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전단흐름 하에 이온교환막 위에서 발생하는 전기수력학적 와류
곽노균(Rhokyun Kwak) 한국가시화정보학회 2018 한국가시화정보학회지 Vol.16 No.1
Ion exchange membrane can transfer only cation or anion in electrically conductive fluids. Recent studies have revealed that such selective ion transport can initiate electroconvective instability, resulting vortical fluid motions on the membrane. This so-called electroconvective vortex (a.k.a. electroconvection (EC)) has been in the spotlight for enhancing an ion flux in electrochemical systems. However, EC under shear flow has not been investigated yet, although most related systems operate under pressure-driven flows. In this study, we present the direct visualization platform of EC under shear flow. On the transparent silicone rubber, microscale channels were fabricated between ion exchange membranes, while allowing microscopic visualization of fluid flow and ion concentration changes on the membranes. By using this platform, not only we visualize the existence of EC under shear flow, its unique characteristics are also identified: i) unidirectional vortex pattern, ii) its advection along the shear flow, and iii) shear-sheltering of EC vortices.
이온농도분극에 의한 와류현상에서 관찰된 유동전단응력-이동장벽 효과
곽노균(Rhokyun Kwak),Van Sang Pham,강지윤,한종윤(Jongyoon Han) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Rotating shear flow is a common tool to suppress turbulence and control transport in plasma physics. Despite the desires to apply this phenomenon to other fields, it has rarely been observed in non-ionized fluids. Here, we visualize this effect on a generic neutral-fluid situation in electro-membrane systems: electroconvective vortex (EC) under Hagen-Poiseuille (HP) flow, which initiated by ion concentration polarization (ICP) on ion exchange membranes. This is the first demonstration of “flow-shear-induced transport barrier” in electrochemical systems, which was found only in magnetized flow previously. Combining scaling analysis and experiment, we capture the effect by pinpointing the threshold for shear suppression. Selected by balancing velocity fluctuation (with EC vortices) and flow shear (with noslip walls), the threshold is scaled as the dimensionless EC height dec/w=0.618. Stable EC occurs under the threshold, while chaotic EC occurs over the threshold by velocity fluctuation overwhelming flow shear. The threshold we identify here has wide applications to prevent chaotic flows in various electro-membrane systems.
UV 나노임프린팅을 이용한 단일(單一) 나노 브리지 구조의 제작
곽노균(Rhokyun Kwak),정훈의(Hoon Eui Jeong),서갑양(kahp Y. Suh) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
In this paper, a simple vacuum-assisted UV nanoimprint lithography (NIL) method is presented for fabricating well-defined monolithic nanobridges. A droplet of UV-curable resin of polyurethane acrylate (PUA) is dispensed and imprinted to form microscale structures with PDMS mold and subsequent curing for several seconds. Under proper conditions, the top layer is partially cured by the trapped air and provides further nanofabrication upon application of a nanoscale PUA mold at vacuum. It is noted that a pressure gradient is generated between a contacting and a non-contacting parts with the mold, resulting in a flow of the partial cured pre-polymer into the void regions. Finally, monolithic nanobridges is fabricated with high aspect ratio in large area. These nanobridges are potentially useful for smart optical and electronic devices.
전기수력학적 유동불안정성에 의한 와류의 크기 스케일 분석
김귀열(Gwiyeol Kim),곽노균(Rhokyun kwak) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
In this paper, steady electroconvection of ~1 was observed in high voltage regime. By scaling analysis and direct observation, length-scale selection of ~1 EC was researched. As a result, A distinctive behavior was revealed, which is, the steady electroconvection got smaller when applied voltage increased, however, when the voltage reached certain point, the vortex stopped being smaller. And the least size of vortex decreased when bulk ion concentration increased. We observed this behavior to plot the least size of steady electroconvection versus ion concentration and earned a power fitting exponent of -1/5. This is first research that directly observed steady electroconvection of ~1 , and analyzed the least size of electroconvection by direct observation. This discovery will contribute to the understanding of length-scale selection of microscale electroconvection and performance improvement of various electrochemical systems.
전단유동에서 슈미트 수가 전기수력학적 와류에 미치는 영향
강수현(Soohyeon Kang),곽노균(Rhokyun Kwak) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.12
Electroconvective vortices (EC) on ion exchange membranes is one of the most tantalizing problem in electrochemical systems, but its dynamics in 3-D is still vague. Here, we present the Schmidt number dependence of 3-D EC under shear flow. The EC was generated by ion concentration polarization between two ion exchange membranes. The vortex patterns were visualized in a wide range of Schmidt numbers (Sc = 556-56127) under various flow rates (Reynolds number Re = 0-0.8074) and applied voltages (electric Rayleigh number RaE = 0-69717). The mixture of glycerol in water is used to control Schmidt number (0-70 wt%). With two basic vortex patterns under shear flow (longitudinal and transverse rolls), low flow shear (low Re) and/or high electric body force (high RaE) induces transverse rolls between longitudinal rolls; and vice versa. Here, at a given Re, we revealed that a high Sc induces the increase of the critical RaE for the onset of transverse vortices. It is noteworthy that this is the first direct verification of the effect of Schmidt number on EC.
정훈의(Hoon Eui Jeong),곽노균(Rhokyun Kwak),이승석(Seung Seok Lee),서갑양(Kahp Yang Suh) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
A simple method is presented for fabricating micro/nanoscale combined hierarchical structures using a two-step UV-assisted capillary molding technique. This lithographic method consists of two steps: (ⅰ) fabrication of partially cured polymer microstructures using a PDMS mold and (ⅱ) subsequent nanofabrication using a high-resolution polyurethane acrylate (PUA) mold on top of the pre-formed microstructures. Using this technique, various micro/nano hierarchical structures were fabricated with minimum resolution down to 70 ㎚ over a large area with very good reproducibility.