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Direct Visualization of Flow-Induced Anisotropy in a Fatty Acid Monolayer Matthew C. Friedenberg,† Gerald G. Fuller,* Curtis W. Frank, and Channing R. Robertson Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received July 13, 1995. In Final Form: December 11, 1995X Brewster angle microscopy is used to directly visualize the influence of an applied extensional flow on the domain structure and molecular orientation of a docosanoic acid monolayer at the air-water interface. At a surface pressure of 12 mN/m and a subphase temperature of 15 °C (L2 phase), extensional flow causes domain elongation parallel to the extension axis. A frequency domain analysis of the Brewster angle images indicates that the domains undergo an affine deformation in response to flow. AT 20 mN/m (L2′ phase), the flow modifies not only the domain structure of the monolayer but also the azimuthal orientation of the fatty acid molecules. This flow-alignment process is strain-rate dependent. Thus, flow can couple to the monolayer order over a variety of length scales.
Introduction Monolayer and multilayer films form a class of materials known as ultrathin films, having thicknesses on the nanometer scale. Although the technology to prepare such films has been in existence since the 1930s,1 it is only recently that many proposed applications of these molecular assemblies have become feasible. Several industrially important areas are discussed in a review by Swalen et al.2 and include thin film optical devices, sensors, displays, patternable materials, and functionalized surfaces. In addition to the renewed technological importance of these films, they also represent unique model system with which to perform fundamental research on molecular organization in constrained geometries. The Langmuir-Blodgett (LB) technique allows fabrication of ordered molecular assemblies through the deposition of monolayer films from the air-water interface to solid substrates. Many applications of ultrathin films require the fabrication of films with a high degree of orientational order, and it has become apparent over the years that the processing of monolayers at the air-water interface can influence the molecular organization in transferred films. When monolayers are transferred to solid substrates by the standard technique of vertical deposition, the resultant films frequently contain anisotropy in the plane of the substrate. This anisotropy is thought to arise from the influence of velocity gradients in the floating monolayer during vertical deposition.3,4 Alternate methods of film deposition, such as horizontal deposition5,6 and deposition in the moving-wall trough,7 are not found to induce anisotropy in the transferred layers. The anisotropy in vertically-deposited films can appear on several different length scales. On the scale of the * To whom all correspondence should be addressed (E-mail:
[email protected], TEL (415)723-9243, FAX (415)725-7294). † Present address: IBM Almadel Research Center, 650 Harry Road K64/803, San Jose, CA 95120. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848. Blodgett, K. A. J. Am. Chem. Soc. 1935, 57, 1007. Blodgett, K. A. Phys. Rev. 1937, 51, 964. (2) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (3) Daniel, M. F.; Hart, J. T. T. J. Mol. Electron. 1985, 1, 97. (4) Schwiegk, S.; Vahlenkamp, T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25, 2513.
