpendicular to the direction of ion beam irradiation. The consequent mismatch strain between the stiff skin and the PDMS substrate buckles the skin, forming the wrinkle patterns. The induced strains in the stiff skin are estimated by measuring the surface length in the buckled state. Estimates of the thickness and stiffness of the stiffened surface layer are estimated by using the theory for buckled films on compliant substrates. The method provides an effective and inexpensive technique to create wrinkled hard skin patterns on surfaces of polymers for various applications.

Water harvest via dewing.

Harvesting water from humid air via dewing can provide a viable solution to a water shortage problem where liquid-phase water is not available. Here we experimentally quantify the effects of wettability and geometry of the condensation substrate on the water harvest efficiency. Uniformly hydrophilic surfaces are found to exhibit higher rates of water condensation and collection than surfaces with lower wettability. This is in contrast to a fog basking method where the most efficient surface consists of hydrophilic islands surrounded by hydrophobic background. A thin drainage path in the lower portion of the condensation substrate is revealed to greatly enhance the water collection efficiency. The optimal surface conditions found in this work can be used to design a practical device that harvests water as its biological counterpart, a green tree frog, Litoria caerulea , does during the dry season in tropical northern Australia.

Wrinkled, Dual-Scale Structures of Diamond-Like Carbon (DLC) for Superhydrophobicity

We present a simple two-step method to fabricate dual-scale superhydrophobic surfaces by using replica molding of poly(dimethylsiloxane) (PDMS) micropillars, followed by deposition of a thin, hard coating layer of a SiO(x)-incorporated diamond-like carbon (DLC). The resulting surface consists of microscale PDMS pillars covered by nanoscale wrinkles that are induced by residual compressive stress of the DLC coating and a difference in elastic moduli between DLC and PDMS without any external stretching or thermal contraction on the PDMS substrate. We show that the surface exhibits superhydrophobic properties with a static contact angle over 160 degrees for micropillar spacing ratios (interpillar gap divided by diameter) less than 4. A transition of the wetting angle to approximately 130 degrees occurs for larger spacing ratios, changing the wetting from a Cassie-Cassie state (C(m)-C(n)) to a Wenzel-Cassie state (W(m)-C(n)), where m and n denote micro- and nanoscale roughness, respectively. The robust superhydrophobicity of the Cassie-Cassie state is attributed to stability of the Cassie state on the nanoscale wrinkle structures of the hydrophobic DLC coating, which is further explained by a simple mathematical theory on wetting states with decoupling of nano- and microscale roughness in dual scale structures.

Nanoscale patterning of microtextured surfaces to control superhydrophobic robustness.

Most naturally existing superhydrophobic surfaces have a dual roughness structure where the entire microtextured area is covered with nanoscale roughness. Despite numerous studies aiming to mimic the biological surfaces, there is a lack of understanding of the role of the nanostructure covering the entire surface. Here we measure and compare the nonwetting behavior of microscopically rough surfaces by changing the coverage of nanoroughness imposed on them. We test the surfaces covered with micropillars, with nanopillars, with partially dual roughness (where micropillar tops are decorated with nanopillars), and with entirely dual roughness and a real lotus leaf surface. It is found that the superhydrophobic robustness of the surface with entirely dual roughness, with respect to the increased liquid pressure caused by the drop evaporation and with respect to the sagging of the liquid meniscus due to increased micropillar spacing, is greatly enhanced compared to that of other surfaces. This is attributed to the nanoroughness on the pillar bases that keeps the bottom surface highly water-repellent. In particular, when a drop sits on the entirely dual surface with a very low micropillar density, the dramatic loss of hydrophobicity is prevented because a novel wetting state is achieved where the drop wets the micropillars while supported by the tips of the basal nanopillars.

