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KWAN HYOUNG KANG, Ph.D. Research Associate Department of Mechanical Engineering Pohang University of Science and Technology San 31, Hyoja-dong, Pohang 790-784, Republic of Korea Phone: +82-54-279-8201 Fax: +82-54-279-3199 Mobile: 019-345-2632 E-mail: khkang@postech.edu Homepage: http://www.postech.ac.kr/~khkang/

EDUCATION Ph.D. Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), (1993. 3 ~ 1996. 8). Dissertation: “Theoretical and Experimental Investigations of Motion and Containment of Contaminated Free Surface Layer.” Supervisor: Prof. Choung Mook Lee. M.S. Department of Mechanical Engineering, POSTECH (1991. 3 ~ 1993. 2). Dissertation: “Theoretical and Experimental Investigation on Wave-Induced Drift Velocity.” Supervisor: Prof. Choung Mook Lee. B.S. Department of Mechanical Engineering, POSTECH (1987. 3 ~ 1991. 2). Dissertation: “Temperature-Field Measurement of a Tiny Electronic Chip by an Infrared Thermal Video.” Supervisor: Prof. Moo Hwan Kim. EMPLOYMENT Research Assistant Professor (2001.8~Present), Department of Mechanical and

Industrial Engineering, POSTECH Senior Researcher (1999.3 ~ 2000. 8), Samsung Ship Model Basin (SSMB),

Samsung Heavy Industries (SHI) Postdoctorial Fellow (1998.9~1999.2), Korean Institute of Machinery and Materials

(KIMM), Korean Research Institute of Ship and Ocean Engineering (KRISO) Researcher (Postdoctorial Fellow) (1996.9~1998.7), Advanced Fluids Engineering

Research Center, POSTECH WORK EXPERIENCE Research Assistant Professor (2001.8~Present), Department of Mechanical and Industrial Engineering, POSTECH Investigating microfluidic transport phenomena, both theoretically and

experimentally. Electrowetting: development of electromechanical theory for charge-related wetting

phenomena. Undergoing project “Application of electrowetting in microfluidics (2 years).” [Published five papers to Langmuir (American Chemical Society, Journal of Interface and Colloid) as the corresponding author].

Electroosmotic flow inside microchannels: Numerical and experimental study on the electrokinetic flow instability, 1) Analysis of electroosmotic flow around a wedge, 2)

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Numerical analysis and µPIV measurement of electrokinetic instability of electroosmotic flow inside a T-channel.

Microfluidics applications: Theoretical investigation on the liquid junction potential in a slit-like microchannel. (J. of Electroanal. Chem., In press)

PIV measurement of flow inside an evaporating droplet, “Visualization of flow inside a small evaporating droplet,” Presented at the 5th Int’l Conf. on PIV, Busan, Korea, 2003).

Reviewer of “National Research Laboratory (NRL)” project (2001), Korean Institute of Science and Technology Evaluation and Planning (KISTEP). Reviewer of “Dual Use Technology” project (2001), KISTEP, Project title “Development of middle-to-large scale waterjet propulsion systems.” Senior Researcher (1999.3 ~ 2000. 8), Samsung Ship Model Basin (SSMB), Samsung Heavy Industries (SHI) Developed the hydrophone array system for underwater hydroacoustic measurement. Measurement of hydroacoustic noise of underwater vehicles. (Major customer:

ADD(Agency for Defense Development). Principal Investigator of the National Research Laboratory (NRL, Hydroacoustic

Research Laboratory) (2000.5. ~ 2000.8), Project title “Measurement and control of hydroacoustic noise of underwater vehicles.”

Postdoctorial Fellow (1998.9~1999.2), Korean Institute of Machinery and Materials (KIMM), Korean Research Institute of Ship and Ocean Engineering (KRISO) Developed oscillatory hydrofoil system for study of cloud cavitation and effect of air

injection for control of the cavitation. Postdoctorial Fellow (1996.9~1998.8), Advanced Fluids Engineering Research Center, POSTECH Developed bubble tracking algorithm for analysis of bubble cavitation and cavitation

noise. Measurement of cavitation noise of the Schiebe and the hemispherical headform in

the Hyundai Marine Research Institute (HMRI). Experimental and numerical investigation on the effectiveness of the tandem oil

fences. HONOR, ACADEMIC AWARDS, AND SCHOLARSHIPS Presidential Researcher [Principal Investigator of the National Research Laboratory

(NRL, Hydroacoustic Research Laboratory)] (2000.5. ~ 2000.8). Bronze Medal, Research Excellence Awards of Samsung Heavy Industries (2001).

“Project Title: Development of Hydrophone Array System for Underwater Noise Measurement”

Excellent Paper Presentation Award: The 5th KSME-JSME Fluids Engineering Conference, Nagoya, Japan, November 2002.

Postdoctorial Scholarships from Korean Science and Engineering Foundation Post

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Doc. (1998.9~1999.2). MEMBERSHIPS Member, Korean Society of Mechanical Engineers. Member, Society of Naval Architecture of Korea. Member, Korean Society for Marine Environmental Engineers.

JOURNAL REVIEW Langmuir, J. Physics D: Applied Physics RESEARCH EXPERIENCE (based on published papers) Microfluidics: electrokinetics, electromagnetism, electrochemistry, electrical

double layer, surface phenomena (wetting, adhesion, film, particle interaction, DLVO theory), electrowetting theory and experiment, electro-magneto-hydrodynamics (EMHD), electroosmotic flow.

MEMS Fabrication: Taken a short course on “Fundamental theory and practice on CMOS fabrication processes,” ISRC (Inter-University Semiconductor Research Center), Seoul National University, March 15-20. The course covers theory and practice on the photolithography, dry etching, CVD (chemical vapor deposition), metallization, and ion implantation.

Bubble (Droplet) Dynamics: cavitation and bubble dynamics, droplet-flow measurement, Lagrangian tracking of bubble and particle.

