Self-Assembly of Mesoscale Objects in ordered 2D Arrays - The ...

REPORTSSelf-Assembly of Mesoscale Objects intoOrdered Two-Dimensional ArraysNed Bowden, Andreas Terfort, Jeff Carbeck,George M. Whitesides*Regular arrays of topologically complex, millimeter-scale objects were prepared byself-assembly, with the shapes of the assembling objects and the wettability of theirsurfaces determining the structure of the arrays. The system was composed of solidobjects floating at the interface between perfluorodecalin and water and interacting bylateral capillary forces; patterning of the wettability of the surfaces of the objects directsthese forces. Self-assembly results from minimization of the interfacial free energy of theliquid-liquid interface. Calculations suggest that this strategy for self-assembly can beapplied to objects on a micrometer scale.This report describes the directed self-assemblyof small objects (between 1 and 10mm in length) into regular, two-dimensionalarrays. It extends ideas emerging frommolecular self-assembly to much larger objectsand describes an approach for the fabricationof complex systems that has potentialfor application in microelectronics, optics,microelectromechanical systems, anddisplays (1). These self-assembled systemshave analogies to bubble rafts (2, 3) andrelated aggregates formed by spheres on surfaces(4, 5); they differ in that the use ofnonspherical objects with designed shapesand wettabilities makes it possible to generateaggregates with complex structures.The individual objects were made ofpolydimethylsiloxane (PDMS), a hydrophobicpolymer with a surface free energyDepartment of Chemistry and Chemical Biology, HarvardUniversity, 12 Oxford Street, Cambridge, MA 02138,USA.*To whom correspondence should be addressed.( SV) equal to 22 to 24 erg cm 2 (6–8).First, the PDMS was cured in a mold of thedesired shape. Designated surfaces werethen made hydrophilic by oxidation withan O 2plasma (6, 9); surfaces that were tobe hydrophobic were either covered withtape before the oxidation or generated bycutting of the PDMS after oxidation toexpose a fresh hydrophobic surface. Foreach of the objects in Fig. 1, the lower face[the face in contact with the perfluorodecalin(C 10F 18)] was hydrophobic, and thesides were either hydrophobic or hydrophilicaccording to the design summarized inthe insets. The upper face was usually madehydrophilic to prevent any C 10F 18from settlingon top of the objects and causing themto sink into the C 10F 18-H 2O interface. TheC 10F 18wetted the hydrophobic sides andformed menisci; the hydrophilic sides barelyperturbed the C 10F 18-H 2O interface (10,11). The system was placed on a rotaryshaker that oscillated gently in the plane ofthe interface. The frequency of rotation wasadjusted for each system to allow the inplanemovement of the liquid to bring theobjects close to one another and to break upmisformed aggregates; a typical frequencywas 1 to 2 Hz. When two hydrophobic sidesof PDMS came within a critical distance ofone another (5 mm), they moved intocontact. This movement was driven by theminimization of the interfacial free energyof the system, caused by eliminating thecurved menisci at the hydrophobic surfaces.A thin layer of C 10F 18remained betweenthe objects, even at their closest contact(12). Self-assembly was completed after 5to 30 min.A variety of self-assembled arrays can bemade by controlling the hydrophobicity ofthe sides of the objects (Fig. 1A). Thecontrast between Fig. 1B and Fig. 1C demonstratesthe level of control that we canachieve: In Fig. 1B, alternate sides of thehexagons are hydrophobic and the objectsform an open network; in Fig. 1C, all of thesides of the hexagons are hydrophobic andthe objects form a close-packed array.A shape-selective lock-and-key geometryallows the formation of aggregates frommore than one component (Fig. 2, Athrough C). The objects in Fig. 2A havethree favorable choices for contact (Fig.2C). The amplitude of oscillation of thesystem during equilibration was set to belarge enough to break apart pairs interactinghead-to-head and tail-to-tail