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UW ATMS 211 - Lecture Notes

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quently harden the coat. However, this coat-ing method is easily generalized to allow fordifferent ways of hardening the particle coats.With the use of appropriate reagents, thistechnique is also compatible with schemesthat thermally or optically initiate the hard-ening of the coats. Figure 6 shows confocalimages of a polystyrene bead encapsulatedwith an agarose coat that was hardened bylowering its temperature. In Fig. 7, we showan optical micrograph of a poppy seed encap-sulated in a poly(styrenesulfonic acid) coatthat was photopolymerized. The selectivewithdrawal geometry may also be inverted byinserting the straw through the bottom of thewithdrawal container, with the straw tip po-sitioned below the interface. Now the denseraqueous fluid is the primary fluid being with-drawn, and the particles to be coated areplaced in the oil (upper fluid). This inversionextends the applicability of this coating tech-nique to hydrophobic particles in oil-solublereagents as well as to heavy particles in theupper fluid that will sediment to the interfa-cial boundary.The selective withdrawal coating tech-nique complements other currently availabletechniques, such as surface-induced polymer-ization (10), which can also produce coats ofuniform thickness on irregularly shaped par-ticles but which often require modification ofthe particle surface and therefore may not befeasible. In our technique, as illustrated in thedescription of the polyamide coating, the par-ticles can be completely separated from thecaustic reagents that initiate the polymeriza-tion at the outer interface. Moreover, surface-induced polymerization requires the reagentto be stable in the solution being polymer-ized. This is not always possible (as with thepolyamide coat installation, the trichloridereagent is unstable in aqueous solutions), andselective withdrawal can solve this problemby initiating polymerization with a reagentthat is stable in the fluid surrounding the coat.Finally, many polymer coatings are eitherdifficult [e.g., the photopolymerized poly-(styrenesulfonic acid) coat shown in Fig. 7]or impossible (e.g., the thermally hardenedagarose coat shown in Fig. 6) to prepare usingtechniques such as surface-induced polymer-ization and must be hardened in bulk. Forthese cases in particular, selective withdrawalpresents a valuable advantage over currentlyavailable coating techniques.The selective withdrawal technique can bereadily optimized. With a single tube weestimate that 10,000 particles can be coatedper hour. Preliminary experiments havedemonstrated that this technique can bescaled up by using an array of tubes inparallel. Injecting particles directly into theregion below the spout can make the meth-od suitable for particles with higher densitythan the prepolymer. As described above,inversion of the selective withdrawal ge-ometry can extend the applicability of thistechnique to hydrophobic particles in oil-soluble reagents. Because of its flexibilityin polymerization schemes, its ability tocoat particles of many different types, andits ability to tune the thickness of the coats,this technique is an attractive option in arange of applications and a valuable addi-tion to the repertoire of currently availablecoating techniques.References and Notes1. A. J. Mendonca, X. Y. Xiao, Med. Res. Rev. 19, 451(1999).2. S. J. Shuttleworth, S. M. Allin, P. K. Sharma, Synthesis11, 1217 (1997).3. F. Lim, A. M. Sun, Science 210, 908 (1980).4. P. Soon-Shiong, Adv. Drug Deliv. Rev. 35, 259 (1999).5. R. Langer, Acc. Chem. Res. 33, 94 (2000).6. G. H. J. Wolters, W. M. Fritschy, D. Gerrits, R. VanSchilfgaarde, J. Appl. Biomater. 3, 281 (1992).7. T. Yoshioka, R. Hirano, T. Shioya, M. Kako, Biotechnol.Bioeng. 35, 66 (1990).8. E. Mathiowitz, Encyclopedia of Controlled Drug De-livery (Wiley, New York, 1999).9. E. Donath, G. B. Sukhorukov, F. Caruso, S. A. Davis, H.Mohwald, Angew. Chem. Int. Ed. 37, 2202 (1998).10. G. M. Cruise, O. D. Hegre, D. S. Scharp, J. A. Hubbell,Biotechnol. Bioeng. 57, 655 (1998).11. J. R. Lister, J. Fluid Mech. 198, 231 (1989).12. S. Blake and G. N. Ivey, J. Volcanol. Geotherm. Res.27, 153 (1986).13. I. Cohen and S. R. Nagel, in preparation.14. E. E. Timm, U.S. Patent 4,444,961 (1984).15. Even without the inclusion of particles, as originallyshown by Savart [Annal. Chim. 53, 337 (1883)] andRayleigh [Philos. Mag. 34, 177 (1892)], the prepolymerspout will break into droplets that can be hardened,allowing the fabrication of monodisperse particles.16. A. M. Ganan-Calvo, Phys. Rev. Lett. 80, 285 (1997).17. P. B. Umbanhowar, V. Prasad, D. A. Weitz, Langmuir16, 347 (2000).18. O. Valges-Aguilera, C. P. Pathak, J. Shi, D. Watson,D. C. Neckers, Macromolecules 25, 541 (1992).19. We thank H. Rilo and A. Rotamel for early discussionsthat motivated these studies. We also thank C. Lassyand the Confocal Digital Imaging Facility at the Uni-versity of Chicago. Supported by NSF grant DMR-9722646 and NSF Materials Research Science andEngineering Centers Program grant DMR-9808595.19 January 2001; accepted 7 March 2001Anthropogenic Warming ofEarth’s Climate SystemSydney Levitus,1* John I. Antonov,1Julian Wang,2Thomas L. Delworth,3Keith W. Dixon,3Anthony J. Broccoli3We compared the temporal variability of the heat content of the world ocean,of the global atmosphere, and of components of Earth’s cryosphere during thelatter half of the 20th century. Each component has increased its heat content(the atmosphere and the ocean) or exhibited melting (the cryosphere). Theestimated increase of observed global ocean heat content (over the depth rangefrom 0 to 3000 meters) between the 1950s and 1990s is at least one order ofmagnitude larger than the increase in heat content of any other component.Simulation results using an atmosphere-ocean general circulation model thatincludes estimates of the radiative effects of observed temporal variations ingreenhouse gases, sulfate aerosols, solar irradiance, and volcanic aerosols overthe past century agree with our observation-based estimate of the increase inocean heat content. The results we present suggest that the observed increasein ocean heat content may largely be due to the increase of anthropogenic gasesin Earth’s atmosphere.Studies using instrumental data to documenta warming of Earth’s climate system due toincreasing concentrations of greenhouse gas-es (GHGs) have focused on surface air tem-perature and sea surface temperature (1).These variables have proved invaluable fordocumenting an


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