• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1752-1757
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• 2008

Simple and highly efficient droplet merging method is proposed, which enables two nanoliter or picoliter droplets to merge regularly in a straight microchannel. We observe that two droplets of the same size but of different viscosities are merged by velocity difference induced as they are transported with the carrier fluid. To make viscosity difference, the mass ratio of water and glycerol is varied. Two droplets of the same size or of different sizes are generated alternatingly in the cross channel by controlling flowrates. This droplet merging method can be used to mix or encapsulate one target sample with another material, so that it can be applied to cell lysis, particle synthesis, drug discovery, hydrogel-bead production, and so on.

• 한국전산유체공학회:학술대회논문집
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• 2009.11a
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• pp.103-108
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• 2009

The droplet impact and merging process on a flat substrate with contact angle hysteresis is numerically studied. The droplet deformation is determined by an improved level-set method employing a sharp-interface technique for the stress condition at the liquid-gas interface and the contact angle condition at the liquid-gas-solid interline. Based on the computations, the droplet impact and merging pattern is investigated to find the optimal condition in manufacturing a micro-line. The effects of dynamic contact angles and droplet spacing on droplet motion are quantified.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.3
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• pp.178-185
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• 2009

Simple and highly efficient droplet merging method is proposed, which enables two nanoliter or picoliter droplets to merge regularly in a straight microchannel. Using a cross channel with inflows of one oil phase through the main channel and two water phases through the side channels, two droplets of different sizes can be generated alternatingly in accordance with flow rate difference of the water phases. It is shown that for a fixed oil phase flow rate, the flow rate of one water phase required for alternating droplet generation increases linearly with the flow rate of another water phase. By this method, the droplets are merged with 100 % efficiency without any additional driving forces.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.666-669
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• 2008

Droplet in miniaturized microfluidic systems have received much focused attention recently. In this work, electrical charging phenomenon of a conducting water droplet on the electrode under the dc electric field is studied and using this phenomenon droplet actuation method for microreactor applications is experimentally demonstrated. To find effects of key factors, the effects of electric field, medium viscosity, and droplet size are investigated. A scaling law of charging for the conducting droplet is derived from the experimental results. Unlike the case of a perfect conductor, the estimated amount of electrical charge ($Q_{est}$) of a water droplet is proportional to the 1.59 power of the droplet radius (R) and the 1.33 power of the electric field strength (E). (For a spherical perfect conductor, Q is proportional to R2 and E.) It is thought that the differences are mainly due to incomplete charging of a water droplet resulted from the combined effect of electrochemical reaction at electrode and the relatively low conductivity of water. Using this phenomenon, we demonstrate the transport of the charged droplet and fusion of two oppositely-charged droplets. When electric field is subjected sequentially on the electrode, the charged droplet is transported on the electrode. For the visualization of fusion of charged droplets, the precipitation reaction is used. When subjected to a DC voltage, two droplets charged are moving and merging toward each other due to the Coulombic force and chemical reaction is simultaneously occurred by coalescence of droplets. It may be due to the interchange effect of charge. It is shown that the droplet can be used for microreactor where transporting, merging etc. of reagents constitute unit operation.

• Journal of Electrical Engineering and Technology
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• v.7 no.1
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• pp.91-96
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• 2012

We introduce an understanding of silver (Ag) inkjet overlap-printing characteristics from the viewpoints of cohesion between ink droplets and adhesion between an ink droplet and a surface. The printing characteristics were closely monitored by changing the surface energy to elucidate the effect of adhesion and cohesion on printing instability, such as droplet merging and line bulging. The surface energy of the substrate was changed through the hydrophilization of a hydrophobic fluorocarbon-coated surface. The surface energy and ink wettability of the prepared surfaces were characterized using sessile drop contact angle analysis, and printing instability was observed using an optical microscope after drop-on-demand inkjet printing with a 50% overlap in diameter of deposited singlet patterns. We found that the surface energy is not an appropriate indicator based on the experimental results of Ag ink printing on a hydrofluoric-treated silicon surface. The analytical approach using adhesion and cohesion was helpful in understanding the instability of the inkjet overlap-printing, as adhesion and cohesion represent the direct interfacial relationship between the Ag inks used and the substrate.

