micro/nano encapsulation via electrified coaxial liquid jets

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  • DOI: 10.1126/science.1067595, 1695 (2002);295 Science

    et al.I. G. LoscertalesMicro/Nano Encapsulation via Electrified Coaxial Liquid Jets

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  • 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.

    References and Notes1. S. Sachdev, C. Buragohain, M. Vojta, Science 286,

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    205, 234 (1995).8. D. Stauffer, A. Aharony, Introduction to Percolation

    Theory, Revised 2nd Ed. (Taylor and Francis, Bristol,PA, 1994).

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    10. R. J. Birgeneau, R. A. Cowley, G. Shirane, H. Yoshizawa,J. Stat. Phys. 34, 817 (1984) and references therein.

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    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.

    14. K. Kato et al., Phys. Rev. Lett. 84, 4204 (2000).15. A. W. Sandvik, preprint available at http://xxx.lanl.

    gov/abs/cond-mat/0110510.16. A. L. Chernyshev, Y. C. Chen, A. H. Castro Neto,

    preprint available at http://xxx.lanl.gov/abs/cond-mat/0107488.

    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,

    Phys. Rev. Lett. 80, 1742 (1998).21. S. Chakravarty, B. I. Halperin, D. R. Nelson, Phys. Rev.

    B 39, 2344 (1989).22. P. Hasenfratz, F. Niedermayer, Phys. Lett. B 268, 231

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    H. E. Stanley, Phys. Rev. Lett. 60, 1330 (1988).

    24. K. Yamada et al., Solid State Comm. 64, 753 (1987).25. M. Greven, R. J. Birgeneau, Phys. Rev. Lett. 81, 1945

    (1998).26. E. Manousakis, Phys. Rev. B 45, 7570 (1992).27. A. H. Castro Neto, D. Hone, Phys. Rev. Lett. 76

    (1996).28. A. B. Harris, S. Kirkpatrick, Phys. Rev. B 16, 542

    (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,

    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

    (1991).33. O.P.V. and M.G. thank A. Aharony, A. H. Castro

    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.

    15 October 2001; accepted 23 January 2002

    Micro/Nano Encapsulation viaElectrified Coaxial Liquid Jets

    I. G. Loscertales,1* A. Barrero,2* I. Guerrero,2 R. Cortijo,1

    M. Marquez,3,4 A. M. Ganan-Calvo2

    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.

    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

    to provide such compound particles in theappropriate size range.

    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.

    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

    polymer solvent and solidifying the polymeras a capsule. Related methods also incorpo-rate phase separation and similar physical orchemical phenomena (16, 17).

    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).

    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).

    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-

    1Escuela Tecnica Superior de Ingenieros Industriales,Universidad de Malaga, Plaza El Ejido, S/N Malaga29013, Spain. 2Escuela Superior de Ingenieros, Univer-sidad

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