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Langmuir 2007, 23, 11924-11931
Flow-Focusing Generation of Monodisperse Water Droplets Wrapped by Ionic Liquid on Microfluidic Chips: From Plug to Sphere Wei-Han Wang,† Zhi-Ling Zhang,*,† Ya-Ni Xie,† Li Wang,† Song Yi,† Kan Liu,‡ Jia Liu,† Dai-Wen Pang,† and Xing-Zhong Zhao‡ College of Chemistry and Molecular Sciences and State Key Laboratory of Virology, and Department of Physics, Wuhan UniVersity, Wuhan 430072, People’s Republic of China ReceiVed April 22, 2007. In Final Form: July 13, 2007 Generating droplets via microfluidic chips is a promising technology in microanalysis and microsynthesis. To realize room-temperature ionic liquid (IL)-water two-phase studies in microscale, a water-immiscible IL was employed as the continuous phase for the first time to wrap water droplets (either plugs or spheres) on flow-focusing microfluidic chips. The IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), could wet both hydrophilic and hydrophobic channel surfaces because of its dual role of hydrophilicity/hydrophobicity and extremely high viscosity, thus offering the possibility of wrapping water droplets in totally hydrophilic (THI), moderately hydrophilic (MHI), and hydrophobic (HO) channels. The droplet shape could be tuned from plug to sphere, with the volume from 6.3 nL to 65 pL, by adding an orifice in the focusing region, rendering the hydrophilic channel surface hydrophobic, and suppressing the Uw/UIL ratio below 1.0. Three different breakup processes were defined and clarified, in which the sub-steady breakup and steady breakup were essential for the formation of plugs and spheric droplets, respectively. The influences of channel hydrophilicity/hydrophobicity on droplet formation were carefully studied by evaluating the wetting abilities of water and IL on different surfaces. The superiority of IL over water in wetting hydrophobic surface led to the tendency of forming small, spheric aqueous droplets in the hydrophobic channel. This IL-favored droplet-based system represented a high efficiency in water/IL extraction, in which rhodamine 6G was extracted from aqueous droplets to [BMIM][PF6] in the hydrophobic orifice-included (HO-OI) channel in 0.51 s.
Introduction Recently, generation of droplets of one fluid (dispersed phase) in another immiscible fluid (continuous phase) in microchannels is a growing trend in microfluidic analysis and synthesis.1-4 This bottom-up route for precise production of droplets opens a stimulating field for manipulation of nano- or picoliter liquid in chemical4-9 and biological3,10-14 research. In previous studies, different oils, such as perfluorocarbon,5,10,11,13-19 mineral oil,6,12 * Correspondingauthor.Tel.: +86-27-68756759.E-mail:
[email protected]. † College of Chemistry and Molecular Sciences and State Key Laboratory of Virology. ‡ Department of Physics. (1) Joanicot, M.; Ajdari, A. Science 2005, 309, 887-888. (2) deMello, A. J. Nature 2006, 442, 394-402. (3) Zheng, B.; Gerdts, C. J.; Ismagilov, R. F. Curr. Opin. Struct. Biol. 2005, 15, 548-555. (4) Seo, M.; Nie, Z.; Xu, S.; Lewis, P. C.; Kumacheva, E. Langmuir 2005, 21, 4773-4775. (5) Shestopalov, I.; Tice, J. D.; Ismagilov, R. F. Lab Chip 2004, 4, 316-321. (6) De Geest, B. G.; Urbanski, J. P.; Thorsen, T.; Demeester, J.; De Smedt, S. C. Langmuir 2005, 21, 10275-10279. (7) Hung, L. H.; Choi, K. M.; Tseng, W. Y.; Tan, Y. C.; Shea, K. J.; Lee, A. P. Lab Chip 2006, 6, 174-178. (8) Liu, K.; Ding, H. J.; Liu, J.; Chen, Y.; Zhao, X. Z. Langmuir 2006, 22, 9453-9457. (9) Kumemura, M.; Korenaga, T. Anal. Chim. Acta 2006, 558, 75-79. (10) Zheng, B.; Roach, L. S.; Ismagilov, R. F. J. Am. Chem. Soc. 2003, 125, 11170-11171. (11) Roach, L. S.; Song, H.; Ismagilov, R. F. Anal. Chem. 2005, 77, 785-796. (12) Grodrian, A.; Metze, J.; Henkel, T.; Martin, K.; Roth, M.; Ko¨hler, J. M. Biosens. Bioelectron. 2004, 19, 1421-1428. (13) Liau, A.; Karnik, R.; Majumdar, A.; Cate, J. H. D. Anal. Chem. 2005, 77, 7618-7625. (14) Song, H.; Li, H. W.; Munson, M. S.; Van Ha, T. G.; Ismagilov, R. F. Anal. Chem. 2006, 78, 4839-4849. (15) Tice, J. D.; Song, H.; Lyon, A. D.; Ismagilov, R. F. Langmuir 2003, 19, 9127-9133. (16) Zheng, B.; Tice, J. D.; Ismagilov, R. F. Anal. Chem. 2004, 76, 49774982. (17) Tice, J. D.; Lyon, A. D.; Ismagilov, R. F. Anal. Chim. Acta 2004, 507, 73-77. (18) Song, H.; Bringer, M. R.; Tice, J. D.; Gerdts, C. J.; Ismagilov, R. F. Appl. Phys. Lett. 2003, 83, 4664-4666.