Hydroacoustics: hydroacoustic measurement device development hydroacoustic measurement, beam-forming, turbulent wall-pressure measurement, theory on Helmholtz resonators.

General Fluid Dynamics: boundary layer theory, interfacial instability, analysis of floating body motion and measurement, hydrodynamic flow measurement (LDV, visualization method), free surface flow, free-surface wave, discrete vortex method.

Others: mathematical analysis of engineering problems, Finite Element Method (FEM), Boundary Element Method (BEM), Finite Volume Method (FVM), oil spill, oil-fence design, membrane deformation in flow.

RESEARCH PROJECTS AND FUNDING Electrical control of wetting characteristics of liquid droplets and films, POSCO

Technology Development Fund, $10k, (Jan. 2003 ~ Dec. 2003). Microfluidics application of electrowetting, POSCO Technology Development

Fund, $10k, (Jan. 2002 ~ Dec. 2002). Modeling of acoustic field inside Samsung cavitation tunnel and reduction of

radiation noise of propeller driving motor, Samsung Heavy Industries, $240k, (Jan. 2001 ~ Dec. 2001), Project Manager.

Error analysis for experimental results in the Samsung cavitation tunnel and an investigation on wall effect, Samsung Heavy Industries, $250k, (Jan. 2001 ~ Dec. 2001), Project Manager.

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Performance test of a new type pump-jet thruster, Agency for Defense and Development (ADD), $250k, (June 2001 ~ Dec. 2001), Project Manager.

A series test of ducted propellers, Agency for Defense and Development (ADD), $260k, (June 2000 ~ Dec. 2000), Project Manager.

Control and measurement of radiation noise of a underwater vehicle, Korean Ministry of Science and Technology, $250k, (Sep. 1999 ~ Aug. 2000), Project Manager.

Control and measurement of radiation noise of a underwater vehicle, Korean Ministry of Science and Technology, $250k, (Sep. 1999 ~ Aug. 2000), Project Manager.

Development of hydrophone array system for underwater noise measurement, Samsung Heavy Industries, $400k, (Jan. 2000~ Dec. 2000), Project Manager.

A model test in a low noise cavitation tunnel, Agency for Defense and Development (ADD), $100k, (June 1999 ~ Dec. 2001).

INVITED TALKS Electrowetting: Fundamental Theory and Applications, Samsung Electro-

Mechanics, Korea, March, 11, 2004. Fluid Mechanics in Microscale and Its Electromagnetic Controls, School of

Mechanical Engineering, Pusan National University, Korea, Jan. 14, 2004. Fluid Mechanics in Microscale and Its Electromagnetic Controls, School of

Mechanical and Aerospace Engineering, Seoul National University, Korea, Nov. 15, 2003.

Fluid Mechanics in Microscale and Its Electromagnetic Controls, Department of Mechanical Engineering, Chung-Ang University, Korea, Dec. 15, 2003.

Electrowetting: Microfluidic Applications and Electromechanical Theory, Autumn’s Meeting of Fluids Engineering Division of KSME, Korea University, Seoul, Dec. 2003.

Electrowetting: Microfluidic Applications and Electromechanical Theory, Department of Chemical Engineering, POSTECH, Sep. 2003.

Measurement of Hydroacoustic Signiture in a Cavitation Tunnel, Department of Mechanical Engineering, POSTECH, Mar. 2001.

Measurement of Radiation Noise of Underwater Vehicles, Agency for Defense Development (ADD) of Korea, May 2000.

Prediction and Control of Cavitation Noise, KRISO (KIMM), May 1998. INTERNATIONAL JOURNAL PAPERS

[J15] K. H. Kang and I. S. Kang “Comment on “Interface profiles near three-phase contact lines in electric fields”” Phys. Rev. Lett. (Submitted).

[J14] K. H. Kang, J. W. Park, I. S. Kang, and K. Y. Huh (2004) “Mechanism of the electrohydrodynamic instability of two-layered miscible fluids in microchannels,” Physical Review E. (Submitted).

[J13] K. H. Kang, S. J. Lee, C. M. Lee, and I. S. Kang (2003) “Visualization of flow inside

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a two-component evaporating droplet,” Measurement Science & Technology (In press). (Corresponding author)

[J12] K. H. Kang and I. S. Kang (2003) “Theoretical investigation on the liquid junction potential in a slit-like microchannel,” J. Electroanalytical Chemistry (Published on web, http:// 10.1016/j.jelechem.2003.11.044). (Corresponding author, SCI, IF=2.027)

[J11] C. M. Lee, D. G. Han, K. H. Kang and S. J. Lee (2004) “Investigation of effectiveness of tandem oil fences under currents,” J. Marine Science and Technology 8, 117-125. (SCIE)

[J10] K. H. Kang and I. S. Kang (2003) “Validity of the Derjaguin approximation in electrostatic effect on the Frumkin—Derjaguin approach,” Langmuir (American Chemical Society, Journal of Interface and Colloid) 19(23), 9962-9967. (Corresponding author, SCI, IF=3.248)

[J9] K. H. Kang, I. S. Kang, C. M. Lee (2003) “Electrostatic contribution to line tension in a wedge-shaped contact region,” Langmuir, 19(22), 9334-9342. (Corresponding author, SCI, IF=3.248).

