Sensors and Actuators B 114 (2006) 350–356
`
`Monodispersed microfluidic droplet generation by
`shear focusing microfluidic device
`Yung-Chieh Tan a,∗
`
`, Vittorio Cristini a,b, Abraham P. Lee a,c
`a Department of Biomedical Engineering, 204 Rockwell Engineering Center, Irvine, CA 92697, USA
`b Department of Biomedical Mathematics, 204 Rockwell Engineering Center, Irvine, CA 92697, USA
`c Department of Biomedical Mechanical Engineering, 204 Rockwell Engineering Center, Irvine, CA 92697, USA
`
`Received 1 December 2004; received in revised form 22 May 2005; accepted 6 June 2005
`Available online 26 July 2005
`
`Abstract
`
`A microfluidic device designed to generate monodispersed picoliter to femtoliter sized droplet emulsions at controlled rates is presented.
`This PDMS microfabricated device utilizes the geometry of the channel junctions in addition to the flow rates to control the droplet sizes.
`An expanding nozzle is used to control the breakup location of the droplet generation process. The droplet breakup occurs at a fixed point
`due to the focused velocity gradient created by the nozzle shape geometry. The system not only creates monodispersed primary droplets with
`sizes controlled by the applied flow rates, but also generates monodispersed submicron droplets. Droplets with radii as less than 100 nm can
`be produced without use of surfactants. Numerical results relating flow rates to the size of primary droplets, satellite droplets and generation
`rates are reported.
`© 2005 Published by Elsevier B.V.
`
`Keywords: Droplets; PDMS; Satellite droplets; Submicron droplets; Channel design
`
`1. Introduction
`
`Cells are biological units that can be viewed as droplet-like
`sensors and actuators that detect and react to small chemical
`changes [1]. Since the 1980s, several groups have begun to
`explore the possibility of designing droplet-based carriers,
`such as surface modified liposomes, to target the delivery
`of drugs to diseased tissues [2]. However, the techniques
`used create large size distributions [3] making it difficult
`to control the encapsulation volume, the reactive surface
`area, and the encapsulation efficiency of drugs and signal-
`ing molecules. In recent years, several groups have devel-
`oped microfluidic technologies that reported generation of
`monodispersed droplets in immiscible fluids [4]. Thorsen et
`al. [5] and Nisisako et al. [6] used T type junction design
`to shear water droplets in oil phase. The sizes of droplets
`and rates of generation are controlled by changing the flow
`
`∗
`
`Corresponding author. Tel.: +1 949 8249926; fax: +1 949 8241727.
`E-mail addresses: ytan@uci.edu (Y.-C. Tan), aplee@uci.edu (A.P. Lee).
`
`0925-4005/$ – see front matter © 2005 Published by Elsevier B.V.
`doi:10.1016/j.snb.2005.06.008
`
`rates. A more recent microfluidic shearing device developed
`by Anna et al. [7] used a cross-junction design, in which
`the dispersed water stream and the continuous oil stream is
`focused into a narrow orifice that connects to a large open-
`ing. In the last decade, droplets have been extensively used
`by various industries for drug development, plastic poly-
`merization, and chemical processings [8]. More recently,
`microfluidic technologies have enabled droplets to be used
`as liquid reaction vessels for screening protein crystalliza-
`tion conditions [9–11], as microtemplates for assisting self-
`assembly of materials [12–14], as molds for curing polymeric
`microspheres [15–18], and as components for microelectri-
`cal actuation [19]. Programmable fluidic assays for sampling
`glucose concentration of human physiological fluids [20,21],
`and DNA analysis [22] have been individually demonstrated
`using droplet-based microfluidic system.
