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Modulated Pattern Formation of Phospholipid Monolayers on Curved Surfaces Jing Yuan and Thomas M. Fischer* Department of Chemistry and Biochemistry, Florida State UniVersity, Tallahassee, Florida 32306 ReceiVed September 13, 2006. In Final Form: January 9, 2007 Langmuir-Blodgett transfer of a dipalmitoylphosphatidylcholine monolayer onto macroscopically curved mica surfaces results in microscopic patterns of the transferred monolayer that differ from those of films transferred onto a flat mica substrate. On curved surfaces a modulated horizontal striped pattern evolves that has a zigzag boundary at the liquid condensed front of the stripe and a continuous straight boundary at the liquid condensed rear. We propose that the sensitivity of the pattern to the macroscopic curvature of the sample is due to a flow-controlled hydrodynamic instability caused by the subphase flow close to the three-phase contact line.
Pattern formation on a solid surface through spatiotemporal self-organization has attracted interest in science and technology due to potential applications ranging from electronic/optical devices to biosensors,1 surface wettability,2 and chromatography.3 Mesoscopic patterns generated by drying solutions or suspensions on solid substrates result in typical ring patterns.4 Evaporationassisted patterning develops near three-phase contact lines and can be controlled via the dewetting conditions. Spin-coated block copolymer films spontaneously form wrinkled stripes on solid substrates.5-7 Shimomura et al.8 reported regular striped and ladderlike patterns by a controlled casting of a dilute polymer solution, where the meniscus was linearly receding during the solvent evaporation. Without requiring any template direction, self-organization makes it possible to manufacture mesoscopic patterned surfaces both efficiently and economically.9,10 Striped and grid patterns are spontaneously generated by a LangmuirBlodgett (LB) transfer of the liquid expanded (LE) phase of a dipalmitoylphosphatidylcholine (DPPC) monolayer from a pure water surface onto a solid substrate.2,11,12 The periodic stripes and channels are found to align either parallel (horizontal stripes) or perpendicular (vertical stripes) to the three-phase contact line. Similar striped patterns were reported in some binary systems such as DPPC/4-(dicyanomethylene)-2-methyl-6-[4-(dimethylamino)styryl]-4H-pyran (DCM),13 DPPC/1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphocholine (DOEPC),14 DPPC/L-Rdilauroylphosphatidylcholine (DLPC),15,16 and lipid/lipopoly* To whom correspondence may be addressed. E-mail: tfischer@ chem.fsu.edu. (1) Carroll, S. B.; Gates, J.; Keys, D. N.; Paddock, S. W.; Panganiban, G. E.; Selegue, J. E.; Williams, J. A. Science 1994, 265, 109. (2) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (3) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (4) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (5) Harrison, C.; Adamson, D. H.; Cheng, Z. D.; Sebastian, J. M.; Sethuraman, S.; Huse, D. A.; Register, R. A.; Chaikin, P. M. Science 2000, 290, 1558. (6) Niu, S.; Saraf, R. F. Macromolecules 2003, 36, 2428. (7) Xuan, Y.; Peng, J.; Cui, L.; Wang, H. F.; Li, B. Y.; Han, Y. C. Macromolecules 2004, 37, 7301. (8) Yabu, H.; Shimomura, M. AdV. Funct. Mater. 2005, 15 (4), 575. (9) Yoo, P. J.; Suh, K. Y.; Park, S. Y.; Lee, H. H. AdV. Mater. 2002, 14, 1383. (10) Lenhert, S.; Zhang, L.; Mueller, J.; Wiesmann, H. P.; Erker, G.; Fuchs, H.; Chi, L. F. AdV. Mater. 2004, 16 (7), 619. (11) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25 (3), 211. (12) Lenhert, S.; Gleiche, M.; Fuchs, H.; Chi, L. F. ChemPhysChem 2005, 6, 2495. (13) Chen, X. D.; Hirtz, M.; Fuchs, H.; Chi, L. F. AdV. Mater. 2005, 17, 2881. (14) Chen, X. D.; Lu, N.; Zhang, H.; Hirtz, M.; Wu, L. X.; Fuchs, H.; Chi, L. F. J. Phys. Chem. B. 2006, 110 (15), 8039.