domain structure (tens of microns), these films can exhibit domain elongation parallel to the dipping direction.3,8,9 On the molecular scale, films of both small and large molecules have been produced in which the molecules are aligned, on average, parallel to the dipping direction.4,5,10-17 Chollet and Messier16 found that deposition-induced alignment of docosanoic acid would only occur if the film was transferred from a phase in which the molecules were tilted from the surface normal. In contrast, studies of fatty acid salts by Zasadzinski and co-workers18 indicate that the strucure of these transferred films often bears no resemblance to that present at the air-water interface and may instead depend quite strongly on the structure of the substrate. Clearly, there are many factors that control structure and orientation in Langmuir-Blodgett films. In situ techniques are required to probe the monolayer at the air-water interface, since any anisotropy that develops here may be imprinted on the substrate after transfer. Fluorescence microscopy and Brewster angle microscopy (BAM) are well-suited for this purpose, as these techniques allow real-time imaging of monolayer films at the air(5) Wijekoon, W. M. K. P.; Karna, S. P.; Talapatra, G. B.; Prasad, P. N. J. Opt. Soc. Am. B 1993, 2, 213. (6) Yang, X. M.; Gu, N.; Lu, Z. H.; Wei, Y. Phys. Lett. A 1993, 183, 111. Nishikata, Y.; Komatsu, K.; Kakimoto, M.; Imai, Y. Thin Solid Films 1992, 210/211, 29. (7) Nishikata, Y.; Komatsu, K.; Kakimoto, M.; Imai, Y. Thin Solid Films 1992, 210/211, 29. (8) Peterson, I. R. Thin Solid Films 1984, 116, 357. (9) Krag, P.; Petrov, A. G.; Sackman, E.; Wunderlich, A. J. Mol. Electron. 1990, 6, 21. (10) Decher, G.; Klinkhammer, F.; Peterson, I. R.; Steitz, R. Thin Solid Films 1989, 178, 445. (11) Minari, N.; Ikegama, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222. (12) Duda, G.; Schouten, A. J.; Arndt, T.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988, 159, 332. (13) Embs, F. W.; Wegner, G.; Neher, D.; Albouy, P.; Miller, R. D.; Willson, C. G.; Schrepp, W. Macromolecules 1991, 24, 5068. Kani, R.; Yoshida, H.; Nakano, Y.; Murai, S.; Mori, Y.; Kawata, Y.; Hayase, S. Langmuir 1993, 9, 3045. Kani, R.; Nakano, Y.; Majima, Y.; Hayase, S.; Yuan, C. H.; West, R. Macromolecules 1994, 27, 1911. (14) Sorita, T.; Miyake, S.; Fujioka, H.; Nakajima, H. Jpn. J. Appl. Phys. 1991, 30, 131. (15) Orthmann, E.; Wegner, G. Angew. Chem., Int. Ed. Engl. 1986, 25, 1105. Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357. (16) Chollet, P. A.; Messier, J. T. Chem. Phys. 1982, 73, 235. (17) Koyama, Y.; Yanagishita, M.; Toda, S.; Matsuo, T. J. Colloid Interface Sci. 1977,61, 438. Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. Blaudez, D.; Buffeteau, T.; Desbat, B.; Escafre, N.; Turlet, J. M. Thin Solid Films 1994, 243, 559. (18) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726.
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water interface. The ability afforded by these techniques to visualize dynamic phenomena in monolayers has provided important insights into the phase behavior and collapse behavior of these ultrathin films.19-21 For example, McConnell and co-workers used fluorescence microscopy to observe quantized shape transitions in lipid domains23 and demonstrated the interplay between line tension, electrostatics, and hydrodynamics in a model that predicts the dynamics of these shape distortions.24 Although fluorescence microscopy requires the presence of an intrinsic or extrinsic fluorophore for its contrast, BAM uses the intrinsic optical properties of the monolayer. The Brewster angle technique is based on the fact that the presence of a monolayer at the air-water interface modifies the local reflectivity of the interface. By imaging a laser beam reflected off the interface at the Brewster angle, where contrast is highest, one can observe structures in the monolayer due to their differential reflectivities. This makes it suitable for a wide variety of monolayer classes, including fatty acids and alcohols, phospholipids, liquid crystals, and polymers. Since the velocity gradients induced by conventional vertical deposition are a spatially inhomogeneous, complex mix of shear and extension, the use of a model flow field allows investigation under well-defined conditions. Schwartz et al.25 have directly visualized the flow of tetradecanoic and pentadecanoic acid domains through a channel using fluorescence microscopy. Benvegnu and McConnell26 have also used channel flow to investigate domain elongation and relaxation in phase-separated monolayers