`When a liquid thread breaks inside the channel, the point
`of droplet breakup varies inside the channel and the breakup
`over the same length of the liquid thread do not always give
`rise to the same number of droplets [23]. Compared to the
`
`1
`
`

`
`Y.-C. Tan et al. / Sensors and Actuators B 114 (2006) 350–356
`
`351
`
`cross-junction design by Anna et al. [7], the design of our
`device aims to focus the droplet break-off location to one
`single point located at the orifice. This is possible since the
`narrowest point incurs the highest shear force. To create the
`narrowest point, an expanding nozzle is used to focus and
`dissipate the force of the flow. This allows fluid velocity to
`increase prior to entering the nozzle and to decrease as flow
`passes through the nozzle. The acceleration and deceleration
`of fluid flow generate a velocity gradient in the flow direction
`that allows the droplet to break continuously at the location
`of maximum sheared stress and pressure point, which pro-
`vides uniform control of droplet sizes and the its subsequent
`generation of satellite droplets.
`
`2. Materials and methods
`
`Oleic acid has been a critical constituent in forming pH
`sensitive liposomes due to its unique property of being sen-
`sitive to the pH of solutions [2,24]. Its short lipid chains
`self-assemble into microstructures ranging from micelles to
`vesicles under basic pHs [25]. The controlled generation
`of droplet in oleic acid can then be used as an alternative
`means to detect the pHs of an unknown aqueous solution, in
`which the microscopic deposition of oleate around a droplet
`would indicate the pH of the aqueous sample. This was
`one of our motivations to use oleic acid as the droplet gen-
`erating fluid. The system setup for droplet generation and
`detection is shown in Fig. 1. In detail, oleic acid (viscosity
`27.64 mPa and interfacial tension 15.6 dyn/cm), purchased
`from Sigma–Aldrich is used as the continuous phase, and
`purified DI water is used as the dispersed phase. The two
`immiscible fluids are injected into the PDMS channel at con-
`stant flow rates controlled by syringe pumps (Pico Plus, Har-
`vard Apparatus). The generated droplets are recorded using
`a high speed imaging camera (Fastcam PCI-10K, Photron
`−1.
`Ltd.) with image capturing rates as high as 10 K frames s
`The droplet generation processeses are recorded with frame
`
`Fig. 1. Schematic of the experimental setup. Water, the dispersed phase, and
`oil, the continuous phase, are injected simultaneously by separate syringe
`pumps. The fluids are delivered directly to the inlets of the microfluidic chip.
`
`Fig. 2. Schematic of the droplet generation channel geometry. The water
`phase enters from the middle channel and is sheared by two oil streams at
`the orifice of the expanding nozzle. The depth of channel is 40 ␮m.
`−1 to 2000 frames s
`−1 and the
`rates ranging from 250 frames s
`diameters of the droplets are measured using an imaging pro-
`gram (Scion Imag, Scion Co.) with a measurement precision
`of ±3 ␮m.
`
`3. Microfluidic channel
`
`The channel is made from bonding PDMS to glass through
`oxygen plasma treatments [26]. The oxidized PDMS channel
`remained hydrophilic until after vapor deposition of tride-
`cafluorocholorosilane (SIT8174.0 Gelest) was achieved [26],
`◦
`which generates a uniform water contact angle of 110
`on the
`surface of the glass. The surface treatment is necessary to gen-
`erate water-in-oil emulsions, and if the surfaces are untreated
`after plasma exposure, the water phase spreads over the inside
`walls of channels regardless of the presence of oil phase. The
`PDMS part of the channel is created by casting PDMS on
`patterned SU-8 molds [26].
`The fabricated microchannel is shown in Fig. 2. The chan-
`nel has three inlets, a pressure reservoir for stabilizing water
`and oil phase, an expanding nozzle for stressing and subse-
`quently relaxing the water and oil streams, and an outlet that
`connects to an open reservoir to allow the generated droplet
`to restore into spherical shape. The channel has a depth of
`40 ␮m, and the nozzle has an orifice width of 48 ␮m, a length
`of 322 ␮m, and an exiting width of 230 ␮m.
