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Langmuir 2003, 19, 8148-8151
Reassessment of Solidification in Fluids Confined between Mica Sheets Yingxi Zhu and Steve Granick* Departments of Materials Science and Engineering, Chemistry, and Physics, University of Illinois, Urbana, Illinois 61801 Received June 27, 2003 Enhanced layering is observed when molecularly thin films of a simple globular molecular fluid (octamethylcyclotetrasiloxane) are confined between step-free sheets of mica cleaved to be free of nanoparticulates produced by using a hot platinum wire. In addition, the linear shear responses depend on history. Films formed by rapid compression or under the action of shear display a large effective viscosity, but films formed by quasistatic compression display unprecedented low friction.
Introduction A liquid confined in one or more dimensions to a thickness less than about 10 molecular dimensions can become increasingly ordered relative to the bulk state. When confinement is between flat surfaces, molecules assemble into discrete layers parallel to these surfaces and their effective shear viscosities, diffusion rates, and relaxation times increase.1-8 It is also reported that, under shear, confined molecules between flat surfaces behave more like a solid than like a liquid, and it is not uncommon to hypothesize the existence of a confinement-induced phase transition.1-8 Although the interplay between liquid layering and the magnitude of friction is also reported for fluids confined between rough metal surfaces,9 most relevant shear experiments have concerned cleaved muscovite mica that was believed to be atomically smooth. Recently, it was discovered that this was not necessarily so; mica commonly prepared by cutting with a hot platinum wire possesses ≈0.05-1% surface coverage of nanometersized particulates, most likely platinum.10-14 This is probably because the vapor pressure of platinum is not negligible at the melting temperature of mica, 1320 °C (≈10-7 Torr, i.e., ≈0.1 monolayers/s). While the incidence of nanoparticles and their size surely differs from laboratory to laboratory and probably from experiment to experiment and they are reported to be dislodged in acidic water, it is not easy to dismiss their conceivable presence in experiments that employed a hot platinum wire to cut
mica substrates. Computer simulations have begun to address the interesting issue of how the topographical roughness might influence the structure, shear dynamics, and phase behavior of confined fluids.15,16 Apart from pioneering studies,10-13,17-19 to the best of our knowledge no prior shear studies of simple liquids employed mica that was cleaved to be demonstrably free of nanoparticulate deposits. Here, we provide the needed data. Mica was cleaved using an adhesive tape, and atomic force microscopy (AFM) showed it to be free of nanoparticles. Using samples cleaved in this way, we find significant differences from accepted data in the literature regarding both layering (force-distance profiles) and responses to shear. This provides a pleasing resolution to previous inconsistencies. It has been puzzling that computer simulations found translational diffusion to be slowed relative to that of the bulk by at most 1-2 orders of magnitude,6-8 that diffusion of tracer dyes showed slowing down by at most 3-4 orders of magnitude,20 and that squeeze-out from one layer to another also implies fluidity,14,18,19 yet shear experiments found confinement-induced solidification. The unprecedented low friction observed here for the films formed by quasistatic compression but not for films formed more rapidly or under the action of large-amplitude shear raises interesting new questions about the influences of surface disorder and external fields on the friction of confined fluids. Experimental Section
* Author to whom correspondence should be addressed. (1) Van Alsten, J.; Granick, S. Phys. Rev. Lett. 1988, 61, 2570. (2) Israelachvili, J. N.; McGuiggan, P. M. Science 1988, 241, 795. (3) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (4) Kumacheva, E.; Klein, J. Science 1995, 269, 816. Kumacheva, E.; Klein, J. J. Chem. Phys. 1998, 108, 6996. (5) Demirel, A. L.; Granick, S. Phys. Rev. Lett. 1996, 77, 2261. Demirel, A. L.; Granick, S. J. Chem. Phys. 2001, 115, 1498. (6) Thompson, P. A.; Grest, G. S.; Robbins, M. O. Phys. Rev. Lett. 1992, 68, 3448. (7) Gao, J. P.; Luedtke, W. P.; Landman, U. Phys. Rev. Lett. 1997, 79, 705. (8) Cui, S. T.; Cummings, P. T.; Cochran, H. D. J. Chem. Phys. 1999, 111, 1273. (9) Ko, J. S.; Gellman, A. J. J. Phys. Chem. B 2001, 105, 5186. (10) Ohnishi, S.; Hato, M.; Tamada, K.; Christenson, H. K. Langmuir 1999, 15, 3312. (11) Kohonen, M. M.; Meldrum, F. C.; Christenson, H. K. Langmuir 2003, 19, 975. (12) Heuberger, M.; Za¨ch, M. Langmuir 2003, 19, 1943. (13) Lin, Z.; Granick, S. Langmuir 2003, 19, 7061. (14) Mugele, F. Private communication. Zilberman, S.; Becker, T.; Mugele, F.; Persson, B. N. J.; Nitzan, A. J. Chem. Phys. 2003, 118, 11160.
