Langmuir 2006, 22, 2397-2398
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Comments Comment on Reassessment of Solidification in Fluids Confined between Mica Sheets
In a recent paper, Zhu and Granick1 (henceforth also referred to as Z&G) reassessed their earlier results on the solvation and friction forces between mica surfaces across liquid octamethylcyclotetrasiloxane (OMCTS) and found significant differences, which they attributed to the existence of platinum particles on their surfaces in their earlier studies.2 Zhu & Granick1 also implied that all previous measurements on OMCTS are flawedsa belief based on the erroneous assumption concerning the way mica surfaces are “commonly prepared” in most laboratories.3 More importantly, since OMCTS has long been used as a model liquid, both in experiments4,5 and in theoretical modeling,6 it is important to establish the true nature of the solvation (oscillatory or structural) and friction (shear or lubrication) forces between molecularly smooth surfaces across this liquid. Zhu & Granick1 show no error bars for the force maxima and minima, or for the distances at the turning points, and all friction traces shown were for shearing distances less than 0.3 nm per molecule. The approach and separation speeds were not given (an important parameter, as discussed below), and the relative crystallographic orientation of the mica lattices (twist angle) were not noted, which has previously been shown to influence oscillatory forces.7,8 We have repeated these experiments, also using platinum-free mica, and find that the earlier results hold, both for the normal oscillatory forces and lateral friction forces. Our results for the normal forces, together with those of other workers using both platinum-cut and platinum-free mica, are shown in Figure 1. Even though there is some variation among the results obtained by different researchers, the forces measured by Z&G1 stand out as having a longer range and larger amplitude (especially the deep adhesive minima) compared to all other measurements, whether using Pt or non-Pt cut mica. Moreover, since Z&G used their liquid OMCTS “as received”, that is, without purifying or distilling it, the range of their oscillations must be seen as a lower limit since impurities are known to suppress, not enhance, oscillations. The source of this discrepancy is not yet known, although we show here that Pt cutting can be ruled out. An important difference in the methodologies used by us and by Z&G, which could explain the different results obtained, concerns the variable-speed motor drive and manual piezoelectric distance control that were used in our experiments, allowing forces to be measured under quasi-equilibrium conditions. In ref * Corresponding author. † University of California. ‡ Australian National University. (1) Zhu, Y.; Granick, S. Langmuir 2003, 19 (20), 8148-8151. (2) Lin, Z.; Granick, S. Langmuir 2003, 19 (17), 7061-7070. (3) Israelachvili, J.; Alcantar, N.; Maeda, N.; Mates, T.; Ruths, M. Langmuir 2004, 20 (9), 3616-3622. (4) Horn, R.; Israelachvili, J. J. Chem. Phys. 1981, 75 (3), 1400-1411. (5) Gee, M.; McGuiggan, P.; Israelachvili, J.; Homola, A. J. Chem. Phys. 1990, 93 (3), 1895-1906. (6) Gao, J.; Luedtke, W.; Landman, U. Phys. ReV. Lett. 1997, 79 (4), 705-708. (7) McGuiggan, P. M.; Israelachvili, J. J. Mater. Res. 1990, 5 (10), 22322243. (8) Israelachvili, J.; Maeda, N.; Rosenberg, K.; Akbulut, M. J. Mater. Res. 2005, 20, 1952-1972.