but toleave together those interacting morestrongly head-to-tail. Head-to-tail contactis favored energetically by two complementaryfactors: it maximizes the area of hydrophobicsurface in close proximity; and it iskinetically stable to dissociation caused bystirring, because the oscillating motion atthe interface—a motion that seems to in-Fig. 1. Crystalline aggregates generated by the self-assembly of (A) crosses,(B) hexagons in an open network, and (C) hexagons close-packed. Panel (A)shows an extended two-dimensional square array formed from crosseshaving hydrophobic ends; the two crosses with arrows pointed toward themwere completely oxidized; these pieces were rejected by the array. Theyclearly show the shape of the individual pieces. Hatched faces in the insetsindicate hydrophobic surfaces and white faces indicate hydrophilic surfaces.The top faces are clear and hydrophilic.http://www.sciencemag.org SCIENCE VOL. 276 11 APRIL 1997 233

fluence the objects primarily by shear—hasminimal influence on pairs of objects onceassembled into a head-to-tail configuration.A third method for self-assembly usesthe area of hydrophobic side surfaces, andthus the strength of the attractive capillaryforce, to direct the self-assembly of differentobjects (Fig. 3, A through D). A mixture oftwo types of PDMS objects with the samesquare bases, but heights that differed by afactor of 5, were agitated at the interfacebetween C 10 F 18 and water. The order of theattractive forces in the system was talltall tall-short short-short. The degreeof agitation was set to allow the tall objectsto form an array; when agitation wasFig. 2. Lock-and-key self-assembly. Lines areformed from one type of component (A) and fromtwo different types of components (B). (C) Thethree possibilities for favorable assembly of twoobjects in (A). Although in the tail-to-tail arrangementin (C) no hydrophobic surfaces are in contact,this interaction was observed to be importantbecause the menisci are brought close enoughtogether to interact. Dark faces in the insets in (A) and (B) indicate hydrophobic sides and white facesindicate hydrophilic sides. The top faces are clear and hydrophilic.stopped, the short objects coagulatedaround this array in a disordered state.The above experiments were carried outwith objects having dimensions of 1 to 10mm. We investigated the lower limits to thesize of the objects that could be assembledby lateral capillary forces at the C 10 F 18 -H 2 Ointerface by calculating the change in interfacialfree energy as two perpendicularsurfaces moved from infinite separation tosome finite separation, d. We calculated theheight h (in meters) of the C 10 F 18 -H 2 Ointerface between the two objects using thelinearized Laplace equation (13) (Fig. 4A) 2 hx 2 1 gh P 0 (1)where (in joules per square meter) is theinterfacial free energy, (in kilograms percubic meter) is the difference in densitybetween the two fluids, the zero for h is setat the C 10 F 18 -H 2 O interface far from theobjects, g (in meters per second per second)is the acceleration due to gravity, and P 0(in pascals) is the change in pressure acrossthe interface at x 0. If we assign a valueof h(0) 0, then the value of P 0 does notenter into the solution of Eq. 1. Using theboundary conditions h(r) t, where t (inmeters) is the thickness of the object and(h/x) x0 0, we find that the solution ofEq.1is2hxt ex/xc d/2xc e x/xc)1e d/xc e d/2xc (2) eFig. 3. Self-assembly based on different heights. Thetwo components are identical except for their heights,and their sides are hydrophobic. The light gray objectsin (A) and (B) have a height of 2.5 mm and thedark gray objects have a height of 0.5 mm. In (A) theobjects have just begun to assemble. After 20 minof agitation, the objects in (B) have segregated intotwo regions: a crystalline central array of the tallerobjects, surrounded by a disordered collection of thesmaller ones. (C) A schematic view of the interaction of the objects of different heights. (D) Side view ofthe three different interactions when two pieces of PDMS with different heights are assembled. Thestrength of interaction increases from right to left.Fig. 4. (A) The coordinate system for Eqs. 1 and 2.The objects have a height of t and a width of w 5t , and their proximate surfaces are separated byd. (B) The logarithm of the change in interfacialfree energy—divided by thermal energy, kT—inbringing two surfaces from infinite separation to afinite separation, d, is plotted for heights from t 1 mm to 100 nm.234SCIENCE VOL. 276 11 APRIL 1997 http://www.sciencemag.org

