Motion Picture Imaging of a Nanometer-Thick Liquid Film Dewetting by

Sep 27, 2008 - a frame rate of 4 frames/s, using ellipsometric microscopy in a vertical objective configuration. The observation revealed that the dew...
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Langmuir 2008, 24, 11645-11650

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Motion Picture Imaging of a Nanometer-Thick Liquid Film Dewetting by Ellipsometric Microscopy with a Submicrometer Lateral Resolution Kenji Fukuzawa,*,† Tomohiko Yoshida,† Shintaro Itoh,† and Hedong Zhang‡ Department of Micro/Nano Systems Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, and Department of Complex Systems Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603 Japan ReceiVed July 3, 2008. ReVised Manuscript ReceiVed August 12, 2008 We visualized the detwetting of a nanometer-thick unstable liquid film on a nanotextured solid surface with a high lateral spatial resolution. The dewetting was imaged as a motion picture at a submicrometer spatial resolution and a frame rate of 4 frames/s, using ellipsometric microscopy in a vertical objective configuration. The observation revealed that the dewetting process significantly depends on the sign of the disjoining pressure Π. When Π is negative, the film rupture due to the spinodal dewetting proceeds to droplet formation in a single step, whereas, when Π is positive, the film rupture due to the spinodal dewetting stops when the pressure of the thicker region balances with that of the thinner region, and then the heterogeneous grooves are nucleated and grow. The dewetting process dependence on the sign of Π can be found in systems other than that reported here because the sign of Π changes at the local maximum of the surface energy.

1. Introduction Dewetting on solid surfaces has attracted the interest of many researchers because it reflects the intermolecular interaction between thin liquid films and solid surfaces.1-10 In addition, the control of the dewetting is also important for nanometer-thick lubricant films in hard disk drives (HDDs) and photoresist films in microelectronics devices.3-7 Dewetting of thin liquid films has been intensively investigated using atomic force microscopy (AFM) observation of a model liquid on flat substrates, and its elementary steps have been clarified. In unstable films, spontaneous rupture of films, called spinodal dewetting, occurs and leads to droplet formation. In metastable films, heterogeneous nucleation and growth of holes occur, which are triggered by the defects or perturbations. Although conventional AFM can reliably follow slow dewetting systems such as highly viscous or melted and quenched thin films, accessible samples are limited because of the slow probe scanning. In addition, the probe contact can possibly trigger film rupture, which modulates the dewetting process as an additional perturbation. The dewetting process dependence on the sign of the disjoining pressure Π was reported * Corresponding author. Address: Department of Micro/Nano Systems Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel: +81-52-789-2747. Fax: +81-52-789-3129. E-mail: fukuzawa@ nuem.nagoya-u.ac.jp. † Department of Micro/Nano Systems Engineering. ‡ Department of Complex Systems Science. (1) de Gennes, P. G.; Brochard-Wyart, F.; Que´re´, D., Gouttes, Bulles, Perles et Ondes; Belin: Paris, 2002. (2) Mate, C. M.; Toney, M. F.; Leach, K. A. IEEE Trans. Magn. 2001, 37, 1821–1823. (3) Waltman, R. J.; Khurshudov, A.; Tyndall, G. W. Tribol. Lett. 2002, 12, 163–169. (4) Xu, L.; Ogletree, D. F.; Salmeron, M.; Tang, H.; Gui, J.; Marchon, B. J. Chem. Phys. 2000, 112, 2952–2957. (5) Guo, Q.; Li, L.; Hsia, Y.-T.; Jhon, M. S. J. Appl. Phys. 2005, 97, 10P3021–10P302-3. (6) Kim, H. I.; Mate, C. M.; Hannibal, K. A.; Perry, S. S. Phys. ReV. Lett. 1999, 82, 3496–3499. (7) Fukuzawa, K.; Shimuta, T.; Yoshida, T.; Mitsuya, Y.; Zhang, H. Langmuir 2006, 22, 6951–6955. (8) de Gennes, P. G. ReV. Mod. Phys. 1985, 57, 827–863. (9) Reiter, G. Phys. ReV. Lett. 1992, 68, 75–78. (10) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. ReV. Lett. 1991, 66, 715–718.