[J8] K. H. Kang, I. S. Kang, C. M. Lee (2003) “Geometry dependence of wetting tension on charge-modified surfaces,” Langmuir, 19(17), 6881-6887. (Corresponding author, SCI, IF=3. 248)

[J7] K. H. Kang, I. S. Kang, C. M. Lee (2003) “Wetting tension due to Coulombic interaction in charge-related wetting phenomena,” Langmuir, vol. 19(8), 5407-5412. (Corresponding author, SCI, IF=3.248)

[J6] K. H. Kang (2002) “How electrostatic fields change contact angle in electrowetting,” Langmuir, vol. 18(26), 10318–10322. (Corresponding author, SCI, IF=3.248)

[J5] K. H. Kang, I. S. Kang, and C. M. Lee (2002) “Effects of uniform magnetic field on a growing or collapsing bubble in a weakly viscous conducting fluid,” Physics of Fluids, vol. 14(1), 29-40. (Corresponding author, SCI, IF=1.697)

[J4] C, M. Lee, and K. H. Kang (1998) “Prediction of oil boom performance in currents and waves,” Spill Science and Technology Bulletin, vol. 4, pp. 257-266. (SCI, IF=0.36)

[J3] C. M. Lee, K. H. Kang, and N. S. Cho (1998) "Trapping of leaked oil with tandem oil-fences with Lagrangian analysis of oil droplet motion," Trans. of the ASME. J. Offshore Mech. and Arctic Eng., vol. 120, pp. 50-55. (SCI, IF=0.108)

[J2] K. H. Kang and C. M. Lee (1996) "Prediction of drift in a free surface," Ocean Engineering, vol. 23, pp. 243~255. (SCI, IF=0.371)

[J1] K. H. Kang and C. M. Lee (1995) "Steady streaming of viscous surface layer in waves," J. Marine Science and Technology, vol. 1, pp. 3~12. (SCIE)

PAPERS IN PREPARATION

[3] K. H. Kang, I. S. Kang, and C. M. Lee (2004) “Electroosmotic flow at an interior corner,” J. Colloid and Interface Science. (In preparation).

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[2] K. H. Kang, S. M. Shin, S. J. Lee, and I. S. Kang “Visualization of the two-layered electroosmotic flow and its EHD instability in T-channels by micro PIV,” J. Visualization (In preparation).

[1] K. H. Kang, S. J. Lee, C. M. Lee, and I. S. Kang “Buoyancy-induced flow inside a two-component evaporating droplet,” Phys. Fluids (In preparation).

DOMESTIC JOURNAL PAPERS [D5] S. K. Chung, K. H. Kang, C. M. Lee, and I. S. Kang (2003) “Analysis of effect of line

tension and electrical double layers on electrowetting phenomena” KSME J. B, 27(7), 956–962. (In Korean).

[D4] J. -W. Ahn, K. H. Kang, I. -H. Song, and K. –Y. Kim (2000) "Measurement of cavitation noise of a hydrofoil and prediction of cavity bubble behavior," J. of the Society of Naval Architects of Korea, vol. 37(4), 40-47. (In Korean).

[D3] K. H. Kang and C. M. Lee (1999) "Behavior of oil-water interface between tandem fences," J. Korean Soc. for Marine Environmental Engineering vol. 2, pp. 70-77.

[D2] C. M. Lee and K. H. Kang (1998) "Analysis of containment capability of oil fence in currents and waves,” J. Korean Soc. for Marine Environmental Engineering, vol. 1, pp. 29-38. (In Korean).

[D1] K. H. Kang and C. M. Lee (1996) "Development of an efficient calculation method of pressure acting on a bluff body and deformation of flexible oil fences in currents,” J. of the Society of Naval Architects of Korea, vol. 33, pp. 22-31. (In Korean).

CONFERENCE PAPERS [C40] K. H. Kang, S. M. Shin, S. J. Lee, I. S. Kang (2003) “Visualization of the two-layered

electroosmotic flow and its EHD instability in T-channels by micro PIV,” Autumn’s Meeting of Korean Society of Visualization, Pohang Univ. Sci. & Tech., Pohang, Korea, 77. 75–78.

[C39] K. H. Kang, J. W. Park, I. S. Kang, and K. Y. Huh (2003) “Electrohydrodynamic instability of two-layered miscible fluids in microchannels,” The 1st Annual Symp. Of the Korean Society of Microsystem on life Science and Chemistry, Oct 29-30, 2003, KAIST, Daejon, Korea. (Presented at poster session).

[C38] K. H. Kang, S. J. Lee, and C. M. Lee (2003) “Visualization of flow inside a small evaporating droplet,” The 5th Int’l Symp. on Particle Image Velocimetry, Pusan, Korea, Paper No. 3242. (In CD-ROM).

[C37] K. H. Kang and C. M. Lee (2003) “Prediction of drift velocity of oil surface layer due to waves,” Spring’s Meeting of Korean Society of Marine Environmental Engineering.

CAVITATION- AND HYDROACOUSTICS-RELATED PAPERS

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[C36] K. H. Kang, J. –W. Ahn, I. –H. Song, and K. –S. Kim (2001) "Analysis of bubble cavitation and control of cavitation noise of hydrofoils," Proc. of the Spring’s Meeting of the Korean Society for Noise and Vibration Engineering, pp. 335-341. (in Korean)

[C35] M. Y. Lee, K. H. Kang, G. T. Boo (2001) "Development of hydrophone array system for underwater noise measurement in water tunnel," Proc. of the Spring’s Meeting of the Korean Society for Noise and Vibration Engineering, pp. 342-349. (in Korean)

[C34] Kang, K. H., Kang, I. S., and Lee, C. M. (1999) "Stability of a cavitation bubble in uniform magnetic field," Proc. of the Annual Spring Meeting of the Society of Naval Architects of Korea, pp. 288-291.

[C33] Kang, K. H, Kang, I. S, and Lee, C. M. (2000) “Effect of uniform magnetic field on a cavitation bubble in a weakly viscous conducting fluid," The First National Congress on Fluids Engineering, Sep 1-2, Muju, Korea, pp. 703-706.

[C32] K. H. Kang, M. Y. Lee, and Y. G. Kim, Y. G. (2000) "Effect of gas-filled cavity on frequency response of a pressure transducer," Proc. of the Spring’s Meeting of the Korean Society for Noise and Vibration Engineering, vol. 2, pp. 785-790. (in Korean)

[C31] Na, Y. C., Kang, K. H., Kim, Y. G., Lee, M. Y. (2000) " Measurement of cavitation-induced pressure fluctuation in a large cavitation tunnel," Proc. of the Spring’s Meeting of the Korean Society for Noise and Vibration Engineering, vol. 2, pp. 792-796.

