Micro/Nano Encapsulation via Electrified Coaxial Liquid Jets

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<ul><li><p>DOI: 10.1126/science.1067595, 1695 (2002);295 Science</p><p> et al.I. G. LoscertalesMicro/Nano Encapsulation via Electrified Coaxial Liquid Jets</p><p> This copy is for your personal, non-commercial use only.</p><p> clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others</p><p> here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles</p><p> ): January 12, 2014 www.sciencemag.org (this information is current as ofThe following resources related to this article are available online at</p><p> http://www.sciencemag.org/content/295/5560/1695.full.htmlversion of this article at: </p><p>including high-resolution figures, can be found in the onlineUpdated information and services, </p><p>254 article(s) on the ISI Web of Sciencecited by This article has been </p><p> http://www.sciencemag.org/content/295/5560/1695.full.html#related-urls11 articles hosted by HighWire Press; see:cited by This article has been </p><p> http://www.sciencemag.org/cgi/collection/app_physicsPhysics, Applied</p><p>subject collections:This article appears in the following </p><p>registered trademark of AAAS. is aScience2002 by the American Association for the Advancement of Science; all rights reserved. The title </p><p>CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience </p><p> on J</p><p>anua</p><p>ry 1</p><p>2, 2</p><p>014</p><p>ww</p><p>w.s</p><p>cien</p><p>cem</p><p>ag.o</p><p>rgD</p><p>ownl</p><p>oade</p><p>d fro</p><p>m </p><p> on J</p><p>anua</p><p>ry 1</p><p>2, 2</p><p>014</p><p>ww</p><p>w.s</p><p>cien</p><p>cem</p><p>ag.o</p><p>rgD</p><p>ownl</p><p>oade</p><p>d fro</p><p>m </p><p> on J</p><p>anua</p><p>ry 1</p><p>2, 2</p><p>014</p><p>ww</p><p>w.s</p><p>cien</p><p>cem</p><p>ag.o</p><p>rgD</p><p>ownl</p><p>oade</p><p>d fro</p><p>m </p><p> on J</p><p>anua</p><p>ry 1</p><p>2, 2</p><p>014</p><p>ww</p><p>w.s</p><p>cien</p><p>cem</p><p>ag.o</p><p>rgD</p><p>ownl</p><p>oade</p><p>d fro</p><p>m </p><p> on J</p><p>anua</p><p>ry 1</p><p>2, 2</p><p>014</p><p>ww</p><p>w.s</p><p>cien</p><p>cem</p><p>ag.o</p><p>rgD</p><p>ownl</p><p>oade</p><p>d fro</p><p>m </p></li><li><p>concentrations and temperature, and arrive atthe unexpected conclusion that Eq. 2, andespecially the heuristic crossover form Eq. 3,provide an excellent description of our data.</p><p>References and Notes1. S. Sachdev, C. Buragohain, M. Vojta, Science 286,</p><p>2479 (1999).2. T. Matsuda, A. Fujioka, Y. Uchiyama, I. Tsukada, K.</p><p>Uchinokura, Phys. Rev. Lett. 80, 4566 (1998).3. M. Azuma, Y. Fujishiro, M. Takano, M. Nohara, H.</p><p>Takagi, Phys. Rev. B 55, 8658 (1997).4. M. Hucker et al., Phys. Rev. B 59, R725 (1999).5. B. Keimer et al., Phys. Rev. B 46, 14034 (1992).6. M. Corti et al., Phys. Rev. B 52, 4226 (1995).7. K. Uchinokura, T. Ino, I. Terasaki, I. Tsukada, Physica B</p><p>205, 234 (1995).8. D. Stauffer, A. Aharony, Introduction to Percolation</p><p>Theory, Revised 2nd Ed. (Taylor and Francis, Bristol,PA, 1994).</p><p>9. M. E. J. Newman, R. M. Ziff, Phys. Rev. Lett. 85, 4104(2000).</p><p>10. R. J. Birgeneau, R. A. Cowley, G. Shirane, H. Yoshizawa,J. Stat. Phys. 34, 817 (1984) and references therein.</p><p>11. C. Yasuda, A. Oguchi, J. Phys. Soc. Jpn. 68, 2773(1999).</p><p>12. Y.-C. Chen, A. H. Castro Neto, Phys. Rev. B 61, R3772(2000).</p><p>13. S. Miyashita, J. Behre, S. Yamamoto, in ComputationalApproaches in Condensed Matter Physics, S. Mi-yashita, M. Imada, H. Takayama, Eds. (Springer, Berlin,1992), pp. 97111.</p><p>14. K. Kato et al., Phys. Rev. Lett. 84, 4204 (2000).15. A. W. Sandvik, preprint available at http://xxx.lanl.</p><p>gov/abs/cond-mat/0110510.16. A. L. Chernyshev, Y. C. Chen, A. H. Castro Neto,</p><p>preprint available at http://xxx.lanl.gov/abs/cond-mat/0107488.</p><p>17. R. J. Birgeneau et al., Phys. Rev. B 59, 13788 (1999).18. Y. J. Kim et al., Phys. Rev. B 64, 4435 (2001).19. M. Greven et al., Z. Phys. B 96, 465 (1995).20. B. B. Beard, R. J. Birgeneau, M. Greven, U.