mer,17 and they vary with the emersion velocity and surface pressure.12,14 Herein, we show that the macroscopically curved three-phase contact line influences the microscale dynamic phenomena, giving rise to a novel zigzag deposition front. We propose that these patterns are flow rather than diffusion controlled and that the sample curvature alters the hydrodynamic flow, which in turn causes the changes in pattern. DPPC (>99%, Avanti Polar Lipids, Inc.) was dissolved in chloroform (HPLC, Fisher Scientific) to 1 mM concentration and spread onto pure water (18.2 MΩ, Barnstead) on a Nima 312D Langmuir trough (Nima Technology, England). Freshly cleaved mica slides (thickness 100 µm) were mounted onto a concave Teflon holder (Figure 1) to transfer the lipid monolayers. A vertical LB transfer of a single monolayer was performed by emersion of the holder after compression of the monolayer to a target surface pressure. The transferred LB films were imaged in tapping mode with an atomic force microscope (Digital Instruments, Nanoscope IV, Dimension 3100, Santa Barbara, CA) equipped with silicon cantilevers (RTESP, Veeco Instruments, CA) with resonant frequencies of 253-306 kHz and spring constants of 20-80 N/m. The effect of the curvature of the substrate on the microscale pattern is obvious from a morphologic comparison between a sample prepared on a flat mica surface and another one prepared on a curved mica surface. The topographic AFM images are shown in Figure 2. In both cases, the surface pressure is 2 mN/m and the emersion velocity is 20 mm/min. Consistent with previous literature reports,14 transferring the DPPC monolayer onto a flat substrate gave rise to a grid pattern with crossed periodic channels, which are either parallel or perpendicular to the three-phase contact line, as shown in Figure 2a. The horizontal stripes have a uniform interstripe distance of λz ) 2.8 µm, and the periodicity of the stripes normal to the contact line is about λx ) 8-12 µm. Although the height of the stripes (1.4 nm) is small compared to the length of a DPPC molecule, fluorescent doping13,14 demonstrated that the stripes (bright areas in Figure 2) are in an ordered condensed phase as dye molecules can only be observed in the channels. Therefore, there must be a phase transition from a homogeneous loosely packed LE phase to an ordered condensed phase during the monolayer transfer, and the grid pattern formation can be (15) Moraille, P.; Badia, A. Langmuir 2002, 18, 4414. (16) Moraille, P.; Badia, A. Langmuir 2003, 19, 8041. (17) Howland, M. C.; Johal, M. S.; Parikh, A. N. Langmuir 2005, 21, 10468.
10.1021/la062688k CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
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Figure 1. Curved (R ) 30 mm) Telflon holder used for LB transfer onto a curved mica slide.
Figure 3. Pattern development along the z-direction for a transferred DPPC LB film on a curved mica surface for different distances z from the film edge, scale bar 5 µm.
Figure 2. AFM images of transferred DPPC LB films on a flat mica (a) and a curved mica (b) surface at a surface pressure of 2 mN/m with an emersion speed of 20 mm/min, scale bar 5 µm.
comprehended as a substrate-mediated condensation of a DPPC monolayer, where the periodic oscillation of the meniscus and the fingering instability coexist simultaneously.12,14 On curved mica substrates at the same surface pressure and using the same emersion velocity, new patterns were observed, typically shown as modulated horizontal stripes with a zigzag boundary at the liquid condensed (LC) front of the stripe and a continuous straight boundary at the liquid condensed rear (Figure 2b). The average distance between neighboring stripes is λz ) 12 µm, much larger than the periodicity of horizontal stripes on the flat mica surface, and the wavelength of the zigzag front is about 4 µm. A statistical analysis of the angle θ of the zigzag front shows an average angle of θ ) 78° with a wide angular deviation of ∆θ ) (25°. Inspection of the morphology over the entire surface of a curved mica slide further showed that the zigzag front and the stripe pattern change with the distance z of the film from the film edge. Figure 3 shows the morphologic development along the emersion direction. In the very beginning region of the transferred monolayer (Figure 3a, z , 100 µm), the pattern is composed of a lot of dispersed dots hundreds of nanometers in diameter and several triangular domains. The triangles are aligned isosceles with their base parallel to the three-phase contact line and the apex pointing toward the film edge. All the bases of the neighboring triangles in the horizontal direction always fall into the same lines, indicating that the pattern formation is under the control of the meniscus oscillation. In this stage, some triangles are connected in a way that the basal vertex of one triangle simultaneously serves as the apex of another triangle. Imaging along the emersion direction, Figure 3b shows that the triangles are getting bigger and the neighboring triangles start to combine as we move further away from the film edge. Further away from the film edge (Figure 3c, z ≈ 3 mm) the triangles merge to form a characteristic zigzag horizontal striped pattern. All the stripes are well-aligned parallel to the three-phase contact line, between which dots and small triangles can still be observed. Comparing
Figure 4. AFM images of transferred DPPC LB films on a flat mica (a) and a curved mica (b) surface at a surface pressure of 6 mN/m and an emersion speed of 20 mm/min, scale bar 5 µm.