[C30] K. H. Kang and M. Y. Lee (2000) “Measurement of flow-induced wall-pressure fluctuation by using a pin-hole mounted hydrophone,” Proceedings of the First National Congress on Fluids Engineering, September 1-2, Muju, Korea, pp. 419-422. (in Korean)

[C29] Lee J. -T., Kang, K. H., Kim, K.-S, Park, Y. -H., Ahn, J. -W. (1999) "Control of cavitation pressure-fluctuation induced by oscillating hydrofoil by using an air-injection method," Proc. of the Annual Spring Meeting of the Society of Naval Architects of Korea, pp. 292-295. (in Korean)

[C28] Na, Y. C., Kang, K. H., Kim, Y. G., Lee, M. Y. (1999) "Measurement of cavitation-induced pressure fluctuation in a large cavitation tunnel," Proc. of the Annual Fall's Meeting of the Society of Naval Architects of Korea, pp. 329-334.

[C27] Ahn, J. -W., Kang, K. H., Song, I. -H., and Kim, K. -Y. (1999) "Measurement of cavitation noise of a hydrofoil and prediction of cavity bubble behavior," Proc. of the Annual Spring Meeting of the Society of Naval Architects of Korea, pp. 282-287.

[C26] K. H. Kang, C. M. Lee, G. Choi, and Y. W. Ahn (1998) "Prediction and measurement of cavitation noise,” Proc. of the Spring’s Meeting of the Society of Naval Architecture of Korea. (in Korean)

[C25] Kang, K. H., and Lee, C. M. (1998) "Prediction of cavitation noise of a body," Proc. of the fourth KSME-JSME Fluids Engineering Conference, Pusan, Korea, pp. 677-680.

[C24] Kang, K. H., Kang, I. S., and Lee, C. M. (1998) "Effects of magnetic field on a growing or collapsing bubble," Proc. of the Annual Autumn Meeting of the Society of Naval Architects of Korea, pp. 75-80.

[C23] Kang, K.H., and Lee, C.M., "Analysis of volume change of a cavitation bubble around a body, Proc. of the Annual Autumn Meeting of the Soc. of Naval Architects of Korea, pp. 518-521, 1997

OTHERS

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[C22] C. M. Lee and K. H. Kang (2003) “Design considerations for oil fences,” Proc. of the Eighth Int’l Marine Design Conf., May 5–8, Athens, Greece, pp. 503–514.

[C21] K. H. Kang, H. C. Lim, C. M. Lee, S. J. Lee and I. S. Kang (2002) “Visualization of flow inside evaporating droplets on a hydrophobic surface,” Proc. of the Fifth JSME-KSME Fluids Engineering Conference, November 17–21, Nagoya, Japan, pp. 458–461. (In CD-ROM).

[C20] K. H. Kang, I. S. Kang, and C. M. Lee (2002) “Finite element analysis of electrical double layers near triple contact lines,” Proc. of the Second National Congress on Fluids Engineering, August 22–24, Muju, Korea, pp. 491–494.

[C19] C. M. Lee and H. K. Kang (2002) “Floating thin sheets in waves: drift force, drift velocity, and wave damping,” Proc. of 21st Int’l Conf. On Offshore Mechanics and Arctic Engineering, June 23–28, Oslo, Norway, Paper No. OMAE2002-28473. (In CD-ROM).

[C18] Lee, C. M., Kang, K. H., and Cho, N. S., "Containment capability of tandem oil-fences," Spring's Meeting of Society of Naval Architects of Korea, 337-340, 1997. (in Korean)

[C17] Kim, Y. G., Kang, K. H., Han, J. M., Lee, M. Y. (2000) "Measurement and visualization of flow field around an axisymmetric body with an angle of attack," Proc. of the Annual Spring's Meeting of the Society of Naval Architects of Korea. (in Korean)

[C16] Y. G. Kim, K. H. Kang, J. M. Hahn, and M. Y. Lee (2000) "Measurement and visualization of flow field around an axisymmetric body with an angle of attack," Proc. of the Third Naval Weapon Systems Development Seminar, pp. 133-136. (in Korean)

[C15] Kang, K. H., and Lee, C. M. (1998) "Behavior of oil-water interface between tandem fences," Proc. of the Annual Spring Meeting of the Korean Society for Ocean Environmental Engineers, pp. 221-226.

[C14] Lee, C. M., Kang, K. H., and Cho, N. S. (1998) "Prediction of oil-droplet motion and containment of spilt oil with tandem fences," Proc. of the fourth KSME-JSME Fluids Engineering Conference, Pusan, Korea, pp. 465-468.

[C13] Lee, C. M., Kang, K. H., and Cho, N. S., "Trapping of leaked oil with tandem oil-fences with Lagrangian analysis of oil droplet motion," Proc. of the 16th Conference on Offshore Mech. and Arctic Eng., Trans. of the ASME, 1997.

[C12] Kang, K.H., Lee, C.M., and Han, D.G., "Degradation of containment capability of oil fences due to deflection in currents," Proc. of the Annual Autumn Meeting of the Soc. of Naval Architects of Korea, pp. 357-360, 1997

[C11] Lee, C.M., and Kang, K.H., "Analysis of containment capability of oil fence in currents and waves," Proc. of the Korean Society for Marine Environmental Engineering, pp. 35-45, 1997.

[C10] Kang, K.H. and Lee, C.M., "Wave-induced drift force on a floating thin sheet and associated wave damping," Proc. of the Third Korea-Japan Joint Workshop on ship and Marine Hydrodynamics, Taejon, Korea, pp. 378-385, 1996.