for a nanometer-thick polar liquid film on a nanotextured surface.7 The observation was conducted with an ellipsometric microscope (EM), which could provide dynamic and noncontact observation. This observation revealed that the dewetting pattern varied according to the sign of Π. The dewetting images of 4.3 and 5.8 nm-thick films and the disjoining pressure obtained from measurement of the surface energy γ (Π ) -dγ/dh) are shown in Figure 1, where h is the film thickness. Uniform droplet formation was observed when Π was negative (the film thickness h ) 4.3 nm), whereas heterogeneous groove growth was observed and uniform droplet formation was not when Π was positive (h ) 5.8 nm). Here, the derivatives of dΠ/dh of both films were positive, which means that the films were unstable.1,11 Although both films were unstable, the dewetting patterns were quite different according to the sign of Π. However, the detailed mechanism of the dewetting process has not been fully clarified because of the limited lateral resolution of the observation method. In this paper, we developed a new type of EM with a high lateral resolution on the order of 0.1 µm. The dewetting process was visualized as a moving picture. The observation revealed the details of the dewetting process. 2. Materials and Methods. In EMs or imaging ellipsometers (IEs), the sample is illuminated with a light at a large incident angle, and this oblique illumination is necessary for improving the ellipsometric image contrast.12,13 An objective lens is set obliquely to the sample surface to correct the light reflected from the sample. This oblique objective setup causes a very narrow field of view when a high-resolution objective lens with a large numerical aperture (NA) is used. When an objective lens with a lateral resolution of 1-2 µm is selected, sample or beam scanning is necessary because of the narrow field of view. Therefore, conventional EMs and IEs are not well suited for providing dynamic visualization with high spatial resolution. To overcome the drawback, various types of EMs that do not use (11) Israelachivili, J. N. Intermolecular and Surface Forces; Academic Press, Inc.: San Diego, CA, 1992. (12) Beaglehole, D. ReV. Sci. Instrum. 1988, 59, 2557–2559. (13) Hennon, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936–939.

10.1021/la802098w CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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Figure 1. Dewetting of polar liquid on a nanotextured surface. (a) Images of dewetting lubricant films with the EM in conventional oblique objective configuration. The thicknesses of the films are shown in the figure. EM images were taken about 20 min after film application. Subnanometer high texture is fabricated in the arrow direction on the substrate magnetic disks. (b) Disjoining pressure obtained from the surface energy measurement.

Figure 2. Schematic of a VEM with a high lateral resolution. Inset illustrates the optical paths through the objective lens.

the oblique objective setup have been proposed.14-16 In these new types of EMs, a high-resolution objective lens is set vertically to the sample surface to avoid a narrow field of view. Moreover, the paths of the oblique illumination light and light reflected from the sample are set in the objective lens. This vertical objective setup provides a diffraction-limited spatial resolution on the order (14) Neumaier, K. R.; Elender, G. R.; Sackmann, E.; Merkel, R. Europhys. Lett. 2000, 49, 14–19. (15) Zhang, Q.; Leger, R. Appl. Opt. 2002, 41, 4443–4450. (16) Zhang, Q.; Leger, R. Appl. Opt. 2002, 41, 4630–4637.

of 0.1 µm and a wide field of view of around 100 × 100 µm2. Here this setup is called vertical-objective-based ellipsometric microscopy (VEM). Figure 2 shows a schematic of our VEM setup. Off-axis Kohler illumination was used to obtain a sufficiently wide field of view. The light from a 100 W mercury arc lamp (U-LH100HG, Olympus) was filtered with a wavelength filter (wavelength: 546.1 nm, width: 10 nm), collimated by a 100-µm diameter pinhole, and focused onto an off-axis point on the back focal plane of the objective lens (NA of 0.95, CFI LU Plan Apo EPI 100X, Nikon).

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of a disk obtained with a three-dimensional optical profiler (NewView 6000, Zygo). Groove-shaped nanotexture in the circumferential direction (horizontal direction in the figure) was observed. The arithmetic average roughness, Ra, of the disk was about 0.2 nm. Polar and nonpolar perfluoropolyether (PFPE) lubricants (Fomblin Zdol4000 and Z03, Solvay Solexis) were used as sample liquids. Their molecular structures are given by Figure 3. Topography of a sample disk taken with a three-dimensional optical surface profiler.