`
`4. Results and discussion
`
`The velocity distributions inside the channel using the
`CFDRC ACE single phase flow simulation with constant
`input and output flow rate conditions indicates that the max-
`imum fluid velocity occurs at the orifice of the expanding
`junction, and this is the same for two phase fluid since the
`relative velocity distribution of the continuous phase shares
`similar characteristics as in single phase fluid system. In two
`other types of geometries: the long straight channel geometry
`and the short channel geometry shown in Fig. 3, the maxi-
`mum velocity occurs across the whole length of the channel
`that results in no velocity gradient in the flow direction. In
`
`2
`
`

`
`352
`
`Y.-C. Tan et al. / Sensors and Actuators B 114 (2006) 350–356
`
`Fig. 3. CFDRC simulation of velocity distributions in channels with different geometries. (a) Orifice connected to long straight channel. (b) Orifice connected
`to a short channel with subsequent rectangular expansion. (c) Orifice connected to an expansion nozzle. This design allows the max velocity to occur at the
`orifice.
`
`the expanding nozzle design, fluid velocity increases before
`entering the nozzle and reaches the maximum velocity at the
`orifice before decreasing as it exits the orifice. As a result,
`a high velocity gradient around the orifice of the nozzle is
`created.
`As illustrated in Fig. 4, normal to the surface of the liquid
`thread, the pressure and shear stress difference of the fluids
`are balanced by the interfacial tension force,
`(pw + µwG) − (po + µoG) = σ
`rt
`
`(1)
`
`,
`
`where po, µo, pw, and µw are the pressures and viscosities
`of the oil and water phase, respectively, σ the interfacial ten-
`sion of the water–oil interface, and rt is the radius of the
`liquid thread. The thread radius decays due to the continu-
`ous disturbances initiated by the extension of liquid thread
`[23]. Shown in Fig. 5, the head of the partially formed droplet
`passes the orifice while the neck of the droplet continues to
`decrease until a sharp point is developed during which the
`droplet detaches from the thread. The breaking of the thread
`occurs on a finite time governed by the shear rate and flow
`
`rates. During this time, a finite amount of liquid volume trav-
`els through the orifice into the expanding nozzle and the tip
`of the thread. Within the liquid thread, due to the difference
`in shear stresses at the orifice and in the expanded part of the
`nozzle, a pressure force is created in the direction of flow. This
`force is combined with the tangential shear stress exerted by
`the oil phase to separate the droplet from the liquid thread.
`In the expanding nozzle geometry, the droplet always
`breaks at the orifice due to designed shear gradient. If the
`exit from the orifice was into a straight channel as in Fig. 3a
`and b, then due to the constant maximum velocity through-
`out the length of the channel, the droplet break-off could vary
`anywhere along inside the parallel channel that may result in
`the breakup of large residual drops as demonstrated in Anna
`et al. [7].
`Since the shape of the droplets are confined by the top
`and the bottom PDMS walls, the droplets are compressed in
`the channel and have an apparent droplet radius that is larger
`than the radius of spherical droplets with the same volume.
`In Fig. 6, comparing the shape of droplet inside and outside
`the channel reveals that the shape is neither a circular disk
`
`Fig. 4. The radius of the liquid thread decreases due to the perturbation caused by the extension of the thread. Initial balance of the pressure and shear forces
`at the interface of the thread determines the initial radius of the thread.
`
`3
`
`

`
`Y.-C. Tan et al. / Sensors and Actuators B 114 (2006) 350–356
`
`353
`
`Fig. 5. (Top) Sequential images of droplet generation. Immediately after the liquid thread grows pass the orifice, the neck of the thread (located at the orifice)
`decreases until a sharp point is developed to allow the thread to detach. (Bottom) Generated droplets are not spherical until reaching the reservoir. In the
`reservoir, the droplet is not restricted by the channel walls and the shape of droplet relaxes into spherical form. The sizes of droplets are adjustable according
`to the applied flow rates.
`
`nor a hemisphere. The in-channel radius of droplet is always
`greater than the radius of the equivalent spherical droplet
`with the same volume, and when droplet reaches a radius of
`∼14 ␮m the in-channel shape of droplet becomes spherical.
`Monodispersed droplets are generated with sizes changing
`according to the flows as shown in Fig. 7. In shear driven flow,
`the capillary number is important for determining the size
`of the droplet after breakup [4,5,27]. The capillary number,
`Ca = µoGrs/σ, is defined as the ratio of shear stress exerted
`on the droplet and interfacial tension of the droplet, where µo
`is the viscosity of the oil phase, G the shear rate, rs the spher-
`ical diameter of the droplet, and σ is the interfacial tension
`between water and oil interface.