Figure 1. A compilation of Zhu & Granicks’ oscillatory force data (2), our current data (b), Manfred Heuberger’s data10 (9), and previously published data (4, 0, O) for OMCTS between mica surfaces.4,17,18 Filled symbols: mica prepared without using Pt. Open symbols: mica prepared with hot Pt wire cutting. Dashed lines/ curves indicate unstable positive slope regimes where the forces cannot be measured. There is an uncertainty (systematic error) of one oscillation when comparing results between difference workers due to the different methods used to defined D ) 0. The random errors (shown by the crosses) are usually much smaller than the systematic error or uncertainty. Inset: our results, shown in greater detail, with error bars for the forces and distances, based on 3 experiments at a total of 7 different contact positions. Surface radius: R ∼ 2 cm; force-measuring spring stiffness: 160-1360 N/m. We used various approach and separation velocities, down to ∼0.1 nm/sec just prior to the jumps (“in” from the maxima and “out” from the minima) until the measured forces no longer changed. The apparatus chamber was sealed and dried with P2O5, and the forces did not change with time (over 3 h). The force curve measured by Heuberger9 (9) was recorded at a fixed approach speed of 0.5 nm/sec and a separation speed of 2.5 nm/sec using an automated fringe detection and force-distance analysis program.
1, a fixed-speed motor drive was used on both approach and separation. This can artificially enhance any repulsive forces on approach and any attractive forces on separation due to hydrodynamic effects.11 Spurious adhesive jumps out can thus be measured.11-13 We also mention that both flipped and nonflipped mica sheets were used before they were silvered (where the latter is required to obtain commensurate surfaces7), resulting in small effects that could be related to incommensurability. Salmeron et al.,14 who developed the Pt-free tape-cleaving method of preparing mica surfaces, also measured forces across OMCTS and reported that they were similar to previous measurements on Pt-cut mica. (9) Luan, B. Q.; Robbins, M. O. Nature 2005, 435 (7044), 929-932. (10) Heuberger, M. Materials Department, ETH, Zurich. Private communication, 2005. (11) Francis, B. A.; Horn, R. G. J. Appl. Phys. 2001, 89 (7), 4167-4174. (12) You, L. C.; Kuhl, T.; Israelachvili, J. Wear 1992, 153 (1), 31-51. (13) Gourdon, D.; Israelachvili, J. Comment on “Superlubricity: a paradox about confined fluids resolved”. Phys. ReV. Lett., in press. (14) Frantz, P.; Salmeron, M. Tribol. Lett. 1998, 5 (2-3), 151-153.
10.1021/la051470f CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
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Regarding theory, Z&G claimed that the unusually symmetric maxima and minima of their force-distance profile (between two curVed surfaces) are expected theoretically, referring to a number of papers. But none of these papers show or mention force profiles between either curved surfaces or across OMCTS. The only simulation across OMCTS is the molecular dynamics (MD) simulation of OMCTS films between two planar surfaces by Gao et al.6 Now, the Derjaguin approximation16 relates the force F(D) between curved surfaces at a separation D to the energy E(D) between two planar surfaces by F(D)/R ) 2πE(D). However, the MD simulation of the energy E(D) between two planar surfaces (Figure 3a in ref 6) exhibits a rapid and highly nonsymmetrical decay with distance, with a range of no more than 3.0 nm, and with no adhesive minima (E < 0) beyond 1.5 nm or 2-3 molecular layers. This is in contrast to the forcedistance profile in Figure 1 of ref 1, which shows a symmetrical linear decay for both the maxima and minima extending out to >10 nm, or “10-15 molecular dimensions”. The claim by Z&G that their force curve is in better agreement with theory than previous studies is therefore disputed. Zhu and Granick1 also obtained “unprecedented low friction” that was too small to be measured, even for OMCTS films as thin as 1 nm. They attributed this ultralow friction to the cleanliness and greater smoothness of their surfaces. However, it is important to point out that Z&G did not measure friction forces, but the low-strain nudging of molecules about their equilibrium positions. To obtain steady-state friction conditions often requires that surfaces be sheared tens or hundreds of micronssmuch more than the size of the confined molecules.15 In the shearing experiments on OMCTS films by Z&G, the strain amplitude was less than 30%, corresponding to less than 