• Journal of the Korean Society of Visualization
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• v.21 no.1
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• pp.26-30
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• 2023

Interaction of a droplet and substrate is important to determine the coating and final deposition pattern in inkjet printing system. In particular, an accurate deposition of the droplet should be guaranteed for high-resolution patterning. In this study, we performed high-speed shadowgraph experiments on droplet train impact in inkjet system. From the high-speed images, we observed an unexpected bouncing phenomenon. We have found two factors affecting bouncing regime; the Weber number and the curvature of deposited droplet. Experimental results indicate that there is a critical curvature diameter of deposited droplet, which splits into bouncing and merging regime. From this result, we obtained a power-law behavior between the Weber number and the curvature. The understanding of bouncing phenomena helps to improve the accuracy and productivity of inkjet printing.

• Proceedings of the KIEE Conference
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• 2007.11a
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• pp.104-105
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• 2007

In this study, characteristics of silver ink-jet printing were investigated under various substrate treatments such as substrate heating, hydrophobic coating, and ultraviolet(UV)/ozone soaking. Fluorocarbon(FC) film was spin-coated on the polyimide (PI) film substrate to obtain a hydrophobic surface. Although hydrophobicity of the FC film could reduce the diameter of the printed droplets, the singlet images printed on the FC film surface showed irregularities in the pattern size and the position of the printed droplet along with droplet merging phenomenon. The proposed UV/ozone soaking of the FC film improved the uniformity of the pattern size and the droplet position after printing and substrate heating was very effective way in preventing droplet merging. By heating of the substrate after UV/ozone soaking of the coated FC film, silver conductive lines of 78-116 ${\mu}m$ line were successfully printed at low substrate temperatures of $40^{\circ}C$.

• Korean Chemical Engineering Research
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• v.52 no.1
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• pp.106-112
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• 2014

Here, we demonstrated the optimum design of pillar microstructure for efficient microdroplet merging. The microfluidic device mainly consisted of programmable microvalves and pillar microstructures. Based on the system, aqueous droplets were continuously generated at T-junction using actuating of integrated programmable microvalaves under the immiscible continuous fluid (mineral oil containing 0.5 wt% Span 80). The principle of merging process depended on the competitive correlation of hydraulic pressure of continuous phase and Laplace pressure of the droplet. We found that the design of the micropillars controls above two pressures. Finally, it was demonstrated that the microfluidic system could be able to efficient biochemical reaction. We expect that the microfluidic system is useful analytical or reaction tools in fundamental science, biotechnology, and chemical engineering.

• Korean Chemical Engineering Research
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• v.48 no.5
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• pp.545-555
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• 2010

Recently, droplet-based microfluidic systems are widely used in various areas ranging from fundamental science including chemistry, biology, and physics to material science and engineering. This article reviews recent development in the droplet based microfluidic system from basic fabrication of tiny device, principle of droplet formation, merging, mixing, control of droplets, and application for the synthesis of novel functional materials. We discuss strong advantages of the droplet based microfluidics in point of control of particle size, morphologies, shapes, and structures.

• 한국전산유체공학회:학술대회논문집
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• 2009.04a
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• pp.159-164
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• 2009

The water droplet motion and the interaction between the droplets in a PEMFC air flow channel with multiple pores, through which water emerges, is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid-gas interface is tracked by a level set method which is based on a sharp-interface representation for accurately imposing the matching conditions at the interface. The method is modified to implement the contact angle conditions on the walls and pores. The dynamic interaction between the droplets growing on multiple pores while keeping the total water flow rate through pores constant is investigated by conducting the computations until the droplet motion exhibits a periodic pattern. The numerical results show that the droplet merging caused by increasing the number of pores is not effective for water removal and that the contact angle of channel wall strongly affects water management in the PEMFC air flow channel.