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정정열(Jung-Yeul Jung),유정열(Jung Yul Yoo),김영원(Young Won Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10
Nano-particles (on the order of 1 to 100 ㎚) contained within the droplet are moved by liquid flow and stacked at the contact line. The self-pinned contact line under the evaporating droplet is very interesting in the field of patterning and separation of particles and biocells<SUP>2-5</SUP>. Models accounting for the nano-particles’ flow and deposit patterns have been reported and verified by various experiments. Here, we report for the first time a phenomenon where micro-particles (on the order of 1 ㎛) in the colloid droplet flow to the center of droplet. There are three modes of fluid and particle flow in the evaporating droplet. In the first mode, a self-pinned contact line is maintained and the fluid and micro/nano-particles flow to the contact line. In the second mode, micro/nano-particles self-assemble at the near contact line, as reported by Jung and Kwak. In the final mode, only micro-particles are advected to the center of the droplet due to movement of the contact line.
정정열(Jung-Yeul Jung),오후석(Hoo-Suk Oh),곽호영(Ho-Young Kwak),최경빈(Kyong-Bin Choi),고창복(Chang-Bok Ko) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5
Convective heat transfers of a nanofluid in rectangular microchannel were measured. An integrated microsystem consisting of a single microchannel on one side and two localized heaters and seven polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al₂O₃) suspended nanofluids with various particle volume fractions were used in experiment to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient with Al₂O₃ suspended nanofluid was measured to be increased up to 30 % compared to the distilled water at a volume fraction of 1.8 vol.% without major friction loss. A new type of convective heat transfer correlation was proposed to correlate experimental data of heat transfer for nanofluids in microchannels.
정정열(Jung-Yeul Jung),곽호영(Ho-Young Kwak) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5
Boiling heat transfer on porous silicon surface was studied in this study. Experiments were conducted to investigate the effects of porous surface with submicron-scale roughness on the boiling heat transfer in subcooled FC-72 at the ambient pressure. Two-types of porous silicon surface were obtained using anodisation with HF (hydro fluoric acid) based electrolyte and DMF (dimethylforamide) based one. Thin film heaters for supplying constant heat flux and thin film temperature sensors were fabricated using conventional MUPs processes with doped polysilicon. Enhanced heat transfer using PSHF chip (porous silicon made using HF based electrolyte) and PSDMF chip (porous silicon made using DMF based electrolyte) were about 40.7 % and 2.3 %, respectively.
정정열(Jung-Yeul Jung),유정열(Jung Yul Yoo) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
In this study, the mechanism of enhanced thermal conductivity is elucidated on the bases of both electric double layer (EDL) and kinetic theory. A novel expression for the thermal conductivity of nanofluids is proposed and verified by applying to Al₂O₃ nanofluids with regard to various temperatures, volume fractions and particle sizes. In dilute nanofluids, the effects of Brownian motion and particle interaction on enhancing the thermal conductivity of nanofluids are quite comparable while the effect of particle interaction due to EDL is more prominent in dense nanofluids. The model presented in this paper shows that particle interaction due to the electrical double layer is the most responsible for the enhancement of thermal conductivity of nanofluids.
정정열(Jung-Yeul Jung),곽호영(Ho-Young Kwak) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4
In this study, micro square heaters having dimensions of 65×65 μ㎡ and 100×100 μ㎡ were fabricated and bubble nucleation experiments on the heaters were performed. Bubble nucleation temperature was also measured using a bridge circuit and the photographs of bubble nucleation and subsequent growth were taken by a camera with a flash unit. Measured bubble nucleation temperatures were found to be closer to the superheat limit of working fluid (FC-72). Also quasi-1D analyses for the square heaters were performed. The quasi-1D analysis yielded proper temperature distribution of the square heater at steady state, however failed to predict the temperature rise up to the steady state. Similar time dependent temperature can be obtained with proper value of thermal diffusivity. For the 100×100 μ㎡ square heater, nucleation of several bubbles was observed while only one bubble was observed to be nucleated on 65×65 μ㎡ heater.
정정열(Jung-Yeul Jung),곽호영(Ho-Young Kwak) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.11
Micropump is very useful component in micro/nano fluidics and bioMEMS applications. In this study, a bubble-powered micropump was fabricated and tested. The micropump consists of two-parallel micro line heaters, a pair of nozzle-diffuser flow controller and a 1 mm in diameter, 400 μm in depth pumping chamber. The two-parallel micro line heaters with 20 μm-width and 200 μm-length were fabricated to be embedded in the silicon dioxide layer of a wafer which serves as a base plate for the micropump. The pumping chamber, the pair of nozzle-diffuser unit and microchannels including the liquid inlet and outlet port were fabricated by etching through another silicon wafer. A glass wafer (thickness of 525±15 μm) having two holes of inlet and outlet ports of liquid serve as upper plate of the pump. Finally the silicon wafer of the base plate, the silicon wafer of pumping chamber and the glass wafer were aligned and bonded (Si-Si bonding and anodic bonding). A sequential photograph of bubble nucleation, growth and collapse was visualized by CCD camera. Clearly liquid flow through the nozzle during the period of bubble growth and slight back flow of liquid at the end of collapsing period can be seen. The mass flow rate was found to be dependent on the duty ratio and the operation frequency. As duty ratio increases, flow rate decreases gradually when the duty ratio exceeds 60%. Also as the operation frequency increases, the flow rate of the micropump decreases slightly.
유전영동과 액적증발 현상을 이용한 마이크로/나노 입자와 생체 입자 분리에 관한 연구
정정열(Jung-Yeul Jung),곽호영(Ho-Young Kwak) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11
In this study, micro/nano particles and bio cells mixed in water solution were separated using the electrical and mechanical force due to dielectrophoresis and evaporating droplet, respectively. Micro patterned electrodes of Au were fabricated on the silicon dioxide layer and were used for generating dielectrophoresis. Polystyrene particles, red blood cells (RBCs) and E-coli were used for separating objects. Micro/nano particles and bio cells were separated by adjusting the amplitude and frequency of applied voltage. Based on our study, one can separate, control and sense the micro/nano particles and bio cells in solutions.