`In our device, since droplet breakup occurs at the orifice
`when the neck of the droplet becomes a singular point, where
`
`the cross-sectional area of the neck is small compared to the
`cross-sectional area of the channel, the shear rate generated
`by the oil phase can be approximated as the velocity of the
`oil flow divided by the width of the channel. With the flow
`velocity approximated as the flow rate of the oil phase divided
`by the cross-channel area, the shear rate is Qo/W2ho, where
`Qo is the total flow rate of the oil phase, and W and ho are
`the width and height of the orifice. If shear stress is the only
`dominant mechanism for breakup, then this predicts that the
`radius of the spherical droplet changes inversely to the flow
`rate, rs ∼ 1/Qo. However, our result shown in Fig. 7 indicates
`that the droplet radius is weakly dependent on the flow rate
`of the oil phase that rs ∼ Q−0.27
`to Q−0.34
`. This weak depen-
`o
`o
`dence corroborates with previous literatures [6,28,29], and it
`is likely to be dependent on various parameters including the
`
`Fig. 6. The generated radius of droplet suspended in oil is compared to
`the measured radius of disc shaped droplets. The curve is fitted with rd =
`0.5136r1.2817
`.
`s
`
`Fig. 7. Droplet size decreases as Qo increases. The curves are fitted
`with: Qw = 0.3 ␮L/min, rs = 36Q−0.266
`; Qw = 0.5 ␮L/min, rs = 33Q−0.29
`;
`Qw = 0.6 ␮L/min, rs = 27Q−0.34
`o
`o
`.
`
`o
`
`4
`
`

`
`354
`
`Y.-C. Tan et al. / Sensors and Actuators B 114 (2006) 350–356
`
`5. Droplet generation rate
`
`Since the stress exerted by the oil phase not only acts to
`pull the liquid thread but also impedes the growth of the liquid
`thread, the growth rate of the thread becomes less dependent
`on Qw as Qo increases. This effect is more apparent for small
`Qw than for larger Qw, and it is characterized by the sharp
`increase in generation time for rs < 20␮m, Qw = 0.3 ␮L/min
`and 0.4 ␮L/min as shown in Fig. 8. In general, droplet gen-
`eration time decreases with the increase of Qw and increases
`with the increase of rs.
`
`Fig. 8. The time required to generate a single drop increases with increase
`in droplet size and decreases with increase in Qw. Evidence that shear stress
`impedes growth of liquid thread is shown by the sharp increase in the droplet
`generation time for Qw = 0.3 ␮L/min.
`
`wetting properties of the walls, the channel geometry at the
`water–oil junction, the applied flow rates, and fluid viscosi-
`ties. The droplet sizes decrease as Qo increases, and increases
`when Qw increases due to the conservation of mass flow of
`the water phase [29].
`
`6. Satellite droplets
`
`Satellite droplets have been observed in many droplet
`generation devices [7,30–34], and studied under numer-
`ous breakup conditions [30–33]. The generation of satellite
`droplet is initiated by the imbalance of capillary forces dur-
`ing the break-off of primary droplet, and the volume of the
`satellite droplet has been reported by Zhang [31] to be less
`than 1% of the parent drop. In our system, due to the gener-
`ation of monodispersed primary droplets and the expanding
`
`Fig. 9. Size of satellite droplet under different magnifications. (Top) The
`satellite droplets travel to the side of monodispersed primary droplets after
`generation. The diameters of the primary droplets are 52.9 ␮m. (Bottom)
`Under 400× magnification, the monodispersed satellite droplets collected
`at the output have diameters of 2.8 ␮m.
`
`Fig. 10. Generation of satellite droplets. (Top) During the formation of pri-
`mary droplets, further extension of the thinned thread causes the formation
`of satellite droplet. (Bottom) Upon breaking the thread, the tip of the thread
`retracts while releasing the satellite droplet at the orifice.