containing a binary mixture of a phospholipid and cholesterol. In a recent publication, we applied polarized optical techniques to study the influence of flow on molecular orientation in Langmuir monolayers containing a hairy-rod polymer.27 A flow device known as a four-roll mill was used to impose an extensional flow in the plane of the monolayer. Our results indicated that the applied flow caused nearly complete alignment of the polymer molecules along the extension axis. In the present study, we use Brewster angle microscopy to directly visualize the influence of an applied extensional flow on the structure and orientation of the docosanoic acid at the air-water interface. We have found that flow can influence the organization of this fatty acid monolayer on two very different length scales: that of the domain structure and that of the molecular orientation. Moreover, the phase state of the monolayer is seen to be a critical determinant of the resultant flow response. Experimental Methods Monolayer Preparation. Monolayers were prepared at the air-water interface using a KSV 5000 System III symmetriccompression Langmuir-Blodgett trough (KSV Instruments, Helsinki). Docosanoic acid (Sigma) was spread at the water surface from a 1.0 mg/mL spreading solution in chloroform (Baker HPLC). The subphase temperature was maintained at 15 °C to minimize free convection in the monolayer. The film was (19) McConnell, H. M. Ann. Rev. Phys. Chem. 1991, 42, 171. (20) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (21) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (22) Ho¨nig, D.; Overbeck, G. A.; Mo¨bius, D. Adv. Mater. 1992, 4, 419. (23) Lee, K. Y. C.; McConnell, H. M. J. Phys. Chem. 1993, 97, 9532. (24) Stone, H. A.; McConnell, H. M. Proc. R. Soc. London, Ser. A 1995, 448, 97. (25) Schwartz, D. K.; Knobler, C. M.; Bruinsma, R. Phys. Rev. Lett. 1994, 73, 2841. (26) Benvegnu, D. J.; McConnell, H. M. J. Phys. Chem. 1992, 96, 6820. (27) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Submitted for publication in Macromolecules.
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Figure 1. Pressure-area diagram of docosanoic acid at 15 °C on a pure water subphase, pH 5.6. compressed to the target surface pressure at a rate of 10 mm/ min and was held at this pressure for 15 min prior to the inception of flow. Four-Roll Mill. A four-roll mill, described previously,27 was used to generate planar extensional flow in the monolayer. This flow device was originally invented by G. I. Taylor to study droplet deformation in emulsions.28 The mill consists of four cylindrical rollers centered on the corners of a square. The rollers are partially immersed in our Langmuir trough. If adjacent rollers are rotated at equal speeds and in opposite directions, a pure straining (extensional) deformation is produced in the monolayer that is homogeneous over a considerable portion of the area between the rollers. This deformation can be characterized by a uniform strain rate, ˘ , which is directly proportional to the angular velocity of the rollers. The axis along which material enters the mill is the compression axis, and the axis along which it exits is the extension axis. By tracking the motion of sulfur particles placed on the compressed monolayer, we have confirmed the presence of hyperbolic streamlines, a characteristic of this type of flow.27 Another important feature of this flow is the existence of a stagnation point at the center of the flow field, where residence times are long, and therefore the material has the greatest opportunity to achieve a steady-state response. Brewster Angle Microscope. Our Brewster angle microscope21 is based on the design of Ho¨nig, Overbeck, and Mo¨bius.22 The microscope was positioned so the reflection occurred at the stagnation point of the four-roll mill. The microscope was equipped with an analyzer in front of the CCD camera to allow imaging of domain anisotropy in the monolayer. This analyzer was set at 60° with respect to the direction of p-polarization to provide good domain contrast. Image-Processing and Analysis. The BAM images of the flowing monolayer were digitized from the live video signal to obtain the best quality images for subsequent analysis. The video was also recorded on videotape for archival purposes using an SVHS videocassette recorder (JVC BRS-601). Frames were captured by a PC-based frame grabber (Data Translation DT38518). Image analysis was carried out on an Apple Power Macintosh computer using ImageFFT and NIH Image software packages (freely available by anonymous FTP from zippy.nimh.nih.gov).
