Jin Woo Yi

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.

Long-Lasting Hydrophilicity on Nanostructured Si-Incorporated Diamond-Like Carbon Films

We investigated the long-lasting hydrophilic behavior of a Si-incorporated diamond-like carbon (Si-DLC) film by varying the Si fraction in DLC matrix through oxygen and nitrogen plasma surface treatments. The wetting behavior of the water droplets on the pure DLC and Si-DLC with the nitrogen or oxygen plasma treatment revealed that the Si element in the oxygen-plasma-treated Si-DLC films played a major role in maintaining a hydrophilic wetting angle of <10° for 20 days in ambient air. The nanostructured patterns with a roughness of ∼10 nm evolved because of the selective etching of the carbon matrix by the oxygen plasma in the Si-DLC film, where the chemical component of the Si-Ox bond was enriched on the top of the nanopatterns and remained for over 20 days.

Effects of Metal-Plasma Source Ion Implantation on Tribological Properties of a DLC Film

Abstract. Effects of ion implantation on tribological properties of a DLC film as a function of ion doses and implanted energies we investigated. Ti ions were implanted on the Si-wafer substrates followed by DLC coating using an ion beam deposition method. The roughness of DLC films on the ion implanted surface was measured by an Atomic Force Microscope (AFM). In order to study tribological properties of the DLC film, we used ball-on-disc type apparatus in the atmospheric environment. From results of wear test, friction coefficient was more stable below 0.1 as increasing ion implanted energies and ion doses.

Wetting and Adhesion Behavior of Undulated a-C:H Film Deposited on Nano-Scale Copper Dot

This paper investigated the wetting and adhesion property of undulated a-C:H surfaces with surface morphology controlled for a reduced real area of contact. The nano-undulated a-C:H films were prepared by radio frequency plasma enhanced chemical vapor deposition (r.f. PECVD) using nanoscale Cu dots surface on a Si (100) substrate. FE-SEM, AFM analysis showed that the after repeat deposition and plasma induced damage with Ar ions, the surface was nanoscale undulated. This phenomenon changed the surface morphology of a-C:H surface. Raman spectra of film with changed morphology revealed that the plasma induced damage with Ar ions significantly suppressed the graphitization of a-C:H structure. Also, it was observed that while the untreated flat a-C:H surfaces had wetting angle starting ranged from 72° and adhesion force of 332.79 nN. Had wetting angle the undulated a-C:H surfaces, which resemble the surface morphology of a cylindrical shape, increased up to 103.6° and adhesion force decreased down to 11 nN. The measurements agree with Hertz and JKR models. The surface undulation was affected mainly by several factors: the surface morphology affinity to cylindrical shape, reduction of the real area of contact and air pockets trapped in cylindrical double asperities of the surface.