`
`5
`
`

`
`Y.-C. Tan et al. / Sensors and Actuators B 114 (2006) 350–356
`
`355
`
`nozzle geometry, monodispersed satellite droplets are gener-
`ated. Fig. 9 shows an example of 52.9 ␮m diameter primary
`droplets with 2.8 ␮m monodispersed satellite droplets. One
`satellite droplet is formed following the generation of every
`primary droplet as indicated in Fig. 10. After generation,
`these satellite droplets either travel in between primary drops,
`on top of the primary drops, or along the sides of primary
`droplets. While the presence of satellite droplets is a promi-
`nent problem in emulsification, we find it as an interesting
`mechanism that can be reliably used to generate monodis-
`persed nanoparticles [35,36]. We have since developed a
`passive satellite filtering technique and a dynamic satellite
`sorting technique that when the satellite droplets are not desir-
`able, it can be passively filtered with a 2D micro-filtering
`design [35,36], and when it is desirable for the production of
`nano-sized droplets, the dynamic satellite sorting technique
`enables the collection of satellites into the desired reservoir
`[34–36]. Through the combination of sorting and the pre-
`sented droplet generation technique, we are able to produce
`droplet size with diameter <100 nm.
`
`7. Conclusion
`
`The expanding nozzle geometry creates a velocity gra-
`dient that precisely dictates the breakup location of the liq-
`uid thread, which allows the droplet generation to be posi-
`tioned for subsequent droplet fusion, fission, and sorting. This
`design provides a versatile feature in designing droplet-based
`Micro Total Analysis System (␮TAS) that requires precision
`control of droplet sizes and generation rates. The sizes of the
`primary and satellite droplets produced from this device are
`monodispersed and the dependency of droplet size on flow
`rates provides the dynamic controls over the encapsulated
`volume making them unique reaction vessels for chemicals
`of picoliters to nanoliters in volume.
`
`Acknowledgements
`
`The authors would like to thank Dr. Rajinder Khosla at
`National Science Foundation for the SGER award and Pen-
`Hsuan Pan for the assistance with the experimental setups.
`
`References
`
`[1] H. Lodish, A. Berk, S.L. Zipursky, P. Matsudaira, D. Baltimore, J.E.
`Darnell, Molecular Cell Biology, W.H. Freeman and Company, New
`York, NY, 2000.
`[2] S. Wright, L. Huang, Adv. Drug Deliv. Rev. 3 (1989) 343–389.
`[3] D.J. Wedlock, Controlled Particle, Droplet, and Bubble Formation,
`Butterworth-Heinemann Ltd., Jordan Hill, Oxford, 1994.
`[4] V. Cristini, Y.-C. Tan, Theory and numerical simulation of droplet
`dynamics in complex flows—a review, Lab Chip 4 (4) (2004)
`257–264.
`
`[5] T. Thorsen, R.W. Roberts, F.H. Arnold, S.R. Quake, Dynamic pattern
`formation in a vesicle-generating microfluidic device, Phys. Rev.
`Lett. 86 (18) (2001) 4163–4166.
`[6] T. Nisisako, T. Torii, T. Higuchi, Droplet formation in a microchannel
`network, Lab Chip 2 (2002) 24–26.
`[7] S.L. Anna, N. Bontoux, H.A. Stone, Formation of dispersions using
`“flow focusing” in microchannels, Appl. Phys. Lett. 82 (3) (2003)
`364–366.
`[8] R. Heusch, B.A.G. Leverkusen, Ullmann’s Encyclopedia of Industrial
`Chemistry, 2000.
`[9] B. Zheng, L.S. Roach, R.F. Ismagilov, Screening of protein crys-
`tallization condictions on a microfluidic chip using nanoliter-size
`droplets, J. Am. Chem. Soc. 125 (2003) 11170–11171.
`[10] B. Zheng, J.D. Tice, R.F. Ismagilov, Formation of arrayed droplets by
`soft lithography and two-phase fluid flow, and application in protein
`crystallization, Adv. Mater. 16 (15) (2004) 1365–1368.