Results Visualization of Docosanoic Acid Monolayers by BAM. Like many long-chain fatty acids, docosanoic acid can form a rich variety of mesophases and crystalline phases at the air-water interface as a function of temperature and surface pressure.29 This polymorphism is evident in the pressure-area isotherm for docosanoic acid at 15 °C (Figure 1), which exhibits a kink at 15 mN/ m. From X-ray diffraction experiments, this kink is associated with a phase transition between two condensed phases (L2 and L2′).29 In both phases, the molecules are packed into a distorted hexagonal lattice and are tilted at some angle with respect to the surface normal. These phases differ, however, in the direction of this tilt with respect to the unit cell. In the low-pressure L2 phase, the (28) Taylor, G. I. Proc. R. Soc. London, Ser. A 1934, 146, 501. (29) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092.
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Figure 2. Schematic representation of lattice structures for docosanoic acid in the (a) L2 and (b) L2′ phases. This schematic drawing represents the projection of the molecule onto the airwater interface.
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Figure 4. Orientation of four-roll mill and Brewster angle microscope for studies of domain deformation in docosanoic acid monolayers. The gray ellipses indicate the locations where the rollers of the mill penetrate the air-water interface. The direction of rotation of the rollers is indicated by the arc-shaped arrows. The dotted line indicates the orientation of the extension axis created by the mill. The thick line represents the propagation direction of the BAM laser beam (unfilled arrowhead) and the polarization direction of the incident beam (filled arrowhead).
Figure 3. Brewster angle micrograph of a docosanoic acid monolayer: 15 °C, 12 mN/m (L2 phase). The angle of the BAM analyzer is 60°. Note that the spatial information in this image is compressed in one dimension by a factor of 0.6 because of the 53.1° viewing angle of the microscope.
molecules tilt toward their nearest neighbor (Figure 2a), while in the high-pressure L2′ phase, the molecules tilt toward the next nearest neighbor (Figure 2b).29 A Brewster angle micrograph of a docosanoic acid monolayer in the L2 phase (12 mN/m) is shown in Figure 3. The monolayer appears highly textured, with a domain length scale on the order of 100 µm. This texture, present in both the L2 and L2′ phases, is similar to that found in stearic acid monolayers.30,31 The domain contrast can be inverted by rotating the analyzer from 60° to 120°, indicating the presence of anisotropy in the optical properties of the monolayer. By analogy with stearic acid monolayers, this texture arises from spatial variations in the tilt orientation of the fatty acid.30,31 Since the probing beam samples only a projection of the refractive index tensor of the monolayer, the reflectivity of an individual domain will depend on the relative orientation between the molecular tilt direction within a domain and the plane of incidence of the microscope. Flow Behavior in the L2 Phase (12 mN/m). Here, the four-roll mill is used to apply an extensional flow to the docosanoic acid monolayer. We specified the extension axis of the four-roll mill to be orthogonal to the plane of incidence defined by the Brewster angle microscope (Figure 4), but the results are equivalent when the flow is parallel to the plane of incidence. Extensional flow was produced by rotating the rollers of the mill at 0.02 (30) Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1992, 210/211, 64. (31) Hosoi, K.; Ishikawa, T.; Tomioka, A.; Miyano, K. Jpn. J. Appl. Phys. 1993, 32, L135.
Figure 5. Flow-induced domain deformation in docosanoic acid monolayers (L2 phase): π ) 12 mN/m, T ) 15 °C. The roller velocity of the mill was 0.02 revolutions/s. The extension axis is along the vertical direction.
revolutions/s for 40 s. Brewster angle images were acquired at 12 s intervals during flow and are presented in Figure 5. Before flow is initiated, the monolayer contains a distribution of domain shapes that is nearly isotropic (Figure 5, t ) 0 s). After flow begins, the domains elongate parallel to the extension axis, which is along the vertical direction in this figure. The extent of deformation becomes greater as the flow progresses. Ultimately, the domain width becomes narrower than the resolution of the Brewster angle microscope (approximately 10 µm), so no further changes are observed with time. By performing a quantitative analysis of the observed domain deformation, we can better characterize the dynamics of this flow-induced structural anisotropy. Such an analysis can be conveniently performed in the frequency domain using two-dimensional fast Fourier transform (FFT) techniques. The power spectrum of the real-space Brewster angle image, which represents the magnitude of the real and imaginary components of the FFT, is analogous to the structure factor in scattering measurements.32 Figure 6 shows the power spectra of the four BAM images from Figure 5.33 At t ) 0 s, the FET image exhibits (32) Moses, E.; Kume, t.; Hashimoto, T. Phys. Rev. Lett. 1994, 72, 2037.