The sample surface was illuminated by an oblique and parallel beam. The light reflected from the sample was imaged through the objective and imaging lenses onto a highly sensitive electron multiplying CCD camera (Cascade II:512, Photometrics). The vertical setting of the objective lens to the sample surface can provide diffraction-limited resolution and a sufficiently wide field of view. Diffraction-limited resolution is determined by the wavelength and the NA of the objective lens and should be 0.35 µm. Observation of a test pattern (1951 USAF test chart) with our VEM showed that a line gap of 0.78 µm, which was the minimum gap on the test chart, could be resolved. This indicates that our VEM can provide submicrometer spatial resolution. In addition, the area in focus was about 85 × 85 µm2, which means that the field of view is wide enough to provide imaging without scanning. The relationship between image intensity and film thickness depends on the angles of the polarization devices: polarizer, quarter wave plate, and analyzer. In our setup, the angles of these devices were set so that the image intensity was proportional to the film thickness. This made it easier to convert from intensity to thickness. The uniformity of the illumination light intensity in the VEM is important. If the spot of the reflection light on the back focal plane forms an image on the CCD, significant nonuniformity occurs. Note that the optical path shown in Figure 2 is the path of the light specularly reflected from the substrate surface, and this light can cause the background spot image. The light diffusely reflected from the fine features of the sample film forms the film image. In many VEM applications, the substrates have flat and mirror-like surfaces, so the specularly reflected light from the substrate is much stronger than the diffusely reflected light from the sample. Therefore, the background nonuniformity caused by the specularly reflected light must be reduced. In our VEM, the focal plane of the imaging lens is set at the back focal plane of the objective lens so that the back focal plane spot of the reflection light forms an image at infinity. In addition to this optical background reduction, the background was digitally reduced by dividing each film image by a reference image. The sample film image recorded as a movie was divided by the reference image, which was an image of the substrate without the film. As in the previous study,7 we used nanometer-thick liquid lubricant films on magnetic disks for HDDs as samples because the disk surface was flat but had a well-defined subnanometer topography and because the interaction between the liquid lubricant and disk had been clarified.7 In the commercial production of magnetic disks, a subnanometer-high structure, which is called nanotexture, is fabricated by abrasive machining of rotating magnetic disks to improve their recording properties. The surface topography of current magnetic disks should be industrially controlled with subnanometer precision all over the disk surface, because the gap between the head and disk must be on the order of 1 nm. Figure 3 shows a surface topography

HO-CH2-CF2-[(O-CF2-CF2)p-(O-CF2)q]-O-CF2CH2-OH, (1) F-CH2-CF2-[(O-CF2-CF2)p-(O-CF2)q]-O-CF2CH2-F (2) where p/q is typically between 2/3 and 1. These lubricants, which are widely used in HDDs, are linear chain polymers having a molecular weight of about 4000 g/mol. The estimated molecular diameter of the gyration of them is about 3 nm for bulk, and the diameter of their linear chain is about 0.7 nm. The lengths of the molecules are about 14 nm. The polar lubricant has a hydroxyl group at each end and is considered to be adsorbed onto the carbon overcoat surface due to the interaction between the end groups and the carbon surface. From the previous study,7 as shown in Figure 1b, the derivative dΠ/dh is positive when the film thickness h is 4-7 nm. Moreover, in this unstable film, Π < 0 when h < 5 nm, whereas Π > 0 when h > 5 nm. The nonpolar lubricant does not have a hydroxyl end group, but the main chain has the same molecular structure as the polar one. It does not exhibit detwetting because the Π of the film is positive and dΠ/dh is negative. The lubricants were applied to sample disks by dip coating. In this method, which is used for the commercial production of HDDs, the substrate is immersed into and then removed from the lubricant solution at a constant rate. The thickness of the liquid film on the substrate can be adjusted with subnanometer resolution by adjusting the removal rate and lubricant solution density. The thickness of the films was measured with a commercially available scanning ellipsometer (MARY-102, Five Lab). Using this thickness, we converted the image intensity into the liquid film thickness on the basis of the linear dependence of the image intensity on the film thickness. In our experiment, the liquid film was applied to only part of the disk. The reference image was obtained for the digital background reduction by averaging 16 frames of a bare disk image.