`[11] B. Zheng, J.D. Tice, L.S. Roach, R.F. Ismagilov, A droplet-based,
`composite PDMS/glass capillary microfluidic system for evaluating
`protein crystallization condictions by microbatch and vapor-diffusion
`methods with on-chip X-ray diffraction, Angew. Chem. Int. Ed. 43
`(2004) 2508–2511.
`[12] G. Yi, T. Thorsen, V.N. Manoharan, M. Hwang, S. Jeon, D.J. Pine,
`S.R. Quake, S. Yang, Generation of uniform colloidal assemblies in
`soft microfluidic devices, Adv. Mater. 15 (15) (2003) 1300–1304.
`[13] G. Yi, V.N. Manoharan, S. Klein, K.R. Brzezinska, D.J. Pine, F.F.
`Lange, S. Yang, Monodisperse micrometer-scale spherical assemblies
`of polymer particles, Adv. Mater. 14 (16) (2002) 1137–1140.
`[14] G. Yi, S. Jeon, T. Thorsen, V.N. Manoharan, S.R. Quake, D.J. Pine,
`S. Yang, Generation of uniform photonic balls by template-assisted
`colloidal crystallization, Synth. Met. 139 (2003) 803–806.
`[15] D. Dendukuri, K. Tsoi, T.A. Hatton, P.S. Doyle, Controlled synthe-
`sis of nonspherical microparticles using microfluidics, Langmuir 21
`(2005) 2113–2116.
`[16] T. Nisisako, T. Torii, T. Higuchi, Novel microreactors for functional
`polymer beads, Chem. Eng. J. 101 (2004) 23–29.
`[17] S. Sugiura, M. Nakajima, H. Itou, M. Seki, Synthesis of polymeric
`microspheres with narrow size distributions employing microchannel
`emulsification, Macromol. Rapid Commun. 22 (10) (2001) 773–778.
`[18] J.R. Millman, K.H. Bhatt, B.G. Prevo, O.D. Velev, Anisotropic par-
`ticle synthesis in dielectrophoretically controlled microdroplet reac-
`tors, Nature 4 (2005) 98–102.
`[19] J. Lee, C. Kim, Surface-tension-driven microactuation based on
`continuous electrowetting, J. Microelectromech. Syst. 9 (2) (2000)
`171–180.
`[20] V. Srinivasan, V.K. Pamula, R.B. Fair, Droplet-based microfluidic
`lab-on-a-chip for glucose detection, Anal. Chim. Acta 507 (2004)
`145–150.
`[21] V. Srinivasan, V.K. Pamula, R.B. Fair, An integrated digital microflu-
`idic lab-on-a-chip for clinical diagnostics on human physiological
`fluids, Lab Chip 4 (4) (2004) 310–315.
`[22] M.A. Burns, C.H. Mastrangelo, T.S. Sammarco, F.P. Man, J.R. Web-
`ster, B.N. Johnson, B. Foerster, D. Jones, Y. Fields, A.R. Kaiser, D.T.
`Burke, Microfabricated structures for integrated DNA analysis, Proc.
`Natl. Acad. Sci. U.S.A. 93 (1996) 5556–5561.
`[23] S.S. Sadhal, P.S. Ayyaswamy, J.N. Chung, Transport Phenomena
`with Drops and Bubbles, Springer-Verlag, New York, NY, 1997.
`[24] X. Guo, F.C. Szoka, Acc. Chem. Res 36 (2003) 335–341.
`[25] K. Edwards, M. Silvander, G. Karlsson, Langmuir 11 (1995) 2429.
`[26] J.C. McDonald, D.C. Duffy, Electrophoresis 21 (1) (2000) 27.
`[27] B.J. Briscoe, C.J. Lawrence, W.G.P. Mietus, Adv. Colloid Interface
`Sci. 81 (1999) 1–17.
`[28] J.D. Tice, A.D. Lyon, R.F. Ismagilov, Effects of viscosity on droplet
`formation and mixing in microfluidic channels, Anal. Chim. Acta
`507 (2003) 73–77.