triolein,20 soybean oil,8,21 oleic acid,22 silicon oil,7,23 hexadecane,24 and tetradecane,12,25 have been employed as a hydrophobic media for separating aqueous droplets, thus prohibiting transfer of species, especially ions, from one phase to the other. In recent years, room-temperature ionic liquids (ILs), known as the combination of organic cations and various anions that may be liquids at room temperature, have been widely used in extraction,26-29 chromatography,30 biomacromolecule stabilization,31-34 phase transfer synthesis,35 and electrochemistry36 for their unique properties such as negligible vapor pressure, (19) Song, H.; Tice, J. D.; Ismagilov, R. F. Angew. Chem., Int. Ed. 2003, 42, 768-772. (20) Nisisako, T.; Torii, T.; Higuchi, T. Lab Chip 2002, 2, 24-26. (21) Xu, Q.; Nakajima, M. Appl. Phys. Lett. 2004, 85, 3726-3728. (22) Tan, Y. C.; Cristini, V.; Lee, A. P. Sens. Actuators, B 2006, 114, 350356. (23) Anna, S. L.; Bontoux, N.; Stone, H. A. Appl. Phys. Lett. 2003, 82, 364366. (24) Thorsen, T.; Roberts, R. W.; Arnold, F. H.; Quake, S. R. Phys. ReV. Lett. 2001, 86, 4163-4166. (25) Dreyfus, R.; Tabeling, P.; Willaime, H. Phys. ReV. Lett. 2003, 90, 144505. (26) Li, C.; Xin, B.; Xu, W.; Zhang, Q. J. Chem. Technol. Biotechnol. 2007, 82, 196-204. (27) Vijayaraghavan, R.; Vedaraman, N.; Surianarayanan, M.; MacFarlane, D. R. Talanta 2006, 69, 1059-1062. (28) Inoue, G.; Shimoyama, Y.; Su, F.; Takada, S.; Iwai, Y.; Arai, Y. J. Chem. Eng. Data 2007, 52, 98-101. (29) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. Chem. Commun. 2001, 135-136. (30) Liu, J. F.; Jo¨nsson, J. A.; Jiang, G. B. TRAC: Trends Anal. Chem. 2005, 24, 20-27. (31) Wang, J. H.; Cheng, D. H.; Chen, X. W.; Du, Z.; Fang, Z. L. Anal. Chem. 2007, 79, 620-625. (32) Baker, S. N.; McCleskey, T. M.; Pandey, S.; Baker, G. A. Chem. Commun. 2004, 940-941. (33) Fujita, K.; Forsyth, M.; MacFarlane, D. R.; Reid, R. W.; Elliott, G. D. Biotechnol. Bioeng. 2006, 94, 1209-1213. (34) Du, Z.; Yu, Y. L.; Wang, J. H. Chem.-Eur. J. 2007, 13, 2130-2137. (35) Me´vellec, V.; Leger, B.; Mauduit, M.; Roucoux, A. Chem. Commun. 2005, 2838-2839. (36) Wang, S. F.; Chen, T.; Zhang, Z. L.; Shen, X. C.; Lu, Z. X.; Pang, D. W.; Wong, K. Y. Langmuir 2005, 21, 9260-9266.
10.1021/la701170s CCC: $37.00 © 2007 American Chemical Society Published on Web 10/05/2007
Generation of Monodisperse Water Droplets
good thermal stability, good extractability for different species, and biocompatibility.26-37 Many of these applications included a water/IL two-phase system, in which transfer of species took place, for example, extraction of dyes26,27 or metal ions29 from wastewater to ILs, and transfer of aqueous biomacromolecules31-34 or nanoparticles35,37 to ILs for stabilization and catalysis, respectively. However, these studies were carried out in macroscale where large volumes of ILs and sample solutions were needed. This is uneconomical, especially for task-specific ILs and biomacromolecules that are difficult to obtain, and is sometimes time-consuming. Introducing ILs in the droplet-based microfluidic system may lead to the development of water/IL two-phase studies in microscale or nanoscale, and thus solve the problems aforementioned. Unfortunately, the work concerning ILs in microfluidics was rarely reported. Recently, Chatterjee et al.38 and Dubois et al.39 developed digital microfluidic chips for manipulating IL droplets in air via an actuation of electrowetting on dielectric (EWOD). In another study, for the purpose of achieving a commercial status of ILs, [BMIM][PF6] and polysulfone were dissolved in dichloromethane as the droplet phase, and then encapsulated by an aqueous solution of gelatin via a two-stage microfluidic approach.40 Both methods, however, employed IL as the dispersed phase (either in air or in the aqueous phase) and did not include any species transfer between two phases. For the purpose of carrying out water/IL two-phase studies in microscale, we are trying to wrap aqueous droplets in waterimmiscible ILs and study the transfer and even exchange of different species between the two phases. In most droplet-based microfluidic work, two typical shapes of droplet, plug (droplet large enough to block the channel) and sphere, were focused on for different application purposes. The generation of aqueous plugs in oil was mostly achieved by twophase crossflow in T-junction microchannels,5,10-19,25 in which a water stream is vertically injected into a flow of waterimmiscible oil in the main channel, where it breaks up into plugs wrapped by the oil. This method requires a low capillary number (Ca) to avoid destruction of plugs by shear.15,19 Perfluorocarbons have been widely used as the continuous phase in formation of aqueous plugs5,10,11,13-19 for the advantage of their low viscosities, which are favorable for achieving low Ca (