3. Results and Discussion Figure 4 shows the relationship between the normalized intensity of the image and the liquid film thickness for the nonpolar lubricant. The image intensity was averaged over the 16 frames of the film images. The figure shows the difference in image intensity between the film and the bare disk region. The intensity was normalized by the value at a thickness of 10 nm. The image intensity is virtually proportional to the film thickness, demonstrating that, with our VEM, the image intensity can be directly converted into film thickness. The typical noise level obtained from the variation from the average thickness was about 0.5 nm. This means that the VEM can provide subnanometer thickness resolution. A movie of the dewetting of a thin liquid film was recorded on a personal computer from 6 min after liquid film application (refer to the Supporting Information). The frame rate (images acquired per unit time) was set at 4 frames/s, in consideration of the time scale of the observed phenomena and the signalto-noise ratio of the image, although a faster rate is possible.

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Figure 4. Relationship between the normalized image intensity of a VEM and the film thickness of a nonpolar lubricant film.

Figure 6. Sequential images of dewetting of the 6.2-nm-thick polar lubricant film taken with VEM. Times of image capture is shown above each image. G1, G2, and G3 are dewetted grooves, and R1 is the ridge next to groove. B1 is a bump (thicker region). Figure 5. Image of the dewetting of a 4.3-nm-thick polar lubricant film taken with a VEM. Time of image capture is 6 min after the film application.

Figure 5 shows the VEM image of the dewetting of a 4.3-nmthick polar lubricant film at the onset of the observation (6 min after the film application). The brighter areas in the images are the regions where the film was thicker. As mentioned above, the film at this thickness is unstable and has a negative disjoining pressure (Figure 1b). The small droplets that uniformly formed were observed at the onset of the observation, and they were almost stable during the observation. This indicates that the film ruptured and formed droplets before the onset of the observation. The thickness of the flat dark region other than droplets was about 3 nm thick. This thickness corresponds to the monolayer thickness of the film. Therefore, this result indicates that the film ruptured and formed droplets on the adsorbed monolayer. Figures 6, 7, and 8 show VEM images of 6.2-nm-thick polar lubricant film on a magnetic disk. The film at this thickness is unstable and has a positive disjoining pressure (Figure 1b). The onset time of the observation was set to t ) 0 s. Figure 7 shows magnified images of the upper left rectangle in Figure 6. Figure 8 shows a cross-sectional view from y ) 0 to 20 µm at line L1 in Figure 6. From the onset of visualization, small bumps (small thicker regions) were observed over the whole sample surface (Figure 6a). The bumps were elliptically shaped along the nanotexture, which was nearly horizontal (x direction) in the figure. These bump formations could not be found in the previous experiment using low-resolution EM.7 It is considered that the small bumps were formed by spinodal dewetting immediately after film application, as droplets were formed in the 4.3-nmthick film, because the film is unstable. However, the thickness of the flat region other than the bump was around 6 nm, not

Figure 7. Magnified images of the upper left rectangle framed in Figure 6. Times of image capture are shown above each image. G1 and G3 are dewetted grooves, and R1 is the rim next to the groove.

monolayer thickness. The dewetting of the 6.2-nm-thick film did not proceed in a single step as did that of the 4.3-nm-thick film. This indicates that the spinodal dewetting proceeds in different ways according to the sign of Π. The bumps were stable and had uniform step heights of about 2 nm measured from the surface

Motion Picture Imaging of Liquid Film Dewetting

Figure 8. Cross-sectional view at L1 in Figure 6.

