Yudi Rahmawan

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.

One-step process for superhydrophobic metallic surfaces by wire electrical discharge machining.

We present a direct one-step method to fabricate dual-scale superhydrophobic metallic surfaces using wire electrical discharge machining (WEDM). A dual-scale structure was spontaneously formed by the nature of exfoliation characteristic of Al 7075 alloy surface during WEDM process. A primary microscale sinusoidal pattern was formed via a programmed WEDM process, with the wavelength in the range of 200 to 500 μm. Notably, a secondary roughness in the form of microcraters (average roughness, Ra: 4.16 to 0.41 μm) was generated during the exfoliation process without additional chemical treatment. The low surface energy of Al 7075 alloy (γ = 30.65 mJ/m(2)) together with the presence of dual-scale structures appears to contribute to the observed superhydrophobicity with a static contact angle of 156° and a hysteresis less than 3°. To explain the wetting characteristics on dual-scale structures, we used a simple theoretical model. It was found that Cassie state is likely to present on the secondary roughness in all fabricated surfaces. On the other hand, either Wenzel or Cassie state can present on the primary roughness depending on the characteristic length of sinusoidal pattern. In an optimal condition of the serial cutting steps with applied powers of ∼30 and ∼8 kW, respectively, a stable, superhydrophobic metallic surface was created with a sinusoidal pattern of 500 μm wavelength.

Surface energy tunable nanohairy dry adhesive by broad ion beam irradiation

We present a simple and shape-controllable method to tune the surface energy and tilting angle of nanohairy structures using broad ion beam irradiation. First, the vertical nanohair arrays made of ultraviolet (UV)-curable polyurethane acrylate (PUA) material were prepared by replica molding. Subsequently, the nanohairs were bent to a wide range of angles from 0 (vertical) to 75° (stooped hairs) by the oblique broad Ar ion beam. The surface energy was tuned with normal oxygen (O2) ion beam irradiation together with aging effect (47.1 to 71.9 mJ m−2). It was observed that the shear adhesion strength was significantly increased from 25.2 to 58.1 N cm−2 on the hydrophilic test surface and from 22.8 to 45.7 N cm−2 on the hydrophobic test surface, respectively, with the variation of surface energy from 47.1 to 71.9 mJ m−2. In addition, the shear adhesion strength was increased from 58.1 to 124.1 N cm−2 and from 45.7 to 110.6 N cm−2 on hydrophilic and hydrophobic test surfaces, respectively, as the tilting angle was varied from 0 to 75° at the highest surface energy of 71.9 mJ m−2. The measured adhesion data were compared with the theoretical adhesion forces based on Johnson–Kendall–Roberts (JKR), modified-JKR, peel zone (PZ), and Kendall peeling models, suggesting that the contact state of nanohairs is a mixture of tip and side contacts.

Beetle-Inspired Bidirectional, Asymmetric Interlocking Using Geometry-Tunable Nanohairs

We present bidirectional, asymmetric interlocking behaviors between tilted micro- and nanohair arrays inspired from the actual wing locking device of beetles. The measured shear adhesion force between two identical tilted microhair arrays (1.5 μm radius, 30 μm height) turned out to be higher in the reverse direction than that in the angled direction, suggesting that the directionality of beetles microtrichia may play a critical role in preventing the elytra from shifting along the middle of insect body. Furthermore, we observed dramatic enhancement of shear adhesion using asymmetric interlocking of various nanohair arrays (tilting angle, δ < 40°). A maximum shear locking force of ∼60 N/cm(2) was measured for the nanohair arrays of 50 nm radius and 1 μm height with a hysteresis as high as ∼3. A simple theoretical model was developed to describe the measured asymmetric adhesion forces and hysteresis, in good agreement with the experimental data.

Multiscale Transfer Printing into Recessed Microwells and on Curved Surfaces via Hierarchical Perfluoropolyether Stamps

A simple method for the formation of multiscale metal patterns is presented using hierarchical polymeric stamps with perfluoropolyether (PFPE). A dual-scale PFPE structure is made via two-step moulding process under partial photocrosslinking conditions. The hierarchical PFPE stamp enables multiscale transfer printing (MTP) of metal pattern in one step within microwells as well as on curved surfaces.

Pitch reduction lithography by pressure-assisted selective wetting and thermal reflow

We report on a new pitch reduction lithographic technique by utilizing pressure-assisted selective wetting and thermal reflow. The primary line-and-space pattern of low molecular weight polystyrene (PS) (Mw=17,300) was formed by solvent-assisted capillary force lithography (CFL), on which a diluted photoresist (PR) solution was selectively filled into the spaces by the application of a slight pressure (200 g cm(-2)). Subsequent removal of the PS pattern by toluene and ashing process led to a line pattern with approximately 50% pitch reduction. It was observed that the size reduction and space to width ratios were controllable by changing PR concentration and ashing time.

3-D hierarchical wrinkled micro-pillars for anti-cells proliferation surfaces

Fabrication of dual-scale roughness in a large area of biocompatible superhydrophobic surfaces was presented in this paper. Three dimensional (3-D) dual-scale roughness was achieved using a combination of top-down process by replica molding to make an ordered micro-pillars on poly-(dimethylsiloxane) (PDMS) as the soft base material and bottom-up process by deposition of diamond like amorphous carbon (DLC) as a hard-coating using radio frequency-chemical vapor desposition (RF-CVD) to produce nanoscale wrinkle structures. The extreme superhydrophobicity of the as-prepared surfaces was used as the culturing template of calf pulmonary artery endothelial (CPAE) cells for 3 days. We discovered that CPAE cells will be more adhered on surfaces with only microstructure compared to hierarchical structures. In particular, the reduced filopodia extension during cell growth was caused by disconnected focal adhesions on the pillar pattern. This limited cell adhesion could prevent undesired growth and proliferation of biological species on the surface of biomedical devices such as stents, implants or even injection syringes.