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홍지우(Jiwoo Hong),강관형(Kwan Hyoung Kang) 한국가시화정보학회 2009 한국가시화정보학회 학술발표대회 논문집 Vol.2009 No.11
It is very important to predict the time response of electrowetting-based devices, such as liquid lenses, reflective displays, and optical switches. We investigated the time response of electrowetting, based on an analytical method to find out characteristic scales and a scaling law for the switching time. For this, spreading process of a sessile droplet was analyzed based on the domain perturbation method. First, we considered the case of weakly viscous fluids. The analytical result for the spreading process was compared with experimental results, which showed very good agreement in overall time response. We derived characteristic scales combining the droplet volume, density, and surface tension. A scaling law was derived from the analytical solution and was verified experimentally.
Jiwoo Hong(홍지우),Taewoong Uhm(엄태웅),In Seok Kang(강인석),Sang-Joon Lee(이상준) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12
Dispensing of tiny droplets is a basic and crucial process in myriad biochemical applications, such as DNA/protein microarray, cell cultures, chemical synthesis of microparticles, and digital microfluidics. This work demonstrates droplet dispensing on immiscible fluids through electric charge concentration (ECC) as a function of flow rates, applied voltages, and gap distances between the nozzle and the surface of oil, exhibiting three main modes (i.e., attaching, uniform, and bursting modes). Through a conventional nozzle with diameter of a few millimeters, charged droplets with volumes ranging from a few μL to a few tens of nL can be uniformly dispensed into the oil chamber without reduction in nozzle size. Based on the fascinating features of the proposed method (e.g., formation of droplets with controllable polarity and amount of electric charge in water and oil system), a simple and straightforward method is developed for microparticle synthesis, such as preparation of colloidosomes and fabrication of anisotropic Janus microparticles. Finally, a combined system consisting of ECC-induced droplet dispensing and electrophoresis of charged droplet (ECD)-driven manipulation systems is constructed. This integrated platform will provide more utility and flexibility in biochemical applications because a charged droplet can be delivered toward the intended position by programmable electric control.
홍지우(Jiwoo Hong),이승준(Seung Jun Lee),이상현(Sanghyun Lee),강관형(Kwan Hyoung Kang) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11
A droplet takes off from the superhydrophobic surface and bounces up, under an alternating electrical actuation. In this work, we carried out a numerical simulation using the phase field method to understand the detailed jumping process of a droplet and to figure out conditions for jumping. We found that resonant electrical actuation allows the droplet to accumulate sufficient energy for jumping and the superhydrophobic surface provides effective energy conversion from surface energy to kinetic energy due to low hysteresis and low adhesion energy. The numerical analyses provide detailed conditions for the droplet jumping in terms of hydrophobicity, droplet volume, and surface tension. Controlled jumping of a droplet on demand (CJDD) may provide three-dimensional droplet manipulations, which could significantly broaden the scope of applications in digital microfluidics area.