of the flat film region. In other words, the film thickness hs in the bump was about 8 nm. The bumps were stable and had a uniform height, suggesting that the thickness in the bump region hs is stable. Using the result shown in Figure 1b, the disjoining pressures of the 6 and 8-nm-thick films are about 2.0 and 1.9 MPa, respectively. This suggests that Π of the bump region balances with that of the flat region, and the two regions are in hydrodynamic equilibrium. When Π is positive, the liquid molecules do not cohere because the interaction between the molecule and substrate is more attractive than that between the molecules. This causes the separation of thinner and thicker regions of the film. The thickness separation proceeds but stops at the time when the pressures are balanced. Thus, the uniform and stable bumps were generated in the 6.2-nm-thick film whose Π is positive. On the other hand, when Π is negative, the interaction between the liquid molecules is more attractive than that between the molecule and substrate. Therefore, the film rupture proceeds to forming droplets in a single step, as it did in the 4.3-nm-thick film. Thus, the difference in the dewetting process according to the sign of Π was clarified by the VEM observation. When Π < 0, the film rupture proceeds to droplet formation in a single step, whereas, when Π > 0, the film rupture stops when the pressures balance. Continuing the observation of the 6.2-nm-thick film, a grooveshaped dewetted region along with a rim was formed (groove G1 and rim R1 in Figures 6b and 7a). The thickness of the dewetted groove (y < 1 µm) h0 shown in Figure 8 was around 3 nm, which corresponds to the monolayer thickness. In addition to the growth, as shown in Figures 6b-e, 7a-e, and Figure 8, the molecules in the rim spread in a direction perpendicular to the nanotexture. The spreading film merged with the small anterior bumps, for example, bump B1 in Figure 6c,d. In Figure 8, after the spreading film reached bump B1 at t ) 90 s, the spreading film had a step height almost equal to the bump height. This confirms that the liquid film is stable at a thickness of hs () 8 nm), due to the pressure balance, as mentioned above. In addition, measuring the local spreading properties such as diffusion coefficient can provide information about the flat film region. If the flat region has a nonuniform surface, spatially variant spreading properties should be obtained, which is inconsistent with the separation of the thinner and thicker (bump) regions. If conventional ellipsometers are used, only the spatially averaged properties, which includes bump merge process, can be obtained, because the spatial resolution is typically submillimeter, whereas the pitch of the bumps is several micrometers. On the other hand, the VEM can provide local spreading properties with a submicrometer lateral resolution. The time dependence of the cross-section area from y ) 0 to 16.8 µm in Figure 8 is shown in Figure 9. The cross-

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Figure 9. Time dependence of cross-section area from y ) 0 to 16.8 µm in Figure 8.

Figure 10. Movement of film edge at lines L1 in Figure 6 (black triangles) and L2 in Figure 7 (white circles).

section area was obtained by multiplying the average thickness in this region and the region length (∆y ) 16.8 µm). The crosssection area was fairly constant over time and the volume per unit length in the x-direction was almost conserved for this region. This indicates that the film spreading can be treated as a onedimensional spreading process in the y-direction. The crosssection area was obtained by converting the image intensity using the linear dependence on the film thickness, as described above. If the intensity is not proportional to the thickness, the crosssection area may not be constant because the thickness distribution changes with time. This result and that shown in Figure 4 confirm that the image intensity is proportional to the film thickness in our VEM. Figure 10 shows the temporal change in the edge of the spreading film at lines L1 in Figure 6 and L2 in Figure 7. In the figure, ∆t ) 0 s corresponds to t ) 82.5 s, and the square of the displacement from the position at t ) 82.5 s (∆y2) is plotted. This plot shows that ∆y2 is proportional to ∆t for the film edge on the line L2. This indicates that the spreading process can be expressed as a diffusion process. The diffusion coefficient D, was estimated to be D ) 0.59 × 10-12 m2/s from the relationship ∆y2 ) 2 D∆t. This coefficient obtained from microscopic spreading has an order of magnitude similar to that obtained from macro-scale spreading measurements.17,18 In Figure 10, the film edge at line L1 seemingly has a different spreading properties. The film edge at line L1 has a tendency similar to that at L2 until ∆t ) 11.25 (t ) 93.75 s, arrow A), at which point the tendency changes. Figures 6 and 8 show that this point (17) Novotny, V. J. J. Chem. Phys. 1990, 92, 3189–3196. (18) Ma, X.; Gui, J.; Smoliar, L.; Grannen, K.; B. Marchon, B.; Jhon, M. S.; Bauer, C. L. J. Chem. Phys. 1999, 110, 3129–3137.