-J. Wiese,</p><p>Phys. Rev. Lett. 80, 1742 (1998).21. S. Chakravarty, B. I. Halperin, D. R. Nelson, Phys. Rev.</p><p>B 39, 2344 (1989).22. P. Hasenfratz, F. Niedermayer, Phys. Lett. B 268, 231</p><p>(1991).23. A. Aharony, R. J. Birgeneau, A. Coniglio, M. A. Kastner,</p><p>H. E. Stanley, Phys. Rev. Lett. 60, 1330 (1988).</p><p>24. K. Yamada et al., Solid State Comm. 64, 753 (1987).25. M. Greven, R. J. Birgeneau, Phys. Rev. Lett. 81, 1945</p><p>(1998).26. E. Manousakis, Phys. Rev. B 45, 7570 (1992).27. A. H. Castro Neto, D. Hone, Phys. Rev. Lett. 76</p><p>(1996).28. A. B. Harris, S. Kirkpatrick, Phys. Rev. B 16, 542</p><p>(1977).29. R. Coldea et al., Phys. Rev. Lett. 86, 5377 (2001).30. H. E. Stanley, R. J. Birgeneau, P. J. Reynolds, J. F. Nicoll,</p><p>J. Phys. C 9, L553 (1976).31. A. Coniglio, Phys. Rev. Lett. 46, 250 (1981).32. C. C. Wan, A. B. Harris, J. Adler, J. Appl. Phys. 69, 5191</p><p>(1991).33. O.P.V. and M.G. thank A. Aharony, A. H. Castro</p><p>Neto, A. L. Chernyshev, A. W. Sandvik, and E. F.Shender for helpful discussions. The work at Stan-ford was supported by the U.S. Department ofEnergy under contract nos. DE-FG03-99ER45773and DE-AC03-76SF00515, by NSF CAREER Awardno. DMR9400372, and by the by the A.P. SloanFoundation.</p><p>15 October 2001; accepted 23 January 2002</p><p>Micro/Nano Encapsulation viaElectrified Coaxial Liquid Jets</p><p>I. G. Loscertales,1* A. Barrero,2* I. Guerrero,2 R. Cortijo,1</p><p>M. Marquez,3,4 A. M. Ganan-Calvo2</p><p>We report a method to generate steady coaxial jets of immiscible liquids withdiameters in the range of micrometer/nanometer size. This compound jet isgenerated by the action of electro-hydrodynamic (EHD) forces with a diameterthat ranges from tens of nanometers to tens of micrometers. The eventual jetbreakup results in an aerosol of monodisperse compound droplets with theouter liquid surrounding or encapsulating the inner one. Following this ap-proach, we have produced monodisperse capsules with diameters varying be-tween 10 and 0.15 micrometers, depending on the running parameters.</p><p>Production and control of droplets and parti-cles of micrometer or even nanometer sizewith a narrow size distribution are of interestfor many applications in science and technol-ogy. Usually, these particles are formed aseither an aerosol or a hydrosol phase. Aero-sols and hydrosols of compound particles,such that each particle is made of a smallamount of a certain substance surrounded byanother one, are of particular importance forencapsulation of food additives (13), target-ed drug delivery (410), and special materialprocessing (11, 12), among other technolog-ical fields. All of them resort to encapsulation</p><p>to provide such compound particles in theappropriate size range.</p><p>The interest in encapsulation may be mo-tivated by a broad spectrum of reasons: toisolate an unstable component from an ag-gressive environment, to avoid decomposi-tion of a labile compound under a certainatmosphere, to deliver a given substance to aparticular receptor, and so forth. Although thesubstance to be encapsulated may be eithersolid or liquid, the encapsulating agent isusually a polymer carrying solution or a melt-ed polymer. The key issue is the way to formthe micrometer/nanometer-sized capsulesfrom the bulk components, with controllableand adjustable coating thickness.</p><p>One of the most widely adopted methodsto obtain micrometer/nanometer capsules isbased on emulsion technology (1315). Twoimmiscible fluids, one carrying the substanceto be encapsulated and the other one carryingthe polymer for the shell, are stirred to forman emulsion. This emulsion is stabilized bypouring it into a third solution (double emul-sion process; DE), thereby extracting the</p><p>polymer solvent and solidifying the polymeras a capsule. Related methods also incorpo-rate phase separation and similar physical orchemical phenomena (16, 17).