Sang Moon Kim

Robust superomniphobic surfaces with mushroom-like micropillar arrays

We present a simple method for the fabrication of robust superomniphobic surfaces with high transmittance (>90%) and durability (<6 months). The method consists of direct micromolding of mushroom-like micropillars and C4F8 gas surface treatment. Such re-entrant structures were found to be highly resistant against wetting by various liquids and oils with a wide range of surface tensions from 22.3 (ethanol) to 72.1 mN m−1 (water). Optimal structural parameters were derived based on the measurements of static contact angle and contact angle hysteresis.

Bioinspired reversible interlocker using regularly arrayed high aspect-ratio polymer fibers.

A reversible interlocker that is inspired by the wing locking device of beetles is presented. It exploits the van der Waals force-assisted binding between high-aspect-ratio polymer fibers. The two-layered interlocker is highly flexible and displays an extremely high shear locking force and easy normal lift-off.

Shape-Controllable Microlens Arrays via Direct Transfer of Photocurable Polymer Droplets

A simple method is presented to form an array of shape-controllable microlenses by partial photocuring of an UV-curable polymer and direct transfer. Using the transferred lens array, nanoscale metal patterns as small as 130-nm gaps are detected under an optical microscope with a distinguishable resolution.

Multiplex lithography for multilevel multiscale architectures and its application to polymer electrolyte membrane fuel cell

The production of multiscale architectures is of significant interest in materials science, and the integration of those structures could provide a breakthrough for various applications. Here we report a simple yet versatile strategy that allows for the LEGO-like integrations of microscale membranes by quantitatively controlling the oxygen inhibition effects of ultraviolet-curable materials, leading to multilevel multiscale architectures. The spatial control of oxygen concentration induces different curing contrasts in a resin allowing the selective imprinting and bonding at different sides of a membrane, which enables LEGO-like integration together with the multiscale pattern formation. Utilizing the method, the multilevel multiscale Nafion membranes are prepared and applied to polymer electrolyte membrane fuel cell. Our multiscale membrane fuel cell demonstrates significant enhancement of performance while ensuring mechanical robustness. The performance enhancement is caused by the combined effect of the decrease of membrane resistance and the increase of the electrochemical active surface area.

Ultra-sensitive Pressure sensor based on guided straight mechanical cracks.

Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

Thermoresponsive switching of liquid flow direction on a two-face prism array

We present a simple approach to reversibly switch the direction of liquid flow on physically symmetric and chemically asymmetric prism structures by exploiting the reversibility of surface wetting properties of a thermo-responsive polymer, poly(N-isopropyl-acrylamide). Such an asymmetric prism array creates a flow path in the direction of the lower critical contact angle. This allows a unidirectional “step flow” across the ridges of prism channels, which can be made reversible with a suitable temperature change.

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.

High-performance Fuel Cell with Stretched Catalyst-Coated Membrane: One-step Formation of Cracked Electrode

We have achieved performance enhancement of polymer electrolyte membrane fuel cell (PEMFC) though crack generation on its electrodes. It is the first attempt to enhance the performance of PEMFC by using cracks which are generally considered as defects. The pre-defined, cracked electrode was generated by stretching a catalyst-coated Nafion membrane. With the strain-stress property of the membrane that is unique in the aspect of plastic deformation, membrane electrolyte assembly (MEA) was successfully incorporated into the fuel cell. Cracked electrodes with the variation of strain were investigated and electrochemically evaluated. Remarkably, mechanical stretching of catalyst-coated Nafion membrane led to a decrease in membrane resistance and an improvement in mass transport, which resulted in enhanced device performance.

In Situ Realization of Asymmetric Ratchet Structures within Microchannels by Directionally Guided Light Transmission and Their Directional Flow Behavior

An asymmetric ratchet structure within microchannels is demonstrated by directionally guided light transmission for controlled liquid flow. A direct and facile method is presented to realize programmed asymmetric structures, which control the fluid direction and speed.

Artificial Slanted Nanocilia Array as a Mechanotransducer for Controlling Cell Polarity

We present a method to induce cell directional behavior using slanted nanocilia arrays. NIH-3T3 fibroblasts demonstrated bidirectional polarization in a rectangular arrangement on vertical nanocilia arrays and exhibited a transition from a bidirectional to a unidirectional polarization pattern when the angle of the nanocilia was decreased from 90° to 30°. The slanted nanocilia guided and facilitated spreading by allowing the cells to contact the sidewalls of the nanocilia, and the directional migration of the cells opposed the direction of the slant due to the anisotropic bending stiffness of the slanted nanocilia. Although the cells recognized the underlying anisotropic geometry when the nanocilia were coated with fibronectin, collagen type I, and Matrigel, the cells lost their directionality when the nanocilia were coated with poly-d-lysine and poly-l-lysine. Furthermore, although the cells recognized geometrical anisotropy on fibronectin coatings, pharmacological perturbation of PI3K-Rac signaling hindered the directional elongation of the cells on both the slanted and vertical nanocilia. Furthermore, myosin light chain II was required for the cells to obtain polarized morphologies. These results indicated that the slanted nanocilia array provided anisotropic contact guidance cues to the interacting cells. The polarization of cells was controlled through two steps: the recognition of underlying geometrical anisotropy and the subsequent directional spreading according to the guidance cues.