© Copyright 1998 American Chemical Society
MAY 12, 1998 VOLUME 14, NUMBER 10
Letters Imaging Liquid Structures on Inhomogeneous Surfaces by Scanning Force Microscopy T. Pompe, A. Fery, and S. Herminghaus* Max Planck Institut fu¨ r Kolloid und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12489 Berlin, Germany Received November 17, 1997. In Final Form: February 23, 1998 Scanning force microscopy in tapping mode is used for imaging low-viscosity liquid structures. Droplets on a substrate the wettability of which had been artificially structured in the submicron range are investigated. From the observed distortions of the contact line, we obtain contact line tensions in the theoretically expected range.
Various common wetting phenomena, such as contact angle hysteresis or contact line pinning, are dominated by microscopic heterogeneities of the wettability and/or the topography of the substrate. Understanding the interaction of the liquid with these inhomogeneities is thus crucial for understanding wetting of real (as opposed to ideal) surfaces. The characteristic length scales of these phenomena are usually below the micrometer range. At these small scales, effects of the contact line tension are also expected to play a significant role, which makes an investigation of liquid profiles on the nanometer scale even more tempting. However, optical techniques usually employed in studies of wetting phenomena are not appropriate here due to their poor spatial resolution. As far as resolution is concerned, scanning force microscopy (SFM) should be the method of choice. However, one might anticipate that the interaction of the scanning tip with the liquid to be imaged will distort, or even destroy, the liquid structure of interest. In fact, there has been quite a lot of work toward noncontact imaging techniques for liquids,1 trading off a serious reduction in resolution. In the first part of our work, we show that it is possible to image low-viscosity polar liquids by SFM in tapping mode, provided certain precautions are observed2 in (1) Hu, J.; Carpick, R. W.; Salmeron, M.; Xiao, X. D. J. Vac. Sci. Technol., B 1996, B14 (2), 1341.
contrast to earlier work concerned with liquids behaving like viscoelastic solids.3,4 In the second part, we demonstrate the application of this technique to a liquid droplet on a surface the wettability of which had been artificially structured. In tapping mode, the tip of the SFM vibrates with typical amplitudes of 10-100 nm at frequencies usually in the high kilohertz range. The damping of the vibration amplitude due to the interaction with the substrate serves as a feedback signal. The significance of this mode for imaging liquid surface profiles rests on the fact that the times of contact between the tip and the liquid are very short, far below 1 µs per oscillation period, thus reducing substantially the possible distortions of the profile. In our work, a Digital Instruments (Santa Barbara, CA) Nanoscope IIIa was used, with standard tapping cantilevers (model TESP, Nanoprobe), with resonance frequencies around 250 kHz. The radius of curvature of the tip was 5-10 nm, as specified by the manufacturer. To demonstrate the technique, we have imaged droplets of an aqueous solution of CaCl2 which was equilibrated (2) Fery, A.; Reim, D.; Herminghaus, S. Ultramicroscopy 1997, 69 (3), 211. (3) Sheikow, S. S.; Eckert, G.; Ignateva, G.; Muzafarov, A. M.; Spickermann, J.; Ra¨der, H. J.; Mo¨ller, M. Macrolmol. Rapid Commun. 1996, 17, 283. (4) Sheiko, S. S.; Muzafarov, A.; Winkler, R. G.; Getmanova, E. V.; Eckert, G.; Reinecker, P. Langmuir 1997, 13, 4172.
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Figure 2. Resonance curves of the cantilever at different distances from the surface of a droplet of aequous CaCl2 solution. The frequency of the signal exciting the cantilever motion was varied, while its amplitude was kept constant. Simultaneously, the response amplitude of the cantilever was monitored. The solid line marks the resonance curve of the free cantilever. The open circles mark a resonance curve as it was obtained after an approach with an amplitude damping to 90% of the free amplitude; the filled circles represent a curve as obtained after an approach with an amplitude of 70% of the free amplitude. Figure 1. Line scans of the same liquid feature using different amplitude damping for imaging: (a) “light” tapping (typical amplitude ) 90-95% of free amplitude); (b) amplitude for imaging reduced to 80-90% of free amplitude; (c) amplitude below 80% of the free amplitude.