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Figure 6. Two-dimensional FFT of the Brewster angle images in Figure 5.
a gray, circular feature at the center of the image. The isotropic nature of this pattern signifies that the characteristic length scale of the domain structure is essentially the same in all directions. After extensional flow commences, this feature becomes elliptical, narrowing in the direction parallel to the extension axis and elongating perpendicular to the extension axis. Thus, the spatial frequencies become smaller (domains become elongated) parallel to the flow direction, the frequencies become higher (domains become compressed) perpendicular to the flow direction. In the images at 24 and 36 s, the elliptical pattern is oriented at an angle of a few degrees with respect to the horizontal. This arises from a small angular misalignment between the flow axis and the plane of incidence of the Brewster angle microscope. The power spectra can be further characterized in terms of a major and minor axis, calculated from the shape of isointensity contours generated at fixed pixel intensity. For this calculation, one must select a large enough threshold intensity to avoid contamination of the aspect ratio from the background noise. For an affine, or volume-preserving planar extensional deformation (which is also area-preserving in this two-dimensional system), it can be shown that the major and minor axes, Rx and Ry, should evolve over time as
ln(Rx) ) ln(Rx0) + ˘ t
(1)
ln(Ry) ) ln(Ry0) - ˘ t
(2)
where ˘ is the rate of deformation, or strain rate, and Rx0 and Ry0 are the true major and minor axes, respectively, at t ) 0. In Figure 7, ln(Rx) and ln(Ry) are graphed versus time for the initial 24 s of the flow deformation. Both graphs have the expected linear form, and the slopes of the best-fit lines provide an estimate of the strain rate. For the major-axis data, the strain rate is calculated to be 0.021 s-1, and for the minor-axis data, it is calculated to be 0.017 s-1. These values are in good agreement, indicating that the deformation of docosanoic acid domains induced by planar extensional flow is affine. Benvegnu and McConnell25 have calculated the line tension between phase-separated phospholipid domains (33) Before calculating the FFT, a gaussian mask with a transition width of 30% was applied to the image. This mask eliminated the high-frequency artifacts that would invariably be introduced from the nonperiodic nature of the real image. The power spectrum was then calculated and scaled logarithmically so that the resultant intensities ranged between 0 and 255.
Figure 7. Flow dynamics of the (a) major and (b) minor axes calculated from the FFT of docosanoic acid images during an extensional flow deformation. The data are plotted according to the functional form of eqs 1 and 2. Markers represent the experimental data, and the solid lines represent the best linear fits to these data.
Figure 8. Two experimental configurations used in this study. (a) Extension axis of the four-roll mill is parallel to the plane of incidence defined by the Brewster angle microscope. (b) Extension axis is perpendicular to the plane of incidence.
