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We present a novel method of realizing instant hydrophobic and long-lasting superhydrophilic states on stainless steel (SS) surfaces by micropatterning and thermally-induced surface layers (TISL). Heat treatment of SS surfaces in the air for 6 h yields hydrophobic TISLs at 140 °C and hydrophilic TISLs at 300 °C. The result of X-ray photoelectron spectroscopy reveals the correlation of chromium depletion with the hydrophilic TISLs, as well as surface oxidation with the hydrophobic TISLs. We fabricated superhydrophilic SS surfaces holding the wettability over 40 days, and hydrophobic SS surfaces realized in an hour-scale period via the fabrication process comprised of laser beam machining, electrochemical etching, and heat treatment in serial.

We present a straightforward method to realize a wettability-controlled stainless steel surface via wire electrical discharge machining. Samples of AISI 304 stainless steel were prepared, having various depth of groove, while width and pitch of groove were set to be 500 μm and 600 μm. Wetting properties of the fabricated samples were quantified by measuring static contact angle of deionized water and silicone oil, and sliding angle of water droplets. Mechanical robustness test was conducted by comparing static contact angle of water and silicone oil before and after 1000 cycles of abrasion. In order to propose potential applications, we demonstrate oil–water separation and polymer molding for fabricating superhydrophobic surfaces.

Commercial metals have recently been reconsidered as useful materials for functionalized surfaces with controlled wettability, yet the intrinsic wettability of various metal surfaces remain uncertain. In this study, we report that the wettability of a stainless steel (Type 304) surface exhibits a time-dependent transition, initially from a hydrophilic state to a relatively hydrophobic state. The static contact angle of a water droplet was monitored on polished stainless steel surfaces every 24 h, and a rapid increase in the contact angle, from 44° to 81°, was found during the first three days. The contact angle reached 98° in 40 days. To clarify the main factor causing the phenomenon, we compared the contact angles of water stored in four different environments for five days, where the oxygen and moisture levels were controlled. From the difference in contact angles depending on the amount of oxygen, we reached the conclusion that oxygen is likely the main cause of the aforementioned phenomenon. Finally, we demonstrated time-dependent wettability on a micro-grooved stainless steel surface whose initial superhydrophilicity became superhydrophobic after 30 days.

Multiscale, hierarchically patterned surfaces, such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usually used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this review, we highlight recent advances in scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. This review is organized into four sections. First, the role and importance of multiscale, hierarchical structures is described with four representative examples. Second, recent achievements in multiscale patterning are introduced with their strengths and weaknesses. Third, four application areas of wetting control, dry adhesives, selectively filtrating membranes, and multiscale tissue scaffolds are overviewed by stressing out how and why multiscale structures need to be incorporated to carry out their performances. Finally, we present future directions and challenges for scalable, multiscale patterned surfaces.

Modulus-tunable composite micropillars are presented by combining replica molding and selective inking for skin adhesive patch in "ubiquitous"-health diagnostic devices. Inspired from hierarchical hairs in the gecko's toe pad, a simple method is presented to form composite polydimethylsiloxane (PDMS) micropillars that are highly adhesive (∼1.8 N cm(-2) ) and mechanically robust (∼30 cycles).

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.

Bipolar plates of a proton exchange membrane fuel cell (PEMFC) play an important role in removing liquid phase water as a by-product in order to facilitate the reaction between fuel and oxygen. A great amount of effort has been made to improve the performance of a fuel cell such as maximum current density or maximum power, by improving water removability of a bipolar plate. Most of the studies, however, are conducted numerically because of the complexity of analysing gas and liquid and the poor manufacturability of graphite bipolar plates. In this proceeding, we demonstrate that the performance of a PEMFC with metal bipolar plates can be enhanced by reducing the dimension of the channel. Bipolar plates were machined with stainless steel (type 316L) to have three different channel size (1000 μm, 500 μm and 300 μm) and the performance of each assembled cells were tested. As a result, the maximum power density and the maximum current density increased by 25%.

A new page architecture with temporary LSB storing program scheme is presented as a breakthrough solution for sub-40nm FG (floating-gate) MLC NAND flash memories and beyond. Without program speed degradation, the proposed method is able to eliminate 100% BL cell-to-cell and almost 50% WL cell-to-cell coupling interferences which are well known as a most critical scaling barrier for FG NAND flash memories.

Inspired by the exceptional climbing ability of gecko lizards, artificial fibrillar adhesives have been extensively studied over the last decade both experimentally and theoretically. Therefore, a new leap towards practical uses beyond the academic horizon is timely and highly anticipated. To this end, we present a fibrillar adhesive in the form of bridged micropillars and its application to a transportation system with the detachment mechanism inspired by the climbing behaviour of gecko lizards. The adhesive shows strong normal attachment (~30 N cm(-2)) as well as easy and fast detachment within 0.5 s without involving complex dynamic mechanisms or specific stimulus-responsive materials. The fabrication of the bridged micropillars consists of replica moulding of polydimethylsiloxane (PDMS) micropillars, transfer of the PDMS precursor to the heads of the micropillars, and inverse placement on an inert Teflon-coated surface. Owing to the spontaneous interconnections of low viscosity PDMS precursor, bridged micropillars with a uniform capping nanomembrane (~800 nm thickness) are formed over a large area. Interestingly, macroscopic adhesion in the normal direction can be immediately switched between on and off states by changing the two detachment modes of pulling and peeling, respectively. To prove the potential of the fibrillar adhesive for practical use, an automated transportation system is demonstrated for lifting and releasing a mass of stacked glass slides over 1000 cycles of attachment and detachment.

Pure separation and sorting of microparticles from complex fluids are essential for biochemical analyses and clinical diagnostics. However, conventional techniques require highly complex and expensive labeling processes for high purity separation. In this study, we present a simple and label-free method for separating microparticles with high purity using the elasto-inertial characteristic of a non-Newtonian fluid in microchannel flow. At the inlet, particle-containing sample flow was pushed toward the side walls by introducing sheath fluid from the center inlet. Particles of 1 μm and 5 μm in diameter, which were suspended in viscoelastic fluid, were successfully separated in the outlet channels: larger particles were notably focused on the centerline of the channel at the outlet, while smaller particles continued flowing along the side walls with minimal lateral migration towards the centerline. The same technique was further applied to separate platelets from diluted whole blood. Through cytometric analysis, we obtained a purity of collected platelets of close to 99.9%. Conclusively, our microparticle separation technique using elasto-inertial forces in non-Newtonian fluid is an effective method for separating and collecting microparticles on the basis of size differences with high purity.

Platelet separation from blood is essential for biochemical analyses and clinical diagnosis. In this article, we propose a method to separate platelets from undiluted whole blood using standing surface acoustic waves (SSAWs) in a microfluidic device. A polydimethylsiloxane (PDMS) microfluidic channel was fabricated and integrated with interdigitated transducer (IDT) electrodes patterned on a piezoelectric substrate. To avoid shear-induced activation of platelets, the blood sample flow was hydrodynamically focused by introducing sheath flow from two side-inlets and pressure nodes were designed to locate at side walls. By means of flow cytometric analysis, the RBC clearance ratio from whole blood was found to be over 99% and the purity of platelets was close to 98%. Conclusively, the present technique using SSAWs can directly separate platelets from undiluted whole blood with higher purity than other methods.