with ambient humidity (about 50%) on muscovite ruby mica substrates (S&J Trading Inc., Glen Oaks). We show in Figure 1 the impact of variation of the set point amplitude (the amplitude, at fixed drive voltage, to which the feedback loop adjusts the vertical cantilever position). Figure 1a is obtained when the set point amounts to about 95% of the free amplitude, which was 15 nm. The “trace” and “retrace” are obviously in nice agreement. When the set point is reduced, spurious jumps occur, as shown in Figure 1b. They are not reproducible, as the difference between trace and retrace clearly shows. Further reduction results in good agreement of the trace with the retrace again, but the observed droplet profile has changed substantially, as can be seen in Figure 1c. This behavior was found for free amplitudes greater than 5 nm, while for smaller amplitudes, stable imaging was not possible. This phenomenon is known from noncontact AFM5 and attributed to the instability of the cantilever motion against the attractive capillary forces. Increasing the amplitude stabilizes the imaging process. To gain more insight into the imaging mechanism and to decide which of the two “stable” operating modes (Figure 1a or c) is more favorable for obtaining quantitative height information, we measured the resonance curves of the cantilever at different distances from the liquid surface. The frequency of the cantilever excitation signal was varied at fixed amplitude, and the corresponding amplitude of the cantilever vibration was monitored. During the procedure, the position of the cantilever was fixed both laterally and vertically. Figure 2 shows a typical plot of the amplitude versus frequency, as is obtained for the different situations of interest. The open circles correspond to the distance at which the amplitude close to (5) Hartmann, U. Ultramicroscopy 1992, 42, 59.
Figure 3. Comparison of the profile of a drop (aqeuous CaCl2 solution) on a homogeneous surface (mica) as determined by SFM (open squares) with theoretical predictions (solid line). The drop height is large as compared to the range of van der Waals interactions and small as compared to the capillary length of the liquid. Therefore, a spherical cap geometry is expected, which was used for the fit.
resonance (as under imaging conditions such as in Figure 1a) was reduced to about 95%. The full circles represent the resonance curve corresponding to the situation in Figure 1c, which was recorded with a set point amplitude of 70% of the free amplitude. The solid line indicates the free resonance of the cantilever. As can be seen, the resonance curve corresponding to Figure 1a (open circles) is slightly reduced in its peak height with respect to the free oscillation, while the width is unchanged within the accuracy of our measurement. It should be noted that there is a slight shift of the resonance frequency to lower values. In strong contrast, the curve corresponding to Figure 1c (full circles) is strongly reduced in its peak height, even far below 70% of the free amplitude, which would under imaging conditions result in withdrawal of the tip by the feedback system. One also observes a substantially increased resonance width, and the resonance frequency is shifted to higher values. If the resonance curve is
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Figure 4. Topgraphy (left side) and phase image (right side) of a droplet of diethylene glycol on a silicon wafer patterned in wettability (for details on preparation, see text). The height of the droplet is 700 nm, the width of the hydrophobic stripes is around 250, and the width of the hydrophilic stripes is around 550 nm. The topography image shows the elongation of the droplet parallel to the direction of the stripes and the undulation of the contact line normal to the direction of the stripes. In the phase image the hydrophilic regions of the substrate show a higher phase shift as compared to the hydrophobic regions. The liquid regions show a very high phase shift.
recorded at a distance corresponding to that in Figure 1b, the resonance curve jumps erratically between the open circle curve and the full circle curve. An “intermediate state” was not observed. The observed behavior of the cantilever resonance can be understood as follows. For a small set point (as in Figure 1c), there is a permanent liquid bridge between the tip and the liquid drop. This leads to an increased width of the cantilever resonance due to viscous damping in the liquid. Furthermore, the surface tension of the liquid adds to the restoring force of the cantilever, resulting in an increased resonance frequency. In contrast, in the operating mode corresponding to Figure 1a, there are no signs of a liquid bridge. The slight shift to lower frequencies rather points to a dominance of long-range attractive forces (such as the van der Waals force) in the interaction between the liquid and the tip.6,7 The fact that no intermediate state between these two modes of operation could be observed suggests that, at high set point values, the formation of a bridge is indeed completely avoided. It is now interesting to investigate whether the droplet profiles obtained in this mode agree with those expected theoretically. In Figure 3, we show (open squares) the cross section through a droplet of about 3-µm diameter and 160-nm height. It is thus small enough to render gravitational effects on its shape negligible, while it is still large enough to escape almost everywhere the van der Waals tails near the contact line. Consequently, one (6) Ku¨hle, A.; Soerensen, A. H.; Bohr, J. J. Appl. Phys. 1997, 81 (10), 6562. (7) Ancykowski, B.; Kru¨ger, D.; Fuchs, H. Phys. Rev. B 1996, 53 (23), 15485.