</p><p>Other approaches for encapsulation resortto the formation and control of liquid jetswith diameters in the micrometer/nanometerrange. In the electrospray (ES) technique, aconducting liquid is slowly injected throughan electrified capillary tube. When the elec-tric potential between the liquid and its sur-roundings rises to a few kilovolts, the menis-cus at the tube exit develops a conical shape,commonly referred to as the Taylor cone. Athin microthread of liquid is issued from thetip of the Taylor cone, which eventually frag-ments to form a spray of highly chargeddroplets (18). Its most well-known applica-tion has been in mass spectrometry, where ithas been successfully exploited as a way toproduce multiply charged gas phase ions ofhuge biomolecules present in the liquid phase(19). Another outstanding feature of the elec-trospray is its ability to generate monodis-perse droplets whose size can be varied be-tween hundred of micrometers and tens ofnanometers, independent of the diameter ofthe capillary tube (20, 21).</p><p>In the simplest version of selective with-drawal (SW), the exit of a tube is located at aheight S above the interface separating twoimmiscible liquids (or liquid gas). For lowrates of fluid withdrawal Q, only the upperfluid is sucked through the tube. A sufficientincrease in Q, or decrease in S, gives rise to athin spout of the lower liquid surrounded bythe outer fluid (2224).</p><p>In the flow focusing (FF) technique (25,26), a liquid is injected through a capillarytube whose exit is located close to a smallhole drilled in a thin plate. A stream ofanother fluid (gas or liquid) surrounding thetube is forced through the hole. The mechan-</p><p>1Escuela Tecnica Superior de Ingenieros Industriales,Universidad de Malaga, Plaza El Ejido, S/N Malaga29013, Spain. 2Escuela Superior de Ingenieros, Univer-sidad de Sevilla, Camino de los Descubrimientos S/N,41092 Seville, Spain. 3Kraft Foods R&amp;D, The Nano-technology Lab, 801 Waukegan Road, Glenview, IL60025, USA. 4Los Alamos National Laboratory, Chem-ical Sciences and Technology Division, Los Alamos,NM 87545, USA.</p><p>*To whom correspondence should be addressed. E-mail: loscertales@uma.es; barrero@eurus2.us.es</p><p>R E P O R T S</p><p>www.sciencemag.org SCIENCE VOL 295 1 MARCH 2002 1695</p></li><li><p>ical stresses exerted by this stream deform themeniscus attached to the tube exit. Then, themeniscus develops a cusplike shape fromwhose vertex a thin jet is issued. As in theelectrospray case, the jet diameter is alsoindependent of the much larger diameter ofthe capillary tube.</p><p>In the DE methods, the capsules consist of apolymer matrix in which the encapsulated sub-stance is somehow randomly distributed in tinypackets. The mean size of the capsules mayvary between hundreds of nanometers and hun-dreds of micrometers, although the dispersionin sizes presents a relatively high standard de-viation (usually larger than 100%).</p><p>Although each of these methods is able toproduce encapsulated particles with a well-defined size range, good control over thethickness of the coating, or a specific size ofcoated particle, they are unable to accomplishall three of the desired parameters.</p><p>We report a technique that uses electro-hydrodynamic (EHD) forces to generate co-axial jets of immiscible liquids with diame-ters in the nanometer range (27). The spraygenerated from the varicose breakup of the jetconsists of monodisperse compound droplets,which can reach sizes well below the micro-meter range. The basic experimental setup forthe technique is sketched in Fig. 1. Twoimmiscible liquids, red and blue, are injectedat appropriate flow rates through two concen-trically located needles. The inner diameter ofthe inner needle ranges from the order of 1mm to tens of micrometers, whereas its outerdiameter sets limits to the cross section of