Hemocompatibility of surface-modified, silicon-incorporated, diamond-like carbon films

The hemocompatibility of plasma-treated, silicon-incorporated, diamond-like carbon (Si-DLC) films was investigated. Si-DLC films with a Si concentration of 2at.% were prepared on Si (100) or Nitinol substrates using a capacitively coupled radiofrequency plasma-assisted chemical vapor deposition method using a mixed gas of benzene (C(6)H(6)) and diluted silane (SiH(4):H(2)=10:90). The Si-DLC films were then treated with O(2), CF(4) or N(2) glow discharge for surface modification. The plasma treatment revealed an intimate relationship between the polar component of the surface energy and its hemocompatibility. All in vitro characterizations, i.e. protein absorption behavior, activated partial thromboplastin time measurement and platelet adhesion behavior, showed improved hemocompatibility of the N(2-)- or O(2)-plasma-treated surfaces where the polar component of the surface energy was significantly increased. Si-O or Si-N surface bonds played an important role in improving hemocompatibility, as observed in a model experiment. These results support the importance of a negatively charged polar component of the surface in inhibiting fibrinogen adsorption and platelet adhesion.

Extreme water repellency of nanostructured low-surface-energy non-woven fabrics

We report the extreme water repellent nature of non-woven fabrics of PET (polyethyleneterephthalate) whose fiber surfaces are nanotextured with oxygen plasma and coated with a low-surface-energy nanofilm. The surface effectively suppresses vapor condensation and repels condensed water droplets in addition to exhibiting a high contact angle and a low contact angle hysteresis with a millimetre-sized water drop. We also show that the surface maintains its superhydrophobicity after water-vapor condensation and after oil-wetting due to high-aspect-ratio nanohairs on the fibers. The superior water-repellent ability of the plasma treated non-woven fabric can be exploited in a variety of industrial applications including water harvesting and fuel cell water management even under oily contaminations.

Porous Carbon Nanoparticle Networks with Tunable Absorbability

Porous carbon materials with high specific surface areas and superhydrophobicity have attracted much research interest due to their potential application in the areas of water filtration, water/oil separation, and oil-spill cleanup. Most reported superhydrophobic porous carbon materials are fabricated by complex processes involving the use of catalysts and high temperatures but with low throughput. Here, we present a facile single-step method for fabricating porous carbon nanoparticle (CNP) networks with selective absorbability for water and oils via the glow discharge of hydrocarbon plasma without a catalyst at room temperature. Porous CNP networks were grown by the continuous deposition of CNPs at a relatively high deposition pressure. By varying the fluorine content, the porous CNP networks exhibited tunable repellence against liquids with various degrees of surface tension. These porous CNP networks could be applied for the separation of not only water/oil mixtures but also mixtures of liquids with different surface tension levels.

Biocompatible PEG Grafting on DLC-coated Nitinol Alloy for Vascular Stents:

The surfaces of Nitinol (TiNi), a popular metal alloy for arterial stents were thin-coated with diamond-like carbon (DLC) and then grafted with poly(ethylene glycol) (PEG) to increase biocompatibility. The TiNi control, DLC-coated TiNi (TiNi—DLC), and the PEG-grafted TiNi—DLC (TiNi—DLC—PEG) surface characteristics and biocompatibility were evaluated. The hydrophilicity of the TiNi—DLC—PEG significantly increased and the amount of both oxygen and nitrogen on the TiNi—DLC—PEG also increased compared to the TiNi control and TiNi—DLC due to the grafted PEG. The ratio between albumin and fibrinogen was higher on the PEG-grafted surface than the other surfaces when tested with human blood components; the platelet adhesion decreased the most on the TiNi—DLC—PEG surface, indicating improved blood compatibility. For in vivo tests using a rat model, the samples that were implanted for 6 weeks formed fibrous tissue; the tissue layer was much thinner on the PEG-grafted sample than the other two groups. The present results indicate that PEG-grafted TiNi—DLC surface may be effective in enhancing biocompatibility of blood-contacting biomaterials including vascular stents.

Enhancement of electron field emission property with silver incorporation into diamondlike carbon matrix

Effects of silver doping on the electron field emission properties of diamondlike carbon films deposited on silicon substrates by the rf reactive sputtering technique were studied in detail. It was found that the threshold field and effective emission barrier were reduced by Ag doping and the emission current strongly depends on the Ag doping percentage. The threshold field was found to decrease from 6.8to2.6V∕μm with a variation of Ag at. % from 0 to 12.5. The field enhancement factor was calculated and we have explained the emission mechanism.

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.