Handong Cho

Robust superhydrophilic/hydrophobic surface based on self-aggregated Al2O3 nanowires by single-step anodization and self-assembly method.

Superhydrophilic and superhydrophobic surfaces were studied with an eye to industrial applications and use as research tools. Conventional methods involve complex and time-consuming processes and cannot feasibly produce large-area three-dimensional surfaces. Here, we report robust and large-area alumina nanowire structures with superhydrophobic or superhydrophilic properties, generated by an inexpensive single-step anodization process that can routinely create arbitrary three-dimensional shapes. This process is expected to open up diverse applications.

Energy harvesting model of moving water inside a tubular system and its application of a stick-type compact triboelectric nanogenerator

As the first invention to efficiently harvest electricity from ambient mechanical energy by using contact electrification, the triboelectric nanogenerator has elicited worldwide attention because of its cost-effectiveness and sustainability. This study exploits a superhydrophobic nanostructured aluminum tube to estimate electrical output for solid-water contact electrification inside a tubular system. The linearly proportional relationship of short-circuit current and open-circuit voltage to the detaching speed of water was determined by using a theoretical energy harvesting model and experimentation. A pioneering stick-type solid-water interacting triboelectric nanogenerator, called a SWING stick, was developed to harvest mechanical energy through solid-water contact electrification generated when the device is shaken by hand. The electrical output generated by various kinds of water from the environment was also measured to demonstrate the concept of the SWING stick as a compact triboelectric nanogenerator. Several SWING sticks were connected to show the feasibility of the device as a portable and compact source of direct power. The developed energy harvesting model and the SWING stick can provide a guideline for the design parameters to attain a desired electrical output; therefore, this study can significantly increase the applicability of a water-driven triboelectric nanogenerator.

Bendability optimization of flexible optical nanoelectronics via neutral axis engineering.

The enhancement of bendability of flexible nanoelectronics is critically important to realize future portable and wearable nanoelectronics for personal and military purposes. Because there is an enormous variety of materials and structures that are used for flexible nanoelectronic devices, a governing design rule for optimizing the bendability of these nanodevices is required. In this article, we suggest a design rule to optimize the bendability of flexible nanoelectronics through neutral axis (NA) engineering. In flexible optical nanoelectronics, transparent electrodes such as indium tin oxide (ITO) are usually the most fragile under an external load because of their brittleness. Therefore, we representatively focus on the bendability of ITO which has been widely used as transparent electrodes, and the NA is controlled by employing a buffer layer on the ITO layer. First, we independently investigate the effect of the thickness and elastic modulus of a buffer layer on the bendability of an ITO film. Then, we develop a design rule for the bendability optimization of flexible optical nanoelectronics. Because NA is determined by considering both the thickness and elastic modulus of a buffer layer, the design rule is conceived to be applicable regardless of the material and thickness that are used for the buffer layer. Finally, our design rule is applied to optimize the bendability of an organic solar cell, which allows the bending radius to reach about 1 mm. Our design rule is thus expected to provide a great strategy to enhance the bending performance of a variety of flexible nanoelectronics.

Conformable superoleophobic surfaces with multi-scale structures on polymer substrates

The preparation of superoleophobic surfaces has attracted considerable attention, but the practical application of previously reported methods is still difficult owing to the required complicated wafer process and lack of adaptability to any target objects. In this study, we successfully developed a highly conformable superoleophobic surface on a flexible polypropylene film using multi-scale structures and surface functionalization. The multi-scale structures are fabricated via the formation of nanowire arrays and attachment of nanoflakes, realized by inexpensive and easy fabrication processes. Then, after deposition of a fluoroalkylsilane coating, the structured surface shows superoleophobicity with high contact angles and low sliding angles for various liquids with surface tensions as low as 23.8 mN m−1. We also realized a patterned surface with extremely different wettabilities by tailoring the surface geometry and surface energy. Further, the wetting properties of the fabricated surface were maintained during repeated bending and twisting tests (10 000 cycles) and under shrinkage deformation (eshrink = 10%). Thus, the proposed strategy represents a facile method to develop easily adaptable surfaces with special wettabilities.

A large-scale water-harvesting device with β-Al(OH)3 microcone arrays by simple hydrothermal synthesis

As the scarcity of fresh water emerges as a major global problem, fog-water harvesting is considered as a sustainable method for obtaining water resources. Although water-harvesting technologies based on special wetting surfaces have attracted considerable interest because of their high efficiency, many problems remain, including high cost and difficulties in the fabrication of special wetting surfaces. Here we report a novel approach to fabricate conical microstructures on aluminum substrates using a cost-effective and scalable hydrothermal synthesis method. Due to their morphological features, the microcone surfaces exhibit extremely high wettability and excellent water-collection capacity. Furthermore, we suggest a practical water-harvesting device that allows an effective and sustainable way to collect fresh water from moist air and has enhanced adaptability to environmental conditions. These features enable us to develop a highly promising water-harvesting system for overcoming problems associated with water shortage.

The Behavioral Characteristics of a Droplet on the Line Patterned Surface Including Water Film

Herein the water film was introduced to the hydrophilic area on the line patterned surface to solve the contradiction caused by surface roughness (high different wettability has advantage to control the droplet but high roughness for that high wettability difference causes obstruction of droplet moving). Thus the droplet on the water film could not be hindered to line direction but restricted to orthogonal direction, effectively. In addition, droplet behaviors according to droplet volume and line thickness were studied. Droplet fell off the line with narrowing the interface between the droplet and the water film on the line. When the droplet fell off the line, the plate angle was designated as a critical plate angle and it used as an indicator of surface capability to control the droplet. As a result critical plate angle increases as droplet volume decreases and line thickness increases.