and their surroundings from the shape relaxation that follows a shear deformation. For docosanoic acid monolayers in the L2 phase, we have observed no significant structural relaxation of the domain anisotropy following cessation of the applied extensional flow. Thus, the line tension between domains is extremely small. This is not surprising, since the domains are chemically indistinguishable. Flow Behavior in the L2′ Phase (20 mN/m). Domain deformation is also observed in the L2′ phase, but the nature of the flow response that we observe with the Brewster angle microscope depends quite significantly on the direction of the flow with respect to the plane of incidence. To illustrate this finding, we present results from an experiment in which extensional flow was generated in the monolayer for a fixed strain, corresponding to 0.1 revolutions of the mill. Two flow directions are compared, corresponding to parallel and perpendicular orientations of the extension axis with respect to the plane of incidence of the microscope (Figure 8). The influence of flow direction on the resulting images was investigated at both 12 (L2 phase) and 20 mN/m (L2′ phase). The Brewster angle micrographs obtained in the L2 phase are shown in Figure 9a. In the left-hand image, the extension axis is parallel to the plane of incidence, while in the right-hand image, it is perpendicular. At this small strain, the deformation of the domains is barely perceptible. Moreover, the domain contrast appears independent of the orientation of the extension axis with respect to the plane of incidence. The image contrast can be graphically displayed as a histogram (Figure 9b), where the x-axis represents the value of the pixel intensity and the y-axis represents the number of pixels with a particular
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Figure 9. L2 phase: (a) Brewster angle micrographs comparing extensional flow parallel (left image) and perpendicular (right image) to the plane of incidence. (b) Histogram of intensity values for these two images. The dashed curve represents the image on the left, and the solid curve corresponds to the image on the right.
intensity (frequency). The histogram was generated from the central 128 × 128 pixels in the image, since the Gaussian intensity profile of the laser beam leads to a loss of contrast toward the edges. The histograms clearly show that in the L2 phase, the intensity distribution is very broad and independent of orientation of the flow direction with respect to the plane of incidence. The marked asymmetry of the intensity distribution may be due to the Gaussian intensity profile of the laser beam, which causes darker pixels to be weighed more heavily. The large frequency observed at a pixel intensity of 255 results from saturation of the CCD camera. In the L2′ phase (20 mN/m), the behavior is qualitatively different (Figure 10). When the extension axis is parallel to the plane of incidence, the domain structure shows a broad distribution of intensities, similar to that observed in the L2 phase. The overall contrast is poorer in the L2′ phase, however, because of the smaller value of the tilt angle at this higher surface pressure.31 When the extension axis is perpendicular to the plane of the incidence, there is a significant loss of contrast in the image, and the domain structure is not so easily resolved. This loss of contrast is seen in the image histograms (Figure 10b), where the breadth of the intensity distribution decreases by a factor of 2 and its peak intensity doubles relative to the parallel orientation. It is evident that the coupling of flow to the monolayer is fundamentally different at 20 mN/m than it is at 12 mN/m. Moreover, this flow effect is phase dependent; it was not observed at either 12 or 14 mN/m, below the L2-L2′ transition at 15 mN/m, but was observed at 16, 20, and 25 mN/m, above the phase transition. Our observations also indicate that the loss of contrast at fixed strain increased markedly as the surface pressure was raised. We have also investigated the strain-rate dependence of this phenomenon. Five strain rates were investigated, from 0.02 to 0.2 s-1 (estimated using the calculated value of the strain rate from the domain deformation experiment). In Figure 11, intensity histograms are shown after
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Figure 10. L2′ phase: (a) Brewster angle micrographs comparing extensional flow parallel (left image) and perpendicular (right image) to the plane of incidence. (b) Histogram of intensity values for these two images. The dashed curve represents the image on the left, and the solid curve corresponds to the image on the right.
Figure 11. Strain-rate dependence of flow-induced molecular orientation in the L2′ phase of docosanoic acid.