[C9] Lee, C.M. and Kang, K.H., "Assessment of oil-fence effectiveness in currents and waves," Int'l Workshop for Operational Oil Spill Modeling, Pusan, Korea, 1996.

[C8] Kang, K.H. and Lee, C.M., "On the threshold leakage velocity of oil under an oil fence," Proc. of the Sixth International Symposium on Practical Design of Ships and Mobile Units, Seoul, Korea, vol. 1, pp. 266~277, 1995.

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[C7] Kang, K.H. and Lee, C.M., "Investigation of oil fence performance in calm water and waves," Proc. of International Conference on Technologies for Marine Environment Preservation, Kyoto, Japan, vol. 2, 974~981, 1995

[C6] Kang, K.H. and Lee, C.M., "On the deformation of flexible oil fence in current," Autumn's Meeting of Society of Naval Architects of Korea, pp. 299~304, 1995. (in Korean)

[C5] Kang, K.H. and Lee, C.M., "On wave-induced drift of spilt oil," Proc. of the Third JSME-KSME Fluids Engineering Conference, Sendai, Japan, pp. 420~425, 1994.

[C4] Kang, K.H. and Lee, C.M., "Wave-induced mass transport in the boundary layer at an oscillating surface," Spring's Meeting of Korea Society of Mechanical Engineers, pp. 503-508, 1994.

[C3] Lee, C.M. and Kang, K.H., "Investigation of drift velocity of spilt oil in waves," Proc. of the Second Japan-Korea Joint Workshop on ship and Marine Hydrodynamics, Osaka, Japan, pp. 275-284, 1993.

[C2] Kang, K.H. and Lee, C.M. "Problems with wave generation in a tank," Spring's Meeting of Society of Naval Architects of Korea, pp. 246-253, 1993. (in Korean)

[C1] Kang, K.H. and Lee, C.M. "Oil-slick drift due to waves and threshold escape velocity beneath a towed oil fence," Workshop on tidal and oil-spill modeling, Pohang, Korea, pp. 37~56, 1993. (in Korean)

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Highlight of Recent Works

Electrohydrodynamic instability of two-layered miscible fluids with different concentrations in microchannels

Kwan Hyoung Kang, Jae Wan Park, and Kang Y. Huh

(To be submitted to Physical Review E) Recently, an interfacial instability has been observed for the DC-powered electroosmotic flow of the two miscible electrolytes layers having different concentrations in a T-channel by Chen and Santiago [1]. A similar but more complex pattern of the flow instability has been demonstrated for the AC-powered electroosmotic flow too [2]. It is truly interesting because it is rather contrary to our common belief that the flow inside a microchannel is generally stable due to the dominant role of the viscous dissipation. As far as we know, there is no reliable explanation on the fundamental mechanism of the instability.

In the present work, it will be shown that the instability is mostly a consequence of the polarization of ionic species due to the conductivity gradient. It is therefore fundamentally similar in nature to the so-called electrohydrodynamic instability. To check the validity of the idea, a numerical analysis for the electrokinetic flow is carried out for a two-dimensional straight channel, under the Nernst–Planck’s framework of the electrochemical systems. That is, the Navier–Stokes equation with the Coulombic body force ( Eeρ ) is analyzed for fluid flow. The following diffusion equation for ionic species and the equation for the electrostatic potential (φ ) are introduced under the electroneutrality assumption:

cDDtDc

eq2∇= (1)

])[(2 cDDF ∇−⋅∇+∇⋅−∇=∇ +−φσφ (2) where +D and −D are the diffusivity of cation and anion, )/(2 −+−+ += DDDDDeq , and F is the Faraday constant. The conductivity (σ ) is proportional to the concentration (c).

The first and the second terms in the right-hand side of Eq. (2) represent the polarization due to the conductivity gradient and the diffusion potential (i.e., the liquid junction potential) [3]. In Eq. (2) the second term is in general very small relative to the first term, and therefore, it is neglected in the simulation. It is noted that the Poisson equation is not specifically solved to obtain the electric field. Instead, the Poisson equation and Eq. (2) is used to replace the charge density in the Navier–Stokes equation by

E⋅∇=∇⋅∇=∇−= σεφσεφερ 2e (3)

This manifests that how the conductivity gradient contributes to the electrical body force.

Highlight of Recent Works

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That is, when the concentration gradient and the electric fields are in parallel, the polarization of charge occurs. The Coulombic force acts in the direction of the electric field, which eventually generates the fluid motion and the subsequent flow instability.

To confirm the validity of the present model, we choose the values of variables corresponding to the experimental condition of Chen and Santiago [1]. Initially, a small lump-like perturbation is given to the concentration distribution. Figure 1 (a) shows the temporal evolution of concentration distribution together with the velocity vector fields. Note on the similarity of the calculated concentration profile with the photographic images in Chen and Santiago [1]. The crest of wave seems to be draggled due to the convective diffusion by counterclockwise vortex.

The time evolution of the instability will be presented. The detailed mechanism of the instability will be discussed based on the numerical results of the flow field, the electric field, and the concentration distributions. The numerical analysis to determine the instability criteria is undergoing and the results will also be presented soon.

(b)

Figure 1. (a) Temporal evolution of the concentration field in which tc is defined as d/uc, d is the channel width, and uc is Helmholtz-Smoluchowski slip velocity; (b) photographic image taken by Chen and Santiago in case of V= 2.5 kV [1]

(a)

References [1] C. H. Chen and J. G. Santiago (2002) “Electrokinetic flow instability in high

concentration gradient microflows,” Proc. 2002 Int’l Mech. Eng. Cong. and Exp., New Orleans, LA, CD vol.1, Paper No. 33563.

[2] S. M. Shin, I. S. Kang, Y.-K. Cho, and G. Im (2003) “Instability of electroosmotic flow under time-periodic electric fields,” Anal. Chem. (submitted).