For the fluid, we employed a globular-shaped molecule (OMCTS, (octamethylcyclotetrasiloxane), widely considered to be a model liquid for comparison with “Lennard-Jones” liquids.4,5,21-23 Its diameter, 0.8-0.9 nm, exceeds the mica lattice size, 0.7 nm. Muscovite mica (ASTM V-2 grade) was silvered on the backside and glued onto a cylindrical disk using the usual (15) Gao, J. P.; Luedtke, W. D.; Landman, U. Tribol. Lett. 2000, 9, 3. (16) Persson, B. N. J.; Samoilov, V. N.; Zilberman, S.; Nitzan, A. J. Chem. Phys. 2002, 117, 3897. (17) Frantz, P.; Salmeron, M. Tribol. Lett. 1998, 5, 151. (18) Mugele, F.; Becker, T.; Klingner, A.; Salmeron, M. Colloids Surf., A 2002, 206, 105. (19) Mugele, F.; Persson, B. N. J.; Zilberman, S.; Nitzan, A.; Salmeron, M. Tribol. Lett. 2002, 12, 123. (20) Mukhopadhyay, A.; Zhao, J.; Bae, S. C.; Granick, S. Phys. Rev. Lett. 2002, 89, 136103. (21) Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1981, 75, 1400. (22) Christenson, H. K.; Blom, C. E. J. Chem. Phys. 1987, 86, 419. (23) Seeck, O. H.; Kim, H.; Lee, D. R.; Shu, D.; Kaendler, I. D.; Basu, J. K.; Sinha, S. K. Europhys. Lett. 2002, 60, 376.
10.1021/la035155+ CCC: $25.00 © 2003 American Chemical Society Published on Web 09/04/2003
Letters
Langmuir, Vol. 19, No. 20, 2003 8149
Figure 1. Force-distance profile of OMCTS between mica at 25.0 ( 0.5 °C. Force, F, normalized by the mean radius of curvature, R (≈2 cm), of the cross cylinders, is plotted against surface separation, D. The inward and outward jumps occurring upon the onset of an instability are indicated by the left and right triangles, respectively. Near the top of each oscillation is written the implied number of confined layers. The dashed lines (guides to the eye) represent regions inaccessible to measurement because the force gradient exceeds the apparatus spring constant. The bottom inset shows the peak-to-peak amplitudes of the oscillations plotted against D on semilogarithmic scales. The mica cylinders were flattened by compressive forces at separations n e 3. The top inset is an AFM image (Digital Instruments D3100 in the tapping mode, 2 × 2 µm), taken using etched silicon tips on cantilevers (Nanoprobe) with a spring constant of 46 N‚m-1 at a tapping frequency of 335 kHz and scan rate of 1 Hz. This featureless area with a background of thermal noise is free of reported10-14 nanoparticles. protocol for surface forces experiments, and then adhesive tape was placed onto it and detached using the method of Frantz and Salmeron.17 A drop of OMCTS was added rapidly. This method of tape-peeling produces freshly cleaved mica and has the further advantage that exposure to ambient air is short. The surfaces were mounted in crossed cylinder geometry and oriented to be free of steps at their contact. Separation was subsequently calculated by optical interferometry using the equations for an asymmetric interferometer.24 There is an absolute uncertainty of (15% owing to the limited wavelength range of our spectrometer’s detector because more adjacent fringes than were available to us are needed for a more accurate thickness determination;12 this absolute uncertainty does not affect relative comparison of the data. Our reference “zero” thickness was the thickness of two sheets in adhesive contact in air. This would understate the actual thickness by ≈0.5 nm if OMCTS liquid displaced adsorbed air, but we have no direct evidence that this occurred. Experiments were performed at 25 °C, close to the OMCTS melting temperature of 18 °C, with P2O5 (a highly hygroscopic chemical) inside the sealed sample chamber. The sample of OMCTS (Fluka, purum grade 99.8%) was used as was received. The radius of curvature of the mica sheets was ≈2 cm, giving a slitlike geometry when the surface separation was molecularly thin.
Results and Discussion A. Force-Distance Profile. As smooth mica surfaces separated by OMCTS were pushed together, fluid drained smoothly until oscillatory forces of alternating attraction and repulsion were first detected at a thickness of ≈1015 molecular dimensions. These oscillatory forces arise from the tendency of fluid to form layers parallel to the surface; the application of pressure caused the fluid to drain in discrete steps corresponding to squeezing out of successive layers. In Figure 1, the force (normalized by (24) Horn, R. G.; Smith, D. T. Appl. Opt. 1991, 30, 59.
the mean radius of curvature) is plotted against separation. There is considerable variability in the literature data concerning force-distance profiles of OMCTS. Relative to a recent survey of what was believed by the authors to be the most reliable force-distance profiles,4 these oscillatory forces display ≈50% more oscillations. Their amplitudes decay with distance approximately exponentially, as is shown in the bottom inset of Figure 1; however, the decay length (n ) 6 ( 1) exceeds previous findings.4,21,22 This large correlation length also exceeds the theoretical expectation for even a hard-sphere liquid25 (a hard sphere liquid presents an upper bound for a molecular liquid), suggesting that the structure may be crystalline or partly crystalline. The period of oscillation was also somewhat larger for the thickest films, suggesting that packing into layers was in those cases less perfect; this agrees with measurements made long ago.21 Force-distance profiles of OMCTS, using mica that had been prepared using a hot Pt wire, have two common features:4,21,22 (a) the peak amplitudes of repulsive oscillatory forces exceed those of attractive oscillatory forces, and (b) the peak amplitudes of repulsive forces grow steeply when the film thickness is less than ≈3 nm. The findings presented in Figure 1 are noteworthy in that the minima and the maxima of the force-distance profile are unusually symmetric and that, at separations