`[29] J.D. Tice, H. Song, A.D. Lyon, R.F. Ismagilov, Formation of droplets
`and mixing in multiphase microfluidics at low values of the Reynolds
`and the capillary numbers, Langmuir 19 (22) (2003) 9127–9133.
`
`6
`
`

`
`356
`
`Y.-C. Tan et al. / Sensors and Actuators B 114 (2006) 350–356
`
`[30] V. Cristini, S. Guido, A. Alfani, J. Rheol. 47 (5) (2003) 1283.
`[31] X. Zhang, Chem. Eng. Sci. 54 (1999) 1759.
`[32] X.D. Shi, M.P. Brenner, S.R. Nagel, Science 265 (1994) 219.
`[33] D.M. Henderson, W.G. Pritchard, L.B. Smolka, Phys. Fluids 9 (11)
`(1997) 3188.
`[34] Y.-C. Tan, J.S. Fisher, A.I. Lee, V. Cristini, A.P. Lee, Design of
`microfluidic channel geometries for the control of droplet volume,
`chemical concentration, and sorting, Lab Chip 4 (4) (2004) 292–298.
`[35] Y.-C. Tan, A.P. Lee, Microfluidic filtering and sorting of satellite
`droplets as the basis of a monodispersed micron and submicron
`emulsification system, Lab Chip, B504497A.
`[36] Y.-C. Tan, A.P. Lee, Droplet sorting by size in microfluidic chan-
`nels, in: Eighth International Conference on Micro Total Analysis
`Systems, Malmo, Sweden, 2004.
`
`Biographies
`
`Yung-Chieh Tan received a B.S. in Bioengineering Premedical from
`University of California, San Diego in 2001. He received his M.S. in 2002
`and is currently pursuing his doctoral degree in Biomedical Engineering at
`University of California, Irvine. His research interests include the control
`of droplets in microfluidic systems, drug encapsulation, and the design
`and formation of artificial cells.
`
`Vittorio Cristini is currently an assistant professor of Biomedical Engi-
`neering and Mathematics at the University of California, Irvine. He is
`also a member of the Chao Family Comprehensive Cancer Center there.
`He earned his Ph.D. degree in Chemical Engineering (2000) from Yale
`
`University, where he also received a M.S. and a M.Phil. He earned his
`degree of Dottore in Ingegneria Nucleare (Nuclear Engineering) from the
`University of Rome, Italy in 1994. Dr. Cristini is an expert in the field
`of multiphase flows and droplet and bubble dynamics, where he has pub-
`lished about twenty papers. For his important work on droplet breakup
`in laminar and turbulent flows, Dr. Cristini was the first recipient of the
`“Andreas Acrivos Dissertation Award in Fluid Dynamics” from the Amer-
`ican Physical Society-Division of Fluid Dynamics. Dr. Cristini has also
`published several papers in the field of crystal and tumor growth.
`
`Professor Abraham Lee is Professor in Biomedical Engineering and
`Mechanical & Aerospace Engineering at the University of California,
`Irvine. Before UCI he held positions at
`the National Cancer Insti-
`tute, DARPA, and Lawrence Livermore National Laboratory (LLNL).
`He has extensive experience in developing micro and nanotechnology
`for biomedical and biotech applications, including the development of a
`micro-mechanical release mechanism for the deployment of embolic coils
`into brain aneurysms. In the area of micro-fluidics, Dr. Lee and his stu-
`dent demonstrated the first AC magnetohydrodynamic (MHD) micropump
`for micro total analysis systems. Both technologies have been licensed
`to companies. Professor Lee’s current research is focused on the devel-
`opment of integrated “digital” micro/nano fluidic chips for the following
`applications: point-of-care diagnostics, “smart” nanomedicine for early
`detection and treatment, automated cell sorting based on electrical sig-
`natures, tissue engineering and stem cells, the synthesis of ultra-pure
`materials, and biosensors to detect environmental and terrorism threats.
`He currently serves as a Subject Editor for the Journal of Microelec-
`tromechnical Systems and International Advisory Editorial Board member
`of Lab on a Chip. Professor Lee has published over 40 peer reviewed
`articles and owns 32 issued patents.
`
`7