Bongsu Shin

Extreme water repellency of nanostructured low-surface-energy non-woven fabrics

We report the extreme water repellent nature of non-woven fabrics of PET (polyethyleneterephthalate) whose fiber surfaces are nanotextured with oxygen plasma and coated with a low-surface-energy nanofilm. The surface effectively suppresses vapor condensation and repels condensed water droplets in addition to exhibiting a high contact angle and a low contact angle hysteresis with a millimetre-sized water drop. We also show that the surface maintains its superhydrophobicity after water-vapor condensation and after oil-wetting due to high-aspect-ratio nanohairs on the fibers. The superior water-repellent ability of the plasma treated non-woven fabric can be exploited in a variety of industrial applications including water harvesting and fuel cell water management even under oily contaminations.

Towards a biologically inspired small-scale water jumping robot

This paper describes the locomotion of a water jumping robot which emulates the ability of the water strider and the fishing spider to jump on the water surface. While previous studies of the robots mimicking aquatic arthropods were focused on recreating their horizontal skating motions, here we aim to achieve a vertical jumping motion. The robot jumps by pushing the water surface with rapidly released legs which were initially bent. The motion is triggered with a latch driven by the shape memory alloy actuator. The robot is capable of jumping to the maximum height of 26 mm. Jumping efficiency, defined as the ratio of the maximum jumping height on water to that on rigid ground, of a currently developed model is 0.26. This work represents a first step toward robots that can locomote on water with superior versatility including skating and jumping.

Rings, Igloos, and Pebbles of Salt Formed by Drying Saline Drops

It is well-known that evaporation of sessile drops with suspended particles like colloids and coffee powders can yield a variety of two-dimensional patterns depending on the particle shapes and internal flow patterns. Here we show that ordered three-dimensional structures can be built via evaporation of saline drops on highly hydrophobic substrates like pristine PP (polypropylene) with micropores and nanostructured low-surface-energy PP. On pristine PP having a high contact angle but a large contact angle hysteresis (CAH) with water, either rings or igloos of salt are formed depending on the salt concentration and evaporation rate. On nanostructured low-surface-energy PP having extreme water repellency with a very low CAH, pebbles of salt are formed regardless of salt concentration and evaporation rate. These observations lead us to conclude that combined effects of solubility, evaporation rate, and mobility of the contact line determine the final three-dimensional shape of the salt precipitate.

Application of fault diagnosis based on signed digraphs and PCA with linear fault boundary

In this paper, we developed a fault diagnosis model based on signed digraph(SDG), support vector machine(SVM) and improved principal component analysis(PCA) method. In PCA, we set linear fault boundaries. By means of the system decomposition based on SDG, the local models of each measured variable are constructed and more accurate and fast models are using an SVM, which has no loss of information and shows good performance, in order to obtain the estimated value of the variable, which is then compared with the measured value in order to diagnose the fault. And then, in order to make fault boundaries linearized, we select particular variables in the local model and express the data through the PC space. In the last analysis for various fault intensities, we diagnose a number of faulty data effectively. To verify the performance of the proposed model, the Tennessee Eastman(TE) Process was studied and the proposed method was found to demonstrate a good diagnosis capability compared with previous statistical methods.

Interfacial waves generated by electrowetting-driven contact line motion

The contact angle of a liquid-fluid interface can be effectively modulated by the electrowetting-on-dielectric (EWOD) technology. Rapid movement of the contact line can be achieved by swift changes of voltage at the electrodes, which can give rise to interfacial waves under the strong influence of surface tension. Here we experimentally demonstrate EWOD-driven interfacial waves of overlapping liquids and compare their wavelength and decay length with the theoretical results obtained by a perturbation analysis. Our theory also allows us to predict the temporal evolution of the interfacial profiles in either rectangular or cylindrical containers, as driven by slipping contact lines. This work builds a theoretical framework to understand and predict the dynamics of capillary waves of a liquid-liquid interface driven by EWOD, which has practical implications on optofluidic devices used to guide light.