[3] K. H. Kang and I. S. Kang (2003) “Theoretical investigation on the liquid junction potential in a slit-like microchannel,” J. Electroanal. Chem. (in review).

Highlight of Recent Works

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Visualization of the two-layered electroosmotic flow and its EHD instability in T-channels by micro PIV

Kwan Hyoung Kang, Sang Min Shin, Sang Joon Lee, and In Seok Kang

(Presented at Korean Society of Visualization, Pohang, Korea, Nov. 2003) 1. Introduction

The flow instability observed in a microfluidic condition by Chen and Santiago [1] is really interesting, considering such a general nature of the flow in the microscale dimensions.

Kang et al. [2] has recently showed numerically that the instability is originated from the concentration gradient. They predicted that strong vortices are generated due to the body force acting on free charges. The vortices induce the peculiar interface pattern such as shown in Fig. 1. An experiment is undergoing to verify the numerical results. In this paper, the preliminary results of the experiment are presented.

2. Experiment

Fabrication of the glass chip. The standard photolithography techniques and the wet-etching process are used for the T-channel fabrication. The 200nm poly-silicon layer is deposited onto the glass substrate as a sacrificial etch mask in hydrofluoric acid. The positive photoresist (AZ 4620, Clariant) and the reactive ion etching equipment are used to pattern T-channel. The deprotected glass areas were etched isotropically in the 50% hydrofluoric acid solution followed by removal of the polysilicon layer. The basic layout of channel width is 50µm and the channel is etched to a depth about 50µm. Micro PIV system. The micro PIV system

used in this work is schematically described in Fig. 4. A cooled CCD camera is used to capture images. It has the spatial resolution of 1024×1280 pixels and the dynamic range of 12bit. For the visualization of particle streaks, the halogen light is illuminated at the region of interest. For the PIV measurement of flow field, a two-head

Fig. 1 SEM image of the etched glass

PC

Microchannel

EXT/GATE

BNC

EXT/GATE

BNC

Delay generator

Cooled CCDcamera

Two-head Nd:YAG laserInvertedmicroscope

MirrorIBM PC

570nm longpass filter

Objective lens

Fig. 2 Micro PIV system

Highlight of Recent Works

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Nd:Yag laser is used. The location of image plane is changed by controlling the location of the focal plane of the camera. As the seeding particle, a fluorescent particle having the mean diameter of 1　m is used. A pair of consecutive images is obtained. 3. Experimental results

Figure 5 shows the streak lines of the seeding particles. Throughout the experiment, 1mM (upper stream) and 10mM (lower stream) of NaCl solution are used. For the T-channel shown in Fig. 5, the instability begins to occur above 2.7kV. Before the onset of instability, a stable laminar flow is observed. After onset of instability, vortical flow pattern can be observed. A typical vortical flow pattern is shown in Fig 5 for the case of 3kV.

Figure 6 shows the PIV data for the different voltages. A vortex structure is clearly shown in Fig. 6. The vortices are in fact generated periodically and are convected by flow. Note that the condition of Fig. 6 corresponds to that of Fig. 5.

Fig. 5 Visualized particle streaks, 3kV Fig. 6 PIV data

4. Concluding remarks

The existence of the strong vortical flow pattern predicted by Kang et al. [2] is confirmed. It is in general difficult to generate vortical flow for the creeping flow. This kind of vortex-generation mechanism may be utilized as a simple means to enhance the mixing of heterogeneous fluids. References [1] C. H. Chen and J. G. Santiago (2002) Proc. 2002 Int’l Mech. Eng. Cong. and Exp., New

Orleans, LA, CD vol.1, Paper No. 33563. [2] K. H. Kang, J. W. Park, I. S. Kang, and K. Y. Huh, 2003, “Electrohydrodynamic instability

of two-layered miscible fluids with different concentrations in microchannels,” Phys. Rev. E. (Submitted).

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Visualization of flow inside a small evaporating droplet

Kwan Hyoung Kang, Sang Joon Lee, and Choung Mook Lee (Submitted to Measurement Science & Technology)

Droplets have many interesting applications associated with microfluidic problems, e.g., DNA molecules imaging, micro-pumps, and ink-jet printing. The details of droplet-related phenomena in micro and nanoscales such as evaporation process, Marangoni effects, contact angles with solid substrates, and electrowetting are not well known. There is a consensus of opinion that the fluid flow inside a droplet may play an important role on the overall transport phenomena. Compared to its importance of fluid flows, only a few systematic investigations have been performed in the past, toward understanding of overall transport phenomena inside a droplet.

The accuracy of the present image restoration method which incorporates the ray-tracing method is checked by using a hemi-spherical Plexiglas lens (see Fig. 1a). In Fig. 1b, the distorted image of 30×15 meshes due to the presence of the hemi-spherical Plexiglas lens is shown. The restored image by using the ray-tracing method is shown in Fig. 1c. The center region is well restored, while the accuracy of image restoration is not so good for the region of r/Ro > 0.75.

In Fig. 2, the flow patterns inside a droplet of ethanol mixture are shown for different ethanol concentrations of 1%, 5%, and 20%. The exposure time are 20sec for (a), (b) and (c) and 2sec for (d), respectively. A pair of vortex is clearly shown in Fig. 2a for the case of 1% mixture. The flow is upward at the center region for all the cases. For the case of 5% mixture, the flow is initially rather unstable at the periphery of the droplet as shown in Fig. 2b. In all the cases considered here, the regular flow patterns such as shown in Figs. 2a and 2c are established after moderate times. A similar flow pattern is also observed when KCl (sodium chloride) mixed with deionized water is used.

It is very intriguing phenomenon that very small amount of additives generates such a change in the fluid motion. It is conjectured that the flow shown in Fig. 2 is either driven by the density gradient inside a droplet or by viscous diffusion, rather than by the Marangoni instability.

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(a) (b) (c)

Fig. 1. Verification of the image-restoring method. (a) Plexiglas lens; (b) original image; (c) restored image.

