Langmuir 1999, 15, 2217-2223
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Friction of the Liquid Crystal 8CB As Probed by the Surface Forces Apparatus Alexander Artsyukhovich,† Leonard D. Broekman, and Miquel Salmeron* Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received April 10, 1998. In Final Form: August 3, 1998 The surface forces apparatus (SFA) was used to study the lubricating properties of an 8CB liquid crystal film confined between mica surfaces. After displacement of most of the liquid crystal by compression, a 10.2 Å thick layer of 8CB was found between mica surfaces in the range of applied pressures from 1 to 20 MPa. Under these conditions, we found that friction depends linearly on contact area in the 1-1.5 MPa range. We found that the critical shear stress was anisotropic, with values of 0.28 MPa for shear along the 8CB molecular axis and 0.42 MPa for shear in the perpendicular direction. This anisotropy was observed through all the ranges of load where slip occurred. At higher loads (1.5-2.7 MPa), the critical shear stresses increase to 0.6 and 1.2 MPa for parallel and perpendicular shear, respectively.
Introduction The connection between tribological properties, such as friction, adhesion, and the molecular scale structure of the rubbing surfaces, is the object of study in the emerging field of nanotribology. One of the most fascinating phenomena observed in nanotribology is the anisotropy of friction1-3 and the related phenomenon of adhesion anisotropy.4 In both cases, the microscopic properties of the material, such as molecular alignment and surface crystallographic orientation, reveal themselves in a macroscopic way. The earlier predictions of friction anisotropy were made on the basis of macroscopically measured single-crystal surface friction.1 The lubricant in our studies of friction anisotropy is octylcyanobiphenyl (8CB), one of the most widely studied liquid crystals (LC). Bulk 8CB exists in three crystal phases. With increasing temperature, 8CB undergoes a phase transition at 21.5 °C from crystalline to smectic A phase (Figure 1). At 33.5 °C there is a second-order phase transition to a Nematic phase, and at 40.5 °C there is a weak first-order transition to an isotropic liquid.5 The 8CB molecules are paired into dimers (1.4 times longer than a single molecule) with antiparallel dipole moments.6 In the smectic A phase, 8CB forms layers spaced by 31.7 Å with dimers aligned perpendicular to these layers.7 Liquid crystals are known for their anisotropic properties. In the smectic A phase, 8CB is strongly birefringent with an optical axis that lies perpendicular to smectic layers.8 The refractive indices for ordinary and extraordinary waves depend on temperature and phase and are † Present address: StorMedia, Inc., 390 Reed Street, Santa Clara, CA 95050.
(1) Enomoto, Y.; Tabor, D. Proc. R. Soc. London A 1981, 373, 405. (2) Bluhm, H.; Schwarz, U. D.; Meyer, K. P.; Wiesendanger, R. Appl. Phys. A 1995, 61, 525. (3) Morita, S.; Fujisawa, S.; Sugawara, Y. Surf. Sci. Rep. 1996, 23, 3. (4) McGuiggan, P. M.; Israelachvili, J. N. Chem. Phys. Lett. 1988, 149, 469. (5) Mondain-Monval, O.; Coles, H. J.; Claverie, T.; Lalanne, J. R.; Marcerou, J. P.; Philip, J. J. Chem. Phys. 1994, 101, 6301. (6) Urban, S.; Bru¨ckert, T.; Wu¨rflinger, A. Z. Naturforsch. 1994, 49a, 552. (7) Idziak, S. H. J.; Safinya, C. R.; Hill, R. S.; Kraiser, K. E.; Ruths, M.; Warriner, H.; Steinberg, S.; Liang, K. S.; Israelachvili, J. N. Science 1994, 264, 1915. (8) Horn, R. G. J. Phys. 1978, 39, 105.
Figure 1. Temperatures of phase transitions in bulk 8CB: 21.5 °C (crystal to Smectic A); 33.5 °C (smectic A to nematic); 40.5 °C (nematic to isotropic). The alignment of dimers is shown schematically. The dimensions of the 8CB dimer are shown below.
1.51 and 1.68 respectively at λ ) 633 nm and 23 °C.8 In our experiment, optical anisotropy provides a convenient way to determine molecular orientation. 8CB also displays significant anisotropy in heat conductivity (by a factor of 2)9 and viscosity.10 An added complication is that viscosity measurements have demonstrated a dependence of 8CB orientation on shear rate.10-12 At low shear rates (250 mm) and the director (long molecular axis) is parallel to the shear direction.10,12 At high shear rates (>300 s-1) or elevated temperatures, 8CB reorients so that its director is perpendicular to the flow direction.11,12 This has also been demonstrated by X-ray diffraction performed in situ with SFA.13 When confined between clean mica surfaces, 8CB adopts a planar configuration in which the dimer axis is parallel (9) Schoubs, E.; Mondelaers, H.; Thoen, J. J. Phys. 1994, 4, C7. (10) Chmielewski, A. G. Mol. Cryst. Liq. Cryst. 1986, 132, 339. (11) Safinya, C. R.; Sirota, E. B.; Bruinsma, R. F.; Jeppesen, C.; Piano, R. J.; Wenzel, L. J. Science 1993, 261, 588. (12) Panizza, P.; Archambault, P.; Roux, D. J. Phys. II 1995, 5, 303. (13) Idziak, S. H. J.; Koltover, I.; Israelachvili, J. N.; Safinya, C. R. Phys. Rev. Lett. 1996, 76, 1477.
10.1021/la980415m CCC: $18.00 © 1999 American Chemical Society Published on Web 02/26/1999
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to the mica surface due to strong anchoring