REPORTSwhere we have made the replacement x c (/g) 1/2 . For infinite d the capillary surfaceis given by a simple exponential decaywith h(x) e (x/x c) .To estimate the change in interfacialenergy as a function of distance, we calculatedthe difference in the arc length (inmeters), defined by h(x) for two surfacesseparated by d and two surfaces separated byan infinite distance; the change in arclength was then multiplied by the width ofthe object w (in meters), and the interfacialfree energy to yield the change in interfacialfree energy (14). As a model system, weassigned a length to the perpendicular surfaceequal to five times the height. Thismodel gave a change in interfacial free energy,W, of W 5t. From thechange in interfacial free energy for heightsfrom t 1 mm to 100 nm (Fig. 4B), weconclude that the energetics for self-assemblyare favorable for objects with t as smallas 100 nm. For the two-dimensional selfassemblyof spheres, the radius at whichW/kT 1 (where kT is the thermal energy)has been calculated to be on the orderof 1 to 10 m (15, 16). Self-assemblydriven by capillary forces between conformalsurfaces should therefore make possiblethe assembly of much smaller objectsthan would be possible with spheres; theability to control the shapes and interfacialproperties of these objects makes itpossible to design the geometries of theresulting arrays.Four factors contribute to the success ofthis strategy for the directed self-assembly ofsmall objects. First, the aggregates are energeticallymore stable than the individualdissociated objects or disordered aggregates.Second, formation of the aggregates is reversiblewhen the system is agitated: formationand dissociation of the aggregates compete.The aggregates are therefore able toreach the energetically most stable form.Third, the hydrophobic sides are attractedto one another over large distances (abouttwo to three times the dimension of theheight), leading to relatively rapid assembly.Fourth, even when two hydrophobicsides are in close proximity, they can movelaterally from side to side, lubricated by theintervening film of C 10F 18, and can thusmaximize the amount of hydrophobic areain contact.REFERENCES AND NOTES___________________________1. There are few methods for assembling arrays ofsmall objects. See (2, 4, 5); J. K. Tu, J. J. Talghader,M. A. Hadley, J. S. Smith, Electron. Lett. 31, 1448(1995); A. Ashkin and J. M. Dziedzic, Science 235,1517 (1987); K. Svoboda and S. M. Block, Annu.Rev. Biophys. Biomol. Struct. 23, 247 (1994).2. A. W. Simpson and P. H. Hodkinson, Nature 237,320 (1972).3. S. T. Schober, J. Friedrich, A. Altmann, J. Appl.Phys. 71, 2206 (1992).4. M. Yamaki, J. Higo, K. Nagayama, Langmuir 11,2975 (1995).5. P. A. Kralchevsky and K. Nagayama, ibid. 10, 23(1994).6. PDMS is easy to fabricate and its solid-liquid interfacialfree energy can be readily controlled by oxidation tomake the surfaces hydrophilic or hydrophobic. Weplaced the PDMS structure into a plasma cleaner for 5min under O 2at a pressure of 0.20 torr to oxidize thePDMS. The oxidation of PDMS is believed to result ina surface that comprises SiOH groups. The contactangle of water on oxidized PDMS is less than 15°.7. M. J. Owen, J. Coatings Technol. 