Fig. 2. Flow inside evaporation droplets for different concentration of ethanol at 20 sec exposure except for (d) at 2 sec exposure. (a) 1%; (b) 5%; (c) 20%; (d) 20%.

In Fig. 3, the velocity vectors calculated by using the raw images and the reconstructed images (corresponding to the case of Fig. 2a) are compared. The most important difference between the two results is that the magnitude of upward velocity component at the center region of the droplet is reduced after the image restoration. Additionally, the center of vorticity is moved a little towards the origin.

x (mm)

z(m

m)

-1.5 -1 -0.5 0 0.5 1 1.50

0.5

1

1.5 0.1mm/s

x (mm)

z(m

m)

-1.5 -1 -0.5 0 0.5 1 1.50

0.5

1

1.5 0.1mm/s

(a) (b)

Fig. 3. Restoration of velocity vectors for the case of 1% mixture. (a) before restoration; (b) after restoration.

(a) (b)

(c) (d)

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Theoretical investigation on the liquid junction potential in a slit-like microchannel

Kwan Hyoung Kang* and In Seok Kang†

(Published on web to the J. Electroanalytical Chemistry)

The liquid junction potential (LJP) is generated at the interface of the two electrolyte solutions of different ionic concentrations (see Fig. 1). It can be applied to generate electricity (Fig. 2) and it can also be applied to passive control of the trajectory of the charged micro- and nanoparticles in the micro total analysis systems.

In this work, an analytical model is provided to predict the LJP and the associated electrostatic field generated by the contacting two electrolyte layers in a slit-like channel, for the case of the simple 1:1 electrolyte system. The one-dimensional Nernst–Planck equation is analyzed to investigate the temporal evolution of the concentration distribution.

1M NaCl

0.1M NaCl

Na+ Cl- Na+

Cl-Na+

Na+

Na+

Cl-Cl-

Cl-Na+ Na+

Cl-Cl-

(a) isolated (b) diffusion (c) ion distribution Fig. 1 Diffusivity difference of ionic species induces the concentration polarization.

(a) µFuel cell in a microchannel [1] (b) Concentration fuel cell in a channel [2]

Fig. 2 Application of the LJP for the membraneless fuel cell As a result, comprehensive analytical formulas for the LJP, the electric field, and the

charge density are obtained. The LJP is obtained by introducing the concentrations at the boundary surfaces to the Planck’s equation written for the finite domain. The analytical result

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for the LJP is compared with the existing experimental result of Lagger et al. [2], which shows a reasonable agreement as shown in Fig. 3.

Of particular interest is the influence of the initial concentration ratio and the thickness ratio of the two electrolyte layers. It is shown that there exist limiting profiles in the temporal evolution of the LJP, with respect to the variation of the concentration ratio and the thickness ratio, respectively. The implication of the electric field produced by the charge separation is discussed concerning the behavior of the charged particles in the micro total analysis systems.

τ

ln(c

h/cl)

/ln(

c h0/c

l0)

0 0.1 0.20

0.5

1

ch0/cl0 = 104

ch0/cl0 = 10

Fig. 3. Validation of the theory. x and y-axes represents the non-dimensional time and the LJP. The experiment of Lagger et al. [2] is performed in a mini channel shown in Fig. 2(b). The lines indicate the theoretical prediction and the symbols represent the experimental results obtained for different concentrations and the location of the electrode in the channel.

References [1] R. Ferrigno, A. D. Strook, T. D. Clark, M. Mayer, G. M. Whitesides “Membraneless

Vanadium Redox Fuel Cell Using Laminar Flow,” J. Am. Chem. Soc. 124 (2002) 12930. [2] G. Lagger, H. Jensen, J. Josserand, H. H. Girault “Hydro-voltaic cells. Part I.

Concentration cells,” J. Electroanal. Chem. 545 (2003) 1.

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Establishment of the electromechanical theory for charge-related wetting phenomena including the electrowetting phenomenon

Kwan Hyoung Kang,* In Seok Kang,† and Choung Mook Lee*

[Langmuir 18, 10318–10322 (2002); Langmuir, 19, 5407-5412 (2003); Langmuir, 19, 6881-6887 (2003); Langmuir, 19(23), 9962-9967 (2003)]

The macroscopic contact angle of liquid layers (film or droplet) is significantly influenced by externally applied electric potential (Fig. 1a, case I), degree of surface charge on substrates, and the pH value of the solution (Fig. 1b, case II). These electrical modifications of wettability has important consequence on the behavior of liquid layer such as spreading, film instability, adhesion and subsequent spreading of biological cells and membranes, and mineral separation by floatation. The electrical control of contact angle on dielectric substrates or on self-assembled monolayers (Fig. 1c, case III), which is sometimes called electrowetting or more distinctly electrowetting-on-dielectrics, draws much attention recently for its potential applications in microfluidic control. By use of electrowetting-on-dielectrics, nano- or microliter volumes of (nearly any kinds of) electrolyte liquids can be controlled very quickly and reversibly with low power consumption.

(b) (c)(a)

+ + + + + + + +- - - - - - - -

+ + + + + + + + +- - - - - - - -

+ + + + + + + +

- - - - - - - -

Fig. 1. Three typical cases of electrical effect on wetting. (a) Droplet on an electrode (case I). (b) Droplet on a charged substrate (case II). (c) Droplet on a dielectric substrate (case III).

The thermodynamic approach is somewhat indirect in offering a specific

understanding of detailed physics associated with electrical effect on wetting. Furthermore, the approach is based on the equilibrium assumption, and therefore, it cannot be applied to analyze the dynamical problems. In that sense, a more direct complementary approach is necessary. Kang et al. has newly established the electromechanical approach in which the Maxwell stress tensor is introduced to analyze the wetting and spreading of droplet in the mechanical viewpoint. It is demonstrated by them [1-3] that the electromechanical approach in many cases can provide more detailed information and it can constitute a potential backbone for direct analysis and understanding on charge-related wetting problems.