0.3 nm per molecule, at which point the (sinusoidal) motion was reversed. The confined molecules were probably never driven out of their potential wells, which could explain the ultralow shear forces observed. Indeed, Z&G mention that they “observed similar enhanced friction”, that is, similar to what was previously measured “under the concomitant action of large-amplitude shear”, but neither give quantitative details nor show any friction traces. The numerous oscillations in their normal forces (Figure 1) “suggesting that the structure may be crystalline or partly crystalline” also appear to be inconsistent with the high fluidity of even their thinnest layers. Our own repeat experiments on the friction forces between Pt-free mica separated by 2, 3, and 4 molecular layers are shown in Figure 2, together with some typical friction traces. The magnitude of the friction forces is found to be quantized with the number of layers, as previously found for OMCTS and other liquids.5,7,17 The only possibly significant difference seen when compared to the earlier experiments is the absence of stick-slip, compared to the small stick-slip (an amplitude of ∼10% of the friction force) measured in ref 5. This is probably due to the ∼3 times stiffer friction force-measuring spring used in our current experiments (104 N/m rather than 3.5 × 103 N/m in the previous studies),19 or to a genuine effect arising from a submonolayer of Pt atoms in the mica surface lattice in the previous experiments, sitting in the holes vacated by half of the potassium ions during cleaving. Friction forces are known to be much more sensitive (15) Drummond, C.; Israelachvili, J. Macromolecules 2000, 33 (13), 49104920. (16) Christenson, H.; Horn, R.; Israelachvili, J. J. Colloid Interface Sci. 1982, 88 (1), 79-88. (17) Kumacheva, E.; Klein, J. J. Chem. Phys. 1998, 108 (16), 7010-7022. (18) Christenson, H. J. Dispersion Sci. Technol. 1988, 9 (2), 171-206. (19) Rabinowicz, E. Friction and Wear of Materials, 2nd ed.; Wiley-Science: New York, 1995. Figure 4.44 on page 110 shows that a change in k by a factor of 3 can change the critical velocity for stick-slip by an order of magnitude.
Comments
Figure 2. Friction force, F|, as a function of sliding speed, V, in dry OMCTS between two Pt-free mica surfaces in dry nitrogen atmosphere. The crystallographic axes of the two mica sheets were not aligned. Only smooth sliding friction was observed at all the speeds studied. As the load was increased, the friction force increased, and the number of OMCTS layers, n, decreased from n ) 4 to n ) 1 layers. The data for each n were recorded at constant load as V was changed. Inset: Measured friction traces for an OMCTS film between two mica surfaces at two different crystallographic twist angles of θ ) 90 ( 1° and θ ) 0 ( 1° at about a 35 mN load (the method of alignment was similar to that described in ref 7), based on two experiments at a total of four different contact positions. Note the stiction spikes on start-up for aligned sheets and their absence for nonaligned sheets.
to the fine sub-angstrom structure of a surface than adhesion and solvation forces.8,9 In conclusion, we find that neither the oscillatory forces nor the magnitude of the steady-state friction or shear forces are affected by preparing mica in a different way from those previously used by most researchers, and therefore disagree with the claim1 that there is a need for any reassessment of the solidification in fluids confined between solid surfaces. The results of Z&G,1 especially the deep adhesive minima out to 15 molecular dimensions, are likely to be due to instrumental limitations of their apparatus that do not allow for quasi-static measurements to be made except by relying on uncontrollable and unquantifiable thermal drift. Our results, together with those of others using both Pt-cut and Pt-free mica cutting, display (1) similar asymmetric oscillations that extend no more than 8-10 molecular layers, and (2) high, quantized friction forces at discrete numbers of molecular layers. Acknowledgment. We are grateful to Manfred Heuberger for providing the normal force run data shown in Figure 1. This work was supported by DOE grant DE-FG02-87ER45331. Jacob Israelachvili,*,† Nobuo Maeda,‡ and Mustafa Akbulut†
Department of Chemical Engineering, UniVersity of California, Santa Barbara, California 93106, and Department of Applied Mathematics, Research School of Physical Sciences, Australian National UniVersity, Canberra, ACT 0200, Australia ReceiVed June 3, 2005 In Final Form: August 25, 2005 LA051470F