parts b and c with part a of Figure 3, it is obvious that the base of the triangles increased while the density of the dots decreased. If we stop the emersion instead of lifting the whole sample off the water surface, patterns like Figure 3d can be observed, representing the boundary morphology of the deposited monolayer. We ascribe the formation of such a homogeneous monolayer to an equilibrium deposition of DPPC molecules onto the mica surface. As another boundary of the striped pattern, the zigzag front also appeared as a start edge of the homogeneous monolayer, further confirming that this zigzag pattern survives on the whole region of the sample surface. The difference in the liquid condensed front and rear shows that zigzag fingering only occurs upon condensation of the monolayer on the substrate and not during the period of the oscillating meniscus where the deposited monolayer is diluted. Similarly, we investigate the curvature contribution at a higher surface pressure of 6 mN/m. On a flat mica surface, vertical stripes were observed with an average width of 4-6 µm (Figure 4a). The formation of a vertical striped pattern has been explained as a consequence of a fingering instability, which results from a temperature- or a surface-tension-gradient-induced convectional flow.18 Comparing part b with part a of Figure 4, although a vertical striped pattern with a periodicity similar to that of the pattern in Figure 4a can also be distinguished on a curved mica surface in Figure 4b, the stripes in Figure 4b are obviously not continuous but composed of closely packed segments with a periodicity of about 500 nm along the emersion direction. In contrast to the result at low surface pressure, the periodicity here is much smaller than that of horizontal stripes on a flat mica (18) Diez, J. A.; Kondic, L. Phys. ReV. Lett. 2001, 86, 632.
Pattern Formation of Monolayers on CurVed Surfaces
surface, which might be correlated to a faster meniscus oscillation during LB transfer. The above results indicate that the curvature plays an important role in the pattern formation during the LB transfer. A hydrodynamic mechanism proposed by Bruinsma et al.19 and confirmed by Muruganathan and Fischer20 based on Marangoni flow describes the growth instabilities in supersaturated Langmuir monolayers on the air/water interface that are forced to undergo an LE to LC transition. Bruinsma et al.19 predict a Marangoni instability of the LE/LC phase boundary which is intrinsic to Langmuir monolayers, and it is not controlled by the expulsion of chemical impurities from the liquid condensed phase. In contrast to the Mullins Sekerka instability in three dimensions21 the hydrodynamic transport of the insoluble surfactants overwhelms the passive diffusion and provides a mechanism for fingering instabilities. The differences between the Mullins Sekerka instability in three dimensions and Bruinsma et al.’s instability is the presence of significant hydrodynamic flow in the subphase close to the phase boundary such that the monolayer motion is dominated by the advection from the subphase, not diffusion. If we argue that this is true also for the LB transfer (19) Bruinsma, R.; Rondelez, F.; Levine, A. Eur. Phys. J. E 2001, 6, 191. (20) Muruganathan R. M.; Fischer, Th. M. J. Phys. Chem. B 2006, 110, 22979. (21) Mullins, W. W.; Sekerka, R. F. J. Appl. Phys. 1963, 34, 323.
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onto a solid substrate, we might understand the effect of the curvature of the sample on the pattern. The radius of curvature of the sample is considerably larger than any diffusive or capillary length scale and therefore will not affect any diffusive process or oscillating meniscus. The hydrodynamic flow of the monolayer is a long-range effect that is sensitive to the geometry. If the instability is flow controlled as predicted for the solidification of monolayers on the air/water interface by Bruinsma et al., we would expect the pattern to be sensitive to the flow and thus to the macroscopic geometry. In summary, we have demonstrated the possibility of modulating the dewetting pattern of a DPPC LB film by a macroscopic curvature of the substrate. At low surface pressure, a distorted horizontal striped pattern with a zigzag edge at one side and a straight edge parallel to the three-phase contact line at the other side was observed, while at high surface pressure the pattern was shown as discontinuous vertical stripes. In both cases, the patterns differ from those observed on flat mica slides. Our observation provides a new strategy to design and construct complex patterns on a solid surface. Acknowledgment. We thank Dr. Xiaodong Chen for helpful discussion and Dr. Jad Jaber for technical support. LA062688K