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As a first attempt in this direction, Kang [1] considered the case of electrowetting. He showed that the electrowetting phenomenon originated from the Maxwell stress concentrated near the edge of a droplet, rather than from the change of the interfacial tension at the droplet-dielectric interface. Kang et al. [2] has generalized the theory and showed how the Coulombic interaction at TCL generates wetting tension ( elW ) which enforces a liquid layer to spread, and is related with the (apparent) contact angle (θ ) as

γθθ elW

+= ocoscos

where oθ is the contact angle without the electrical effect, and γ is the interfacial tension at the interface of droplet and surrounding fluid. Concrete analytical expressions of wetting tension for the three practically important cases shown in Fig. 1 are derived (eqs. 9, 11, and 17), and the wetting tensions are compared with those of the thermodynamic counterpart. It is firstly found that the wetting tension in the case of Fig. 1(b) is dependent on the interface profile near three-phase contact line. The geometry-dependency of the wetting tension is unusual in many wetting problems, and the problem is separately considered in Kang et al. [3]. The Frumkin–Derjaguin theory relates the macroscopic contact angle of a droplet with the disjoining pressure of the thin film in equilibrium with the droplet. The disjoining pressure is obtained by way of the celebrated Derjaguin approximation. The validity of the Derjaguin approximation has been believed for a long time to be limited within small contact angles. Kang et al. [4] checked the validity of the Derjaguin approximation by using the electromechanical theory. It is shown (for the constant potential case) that, if the modification of the interfacial energy at the liquid–surrounding fluid interface by the electrocapillary effect is separately considered, the Derjaguin approximation gives an exact interaction free energy, regardless of the magnitude of the contact angle. For the constant charge case, on the contrary, the interaction free energy derived based on the Derjaguin approximation for prediction of the contact angle is shown to have evident deficiency except for the case of vanishing surface charge density at the liquid surface. [1] K. H. Kang (2002) “How electrostatic fields change contact angle in electrowetting,

Langmuir (American Chemical Society, Journal of Interface and Colloid), vol. 18, pp. 10318–10322.

[2] K. H. Kang, I. S. Kang, C. M. Lee (2003) “Wetting tension due to Coulombic interaction in charge-related wetting phenomena,” Langmuir, vol. 19(8), 5407-5412.

[3] K. H. Kang, I. S. Kang, C. M. Lee (2003) “Geometry dependence of wetting tension on charge-modified surfaces,” Langmuir, 19, 6881-6887.

[4] K. H. Kang and I. S. Kang (2003) “Validity of the Derjaguin approximation in electrostatic effect on the Frumkin—Derjaguin approach,” Langmuir, 19(23), 9962-9967.

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Electrostatic contribution to line tension in a wedge-shaped contact region

Kwan Hyoung Kang, In Seok Kang, and Choung Mook Lee (Langmuir 19, 9334-9342, 2003)

□ 연구 배경 및 필요성

Line tension이란 surface tension과 관련된 표면 에너지에 포함되지 않는

contact line에 국부적으로 존재하는 에너지 성분과 관련되어 있다. Line

tension은 그 크기에 대해 이론과 실험이 매우 불일치함으로 인하여

현재까지도 그 크기 및 영향에 대해 논란이 많다.

Molecular interaction에 의해 발생하는 line tension은 보통 10-10 N

정도로 매우 작은 것으로 알려져 있으나 그 영향이 액적의 반경에

반비례하므로 시스템의 크기가 작아질수록 그 중요성이 증가한다.

Line tension은 특히 CNT, nanoparticles 관련된 wetting 문제에서 매우

중요해지게 된다. 예들 들어 CNT의 wetting character는 수소 등과 같은

물질의 저장과 CNT를 이용한 표면 특성의 제어, nanochannel 특성 이해

등에 매우 중요한 부분이다.

Figure 1a는 carbon nanotube (CNT) 안에 액체가 채워져 capillary를

형성하고 있는 것을 보여주고 있다. Figure 1b는 CNT 외벽의 wetting

character를 Molecular Dynamics Simulation을 통하여 해석한 결과를

보여주고 있다.

(a) (b)

Fig. 1. (a) Capillarity in CNT (Megaridis et al. (2002) Phys. Fluids 14, L5-

L8). (b) Molecular Dynamics Simulation to analyze the CNT wetting

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problem (Werder et al. (2003) J. Phys. Chem. B 107, 1345-1352)

CNT나 nanoparticle의 wetting 특성에 대한 연구는 전세계적으로도 매우

초보적인 단계에 있다. 특히 CNT, nanoparticles 자체가 하전된

상태이거나 많은 응용 대상 액체가 전해질인데 전기가 wetting에 미치는

영향에 대해서는 그간 뚜렷한 연구 성과가 없다.

□ 연구 내용 및 결과

본 연구에서는 전기가 발생시키는 electrostatic line tension에 대한

이론적인 예측 결과를 최초로 도출하였다.

이를 위하여 액적의 three-phase contact line 부근의 electrical double

layer를 선형 이론을 통하여 해석하였다. Line tension은 시스템의 total

에너지에서 surface tension과 관련된 bulk energy를 빼서 구하였다.

이론 해석의 검증을 위하여 Poisson–Boltzmann 방정식을 수치 해석하여

별도로 line tension을 구하여 비교하였다 (Fig. 2a). 그 결과 선형해와

수치해가 낮은 전압에 대하여 잘 일치함을 알 수 있었다.

α (deg)

τ elβ2

/ε

0 90 1800

5

10

α (deg)

τ el,

fτel'(

N)

0 30 60 90

10-12

10-11

10-10

(a) (b)

Fig. 2. (a) 수치 해(symbol)와 선형해(solid line)의 비교. α = contact

angle, elτ = 전기에 의한 line tension. (b) Electrostatic line tension에

대한 sample calculation