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corresponds to the time when the spreading film merged with bump B1. After this merger, the tendencies became similar again (∆t ) 22.5 s, t ) 105 s, arrow B) because the liquid spreads into the flat film region. The dotted line in Figure 10 is a linear approximation of the edge movement at L1 after ∆t ) 22.5 s, using the slope of the edge movement at L2 (diffusion coefficient) as that at L1. Thus, the VEM observation revealed that the local diffusion coefficients are uniform on the flat film region. If the 6.2-nm-thick film separates into a thinner and thicker region due to the disjoining pressure balance as described above, the flat thin film region should be uniform. Thus, these results support the initial thickness separation due to the spinodal dewetting. Subsequently, the growth of groove G2 occurred in the lower region in Figure 6e-i. Moreover, the growth of groove G3 along with the rims occurred in the spreading film in the upper region of Figures 6f-i and 7f-i. The grooves grew along the nanotexture. This heterogeneous groove formation is considered to correspond to the hole nucleation on the flat substrate. According to the study of the hole growth,1 for long dewetting times, the speed of the groove growth should be described by considering the balance of the driving force due to the surface energy difference between the dewetted and flat films ∆γV and the viscous energy dissipation per unit time at the rim of the groove 3η(V/hc)2lhc. Here, the velocity, the moving film thicknesses, the width of the rim next to the groove, the liquid viscosity, and the surface energy difference between the dewetted and flat film (spreading film) regions are defined as V, hc, l, η, and ∆γ, respectively. Therefore, the order of the velocity is roughly estimated to be V ) hc∆γ/ 3ηl. The film thicknesses of the dewetted groove and the spreading film were h0 ) 3 nm and hs ) 8 nm, respectively, as mentioned above. The moving film thickness is the difference between the thicknesses in the rim and the dewetted regions, hc ) hr - h0 ) 8 nm, where hr is the thickness of the rim and was around 11 nm. l and ∆γ were about 5 µm and 7 mJ/m2, respectively. The viscosity of nanometer-thick liquid films is known to be different from the viscosity of the bulk. It was reported that the viscosity of the polar lubricant with a thickness of 8 nm is about 3 times larger than that of the bulk (η ) 0.18 Pa s).19 Using these values, we estimated the velocity to be V ) 6.9 µm/s. The temporal change in the forefront of groove G3 (arrows in Figure 7f-i) is shown in Figure 11. The forefront moved at an almost constant speed of about 4.0 µm/s. The order of the estimated velocity agrees with that of the experimental one. This indicates that the dewetted groove grew as a dewetted hole on the flat substrate. In addition, in Figure 6g-i, the successive generation of the dewetted groove, which was triggered by the groove G3, was observed. This indicates that once the dewetted groove nucleated, the dewetted region enlarges through the successive growth of the groove. Thus, the dewetting process of the unstable films on the nanotextured surface is significantly affected by the sign of Π. The process is summarized as follows: when Π < 0, the film rupture proceeds to droplet formation as a result of the spinodal dewetting, whereas, when Π > 0, the film rupture due to the spinodal dewetting proceeds but stops when the pressures balance, and then the grooves are heterogeneously nucleated and grow. In general, liquid films are unstable in the vicinity of the local (19) Karris, T. E.; Marchon, B.; Flores, V.; Scarpulla, M. Tribol. Lett. 2001, 11, 151–159.

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Figure 11. Movement of the forefront of groove G3 (arrows in Figure 7f-i).

maximum of the surface energy γ. Let the film thickness h ) hm at the local maximum. Since Π ) -dγ/dh, Π when h < hm should be negative, whereas that when h > hm should be positive in the vicinity of the local maximum, as shown in Figure 1b. Therefore, the dewetting process dependence on the sign of Π can generally be found in systems other than that reported here.

4. Summary Using the VEM, we dynamically visualized the dewetting of nanometer-thick unstable liquid films on a nanotextured solid surface at a submicrometer spatial resolution and a frame rate of 4 frames/s. The observation revealed that the dewetting process significantly depends on the sign of Π. When Π is negative, the film rupture due to the spinodal dewetting proceeds to droplet formation in a single step, whereas, when Π is positive, the film rupture due to the spinodal dewetting stops when the pressure of the thicker region balances with that of the thinner region, and then, the grooves are heterogeneously nucleated and grow. This type of dewetting is not limited to our experiment because the difference in the dewetting process can be explained by Π and physical heterogeneities. The dewetting process dependence on the sign of Π can be found in systems other than that reported here because the sign of Π changes at the local maximum of the surface energy. Acknowledgment. This work was supported in part by SENTAN, the Japan Science Technology Agency (JST), the Japanese Ministry of Education, Culture, Sports, Science and Technology under Grant No. 20360077, and the Storage Research Consortium. Supporting Information Available: A video clip of dewetting 6.2-nm-thick polar lubricant on a magnetic disk taken with a verticalobjective-based ellipsometric microscope (VEM) at a submicrometer resolution. The image size is about 85 × 85 µm2. It shows directly visualized dewetting for t ) 60-165 s. The frame rate was 4 frames/s. The playback time is about 21 s on a PC, so, the movie is played at about 5 times the speed. Some of the frames are shown in Figures 6 and 7. This information is available free of charge via the Internet at http://pubs.acs.org. LA802098W