theouter needle. The outer needle is connected toan electrical potential of several kilovolts rel-ative to a ground electrode (the extractor).The inner needle is kept to an electrical po-tential that, depending on the conductivity ofthe outer liquid, can be varied from that of theouter needle to that of the extractor. For a</p><p>certain range of values of the electrical po-tential and flow rates (28), a structured Taylorcone is formed at the exit of the needles withan outer meniscus surrounding the inner one(see Fig. 2A). A liquid thread is issued fromthe vertex of each one of the two menisci,giving rise to a compound jet of two coflow-ing liquids (see Fig. 2B). At the minimum jetsection, the two-layered jet shown in Fig. 2Bhas an outer diameter of 4 mm.</p><p>In the case of the experiment in Fig. 2, Aand B, the outer liquid was DuPont pho-topolymer Somos 6120, and the inner onewas colored ethylene glycol (EG). To devel-op this structure, we injected Somos, whichflowed through the annular gap between thetwo needles. We then increased the electricalpotential of the outer needle V1 until theliquid meniscus jumped into a stable cone-jetmode. In this particular example, the electri-cal potential of the inner needle was main-tained at V2 5 V1. The viscosity of the outerliquid is sufficiently high to diffuse the elec-trical stresses acting on the surface into thebulk. Consequently, the liquid motion insidethe Taylor cone is dominated by viscosity, sothat the liquid velocity inside the cone iseverywhere pointing toward the cone apex(29). The second liquid flowed through theinner needle to form a new meniscus insidethe Somos one. The motion of Somos de-formed the EG meniscus to a conical shapeand sucked it from the meniscuss tip to forma thin microthread. This microthread of EGmerged with that of Somos at the tip of theTaylor cone to finally form a two-concentriclayered micro/nano jet.</p><p>To obtain a structured Taylor cone, theEHD forces must act on at least one liquid,although they may act on both. We shall call thedriving liquid the one upon which the EHDforces act to form the Taylor cone. We intro-duced a configuration in which the driving liq-uid flows through the needles annular gap (see</p><p>Fig. 1). However, there is an alternative config-uration in which the driving liquid flowsthrough T2, whereas the second one flowsthrough the annular gap between T1 and T2.That is the case shown in Fig. 2, C and D,where a Taylor cone of water is formed insidea meniscus of a nonconducting liquid such asolive oil. The electrical stresses acting on thecharged waterolive oil interface, which areefficiently transmitted by viscosity toward theolive oil bulk, set the olive oil into motion,toward the water cone vertex. These two co-flowing streams eventually give rise to a coax-ial jet of water coated by oil. As in regularelectrosprays, the motion of the driving liquidin this second configuration does not need to bedominated by viscosity. Olive oil (or any otherliquid insulator) cannot be electrosprayed on itsown in the cone-jet mode, because the lack ofsurface charge density prevents the formationof a steady Taylor cone (30).</p><p>The stability of the structured Taylorcones and the electrified compound jetsstrongly depends on the physical propertiesof the working fluids as well as the liquidflow rates and applied voltages. This com-plex electro-fluid-mechanical scenario israther poorly understood, so that the stabilitydomains for most liquid couples remain to beinvestigated. In the absence of better knowl-edge, we should point out that stable struc-tured Taylor cones have been obtained forliquids satisfying the condition si . so,where s is the liquid-dielectric atmospheresurface tension and subscripts i and o refer tothe inner and outer liquid, respectively.</p><p>Experiments show that the scaling laws(electrical cu...</p></li></ul>


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