would expect the droplet to be shaped as a perfect spherical cap. The solid line represents a spherical cap fit to the data: within experimental accuracy, perfect agreement is found. The same behavior as described above was found for a number of different liquids (aqueous solutions of P2O5, diethylene glycol, triethylene glycol) on various substrates (mica, silicon, glass).2 In principle, the technique is limited to droplets with contact angles less than 60°. For higher contact angles, the side of the tip touches the droplet, causing artifacts at the droplets edges. We will now turn to the application of this technique for imaging a liquid droplet on an inhomogeneous surface, to study the interaction of the contact line with the heterogenities present. For these studies, we used droplets of diethylene glycol instead of aqueous salt solutions for reasons of simplicity of the interaction with the substrate. The disadvantage is that the droplets are not completely stabilized against evaporation and disappear slowly, but we found that there is plenty of time for imaging. The substrates used were silicon wafers with a native oxide layer, cleaned by ultrasonication in ethanole, snow-jet cleaning8,9 and exposure to Caro’s acid.10 Their wettabilities were patterned by microcontact printing11,12 of parallel hydrophobic stripes with a width of 250 nm (8) Sherman, R.; Hirt, D.; Vane, R. J. Vac. Sci. Technol. A 1994, 12 (4), 1876. (9) Sherman, R., Adams, P., Eds. Precision Cleaning +96 Proceedings, 1996. (10) Boneberg, J.; Burmeister, F.; Scha¨fle, C.; Leiderer, P.; Reim, D.; Fery, A.; Herminghaus, S. Langmuir 1997, 13 (26), 7080. (11) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (12) Kumar, A. Biebuyck, H.; Whitesides, G. M. Langmuir 1994, 10, 1498.
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and a periodicity of 800 nm. The hydrophobic coating consisted of a covalently bound layer of (heptadecafluoro1,1,2,2,-tetrahydrodecyl)dimethylchlorosilane (C12H10F17SiCl) deposited from hexane solution by means of a silicone rubber stamp.13-15 The droplets were created by means of an atomizer (Fleischhacker, Germany) and were deposited on the substrate out of the aerosol phase. For the small droplets of interest here, this method minimizes contact line pinning effects, as opposed to other methods.1 Figure 4 shows a typical image of a droplet of diethylene glycol on a structured surface. The left-hand side of the image shows the topography; the right-hand side shows the corresponding phase portrait. (In the phase portrait, the phase shift between the signal exciting the cantilever motion and the motion actually carried out by the cantilever is plotted for each point of the scanned area.7,16,17) As expected, the drop topography differs from a spherical cap geometry here, in that the liquid tries to avoid the hydrophobic regions. The drop appears elongated in a direction parallel to the stripes. In the phase image, the hydrophilic regions can be identified by their slightly larger phase shift. On the liquid drop, the phase is spacially constant and strongly different from the value found on the substrate (refs 2 and 3). The undulations of the contact line, due to the wettability structure, enable us to determine the tension of the contact line of the drop.19 On larger length scales, measured contact line tensions are usually orders of (13) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576. (14) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024. (15) John, P. M.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022. (16) Tamayo, J.; Garcia, R. Langmuir 1996, 12 (18), 4430. (17) Winkler, R. G.; Spatz, J. P.; Sheiko, S.; Mo¨ller, M.; Reineker, P.; Marti, O. Phys. Rev. B 1996, 54 (12), 8908. (18) Drelich, J. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 116, 43.
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magnitude larger than the theoretically expected values, which are in the 10-10 N region or less.18 This is because the unavoidable heterogeneity of the substrate distorts the liquid surface near the contact line, just as is seen from our drop in Figure 4. This adds to the total energy of the contact line region and completely masks the “real” line tension, which is due to the peculiarities of the effective interaction potential between the liquid and the substrate. However, when the lateral scale of observation comes close to 100 nm, the real line tension should be observable even if heterogenities are present.19,20 For the drop shown here, we see a strong influence of the 250-nm-wide hydrophobic stripes on the contact line shape. Furthermore, we can determine the contact angles normal to the contact line from appropriate cross sections of the AFM image and also determine the in-plane curvature of the contact line at the same points. It is then straightforward to calculate the contact line tension from the modified Young equation,20,21 and we arrive at (5 ( 3) × 10-10 N on the hydrophobic parts, which is in fact in the theoretically expected range. Experiments are under way to put these results on a stronger basis, by providing more, and more accurate, data on the contact line tension. However, the results presented here already demonstrate the usefulness of the force microscope for the determination of liquid profile surfaces. Acknowledgment. We are indebted to the group of Prof. J. Kottaus (LMU Munich) for providing the silicon masters for the microcontact printing procedure. LA971262Q (19) Boruvka, L.; Gaydos, J.; Neumann, A. W. Colloids Surf. 1990, 43, 307. (20) Drelich, J. Polish J. Chem. 1997, 71, 525. (21) Drelich, J.; Millert, J. D.; Kumar, A.; Whitesides, G. M. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 93, 1.