53, 49 (1981).8. W. A. Zisman, in Symposium on Adhesion and Cohesion,P. Weiss, Ed. (Elsevier, New York, 1962), p.176.9. D. W. Fakes, M. C. Davies, A. Browns, J. M. Newton,Surf. Interface Anal. 13, 233 (1988).10. The perfluorodecalin wet the unoxidized PDMS andformed a meniscus; the water wet the higher energysurface of oxidized PDMS. The capillary forces actingat the oxidized surfaces were very weakly attractivecompared to those of the hydrophobic surfaces,because the PDMS (density 1.05 g ml 1 ) does notsink far enough into the perfluorodecalin (density 1.91 g ml 1 ) to generate a meniscus with significantcurvature at the hydrophilic interfaces. Other fluorinatedalkanes with properties similar to those of perfluorodecalinwere used with equal success.11. C. D. Dashkin, P. A. Kralchevsky, W. N. Paunov, H.Yoshimura, K. Nagayama, Langmuir 12, 641 (1996).12. The evidence that a thin layer of C 10F 18remainedbetween the PDMS solids at their closest contact isindirect: When two flat pieces of PDMS come incontact in water with no C 10F 18present, they stick toeach other strongly, and this process is effectivelyirreversible. If we add C 10F 18to the water after thePDMS solids have come into contact, they remainstuck to one another.13. H. T. Davis, Statistical Mechanics of Phases, Interfacesand Thin Films: Advances in Interfacial Engineering( VCH, New York, 1996).14. In performing these calculations, we used values of 0.05 J m 2 and 900 kg m 3 for the C 10F 18-H 2O interface.15. P. A. Kralchevsky, V. N. Paunov, N. D. Denkov, I. B.Ivanov, K. Nagayama, J. Colloid Interface Sci. 155,420 (1993).16. V. N. Paunov, P. A. Kralchevsky, N. D. Denkov, K.Nagayama, ibid. 157, 100 (1993).17. We thank M. Mammen for helpful comments andsuggestions. Funded by NSF grant CHE-9122331,the Office of Naval Research, and the Defense AdvancedResearch Projects Agency. N.B. was supportedby a predoctoral fellowship from the Departmentof Defense. A.T. thanks the Deutsche Forschungsgemeinschaftfor a research grant.4 November 1996; accepted 24 February 1997Permo-Triassic Boundary Superanoxia andStratified Superocean: Records fromLost Deep SeaYukio IsozakiPelagic cherts of Japan and British Columbia, Canada, recorded a long-term and worldwidedeep-sea anoxic (oxygen-depleted) event across the Permo-Triassic (or Paleozoicand Mesozoic) boundary (251 2 million years ago). The symmetry in lithostratigraphyand redox condition of the boundary sections suggest that the superocean Panthalassabecame totally stratified for nearly 20 million years across the boundary. The timing ofonset, climax, and termination of the oceanic stratification correspond to global bioticevents including the end-Guadalupian decline, the end-Permian extinction, and mid-Triassic recovery.The greatest mass extinction in the Phanerozoicoccurred at the timing of thePermo-Triassic (P-T) boundary; many hypothesesfor the extinction have focused onchanges in the ocean, including developmentof overturn of an anoxic ocean (1, 2).One problem has been that most records ofthe boundary are in shallow-water sedimentaryrocks that formed around the supercontinentPangea. Recently, however, P-Tboundary sections were discovered in deepseacherts that crop out extensively in theJurassic accretionary complex in southwestJapan (3, 4). The cherts represent ancientpelagic sediments primarily deposited in amid-oceanic deep sea of the superoceanPanthalassa and accreted onto the SouthChina (Yangtze) continental margin in theDepartment of Earth and Planetary Sciences, Tokyo Instituteof Technology, O-okayama, Meguro, Tokyo 152,Japan. E-mail: yisozaki@geo.titech.ac.jpMiddle Jurassic (5). The Panthalassa superoceanoccupied nearly 70% of Earth’s surfacein the Late Permian (6). Rocks nearthe P-T boundary are reduced, and arethought to have been deposited in an anoxicenvironment (3, 4, 7, 8). I refer tothese rocks as the P-T boundary unit(PTBU; Fig. 1). Across the PTBU, Paleozoicradiolarians (planktonic protozoans)are completely replaced by distinct Mesozoictypes. Similar rocks have also recentlybeen found in British Columbia, Canada. Iused the sections from Japan and BritishColumbia to evaluate Panthalassa oceandynamics across the P-T boundary.In the Japanese sections, Early to earlyLate Permian and Middle to Late Triassiccherts are composed mainly of siliceous radiolariantests (95% by weight) and aremostly brick red in color. X-ray diffractionand 57 Fe Mössbauer spectroscopy demon-http://www.sciencemag.org SCIENCE VOL. 276 11 APRIL 1997 235