fixed strain (9%) for a flow in which the extension axis is perpendicular to the plane of incidence. There is a strong dependence of the extent of contrast loss on strain rate. At 0.02 and 0.04 s-1, the histograms are indistinguishable from each other and from those taken for flow parallel to the plane of incidence. At 0.08 s-1, the peak position of the distribution shifts to slightly higher intensities. At 0.12 and 0.20 s-1, the width of the distribution narrows and the peak intensity increases. These data suggest the presence of a critical strain rate for flow effects to occur, but this point requires further investigation at higher strains. Although these results suggest the presence of a relaxation time scale on the order of the reciprocal of the strain rate, we note that the observed domain contrast persisted for at least several minutes after flow cessation (longer times have not yet been investigated). Discussion Although the L2′ phase exhibits only subtle differences in molecular packing with respect to the L2 phase, the nature of its flow response is clearly different. Because of its observed dependence on the relationship between
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the flow direction and the plane of incidence, the loss of contrast in the L2′ phase cannot be explained by a local change in surface pressure at the stagnation point of the flow. The only reasonable explanation for this phenomenon is that the docosanoic acid molecules become aligned in the presence of flow. To establish whether such a hypothesis is consistent with the observed images, our results were compared with recent calculations of the expected Brewster angle reflectivity for anisotropic monolayers.34 In that paper, Overbeck and co-workers investigated a hypothetical Langmuir monolayer composed of molecules tilted from the surface normal by 20°. The dielectric tensor of this molecule was assumed to be uniaxial in the molecular frame, with a refractive index of 1.54 in the plane parallel to the long axis of the molecule and refractive index of 1.48 in the plane orthogonal to this direction. These assumptions are reasonable first approximations for a docosanoic acid monolayer. The authors calculated the reflectivity as a function of the azimuthal orientation of the molecular tilt with respect to the plane of incidence, defined by the Brewster angle microscope. As expected, the reflectivity was found to be a strong function of the azimuthal orientation. For alignment parallel to the plane of incidence, Overbeck et al. found that the reflectivity was at its darkest or brightest, depending on whether the tilt was toward or away from the incident beam. For alignment perpendicular to the plane of incidence, the reflectivity possessed an intermediate value and was insensitive to whether the tilt was toward the left or right side of the plane of incidence. These predictions are now compared with the images obtained for flow in the L2′ phase: When the extension axis was parallel to the plane of incidence, we have found that the resulting BAM image had a broad intensity distribution, containing both light and dark domains. This is exactly what should be expected if the docosanoic acid molecules aligned parallel to the extension axis, since the p-polarized incident light can distinguish between molecules that tilt toward the incident beam and those that point away from the incident beam. When the extension axis is perpendicular to the plane of incidence, the resulting image had a narrow intensity distribution centered at an intermediate gray value. This is also what would be expected if the docosanoic acid molecules were aligning parallel to the flow direction, since in this case the (34) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 26.
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p-polarized incident light cannot distinguish between molecules tilting to the left or the right. Thus, the trends in the reflectivity calculations of Overbeck et al. are consistent with our model of flow orientation in the L2′ phase of docosanoic acid. Moreover, the reflectivity approaches a spatially uniform intensity as the strain rate increases, indicating that the coupling of the extensional flow to molecular alignment, at equivalent strains, is stronger at higher strain rates. Summary We have characterized the flow response of docosanoic acid monolayers in two tilted phases, L2 and L2′, through direct observation with the Brewster angle microscope. The BAM images in both phases appear textured due to abrupt changes in the azimuthal orientation of the molecular tilt as one crosses a domain boundary. In the L2 phase, an applied extensional flow induced stretching of the fatty acid domains along the extension axis. The elongated domains do not relax to their original shape, indicating the absence of any significant line tension between domains. An FFT analysis of the domain deformation indicates an affine deformation in response to the flow. In the L2′ phase, the extensional flow additionally couples to the molecular orientation and induces alignment of the tilt azimuth of the fatty acid molecules. This was evident from the large differences in image contrast for orthogonal orientations of the extension axis with respect to the plane of incidence of the Brewster angle microscope. This flow effect was also observed to be strain-rate dependent. Recently published simulations of Brewster angle reflectivity as a function of molecular orientation are consistent with a model in which the flow induces the tilt direction to align parallel to the extension axis. The observed dependence of flow alignment on monolayer phase, surface pressure, and strain rate has profound implications for the production of ordered molecular assemblies by Langmuir-Blodgett deposition. Acknowledgment. The authors acknowledge The Fannie and John Hertz Foundation for fellowship support of M.C.F. In addition, we acknowledge support from the Center for Materials Research at Stanford University and the NSF MRSEC Program through the Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA). We also thank Professor Hans C. Andersen in the Department of Chemistry for helpful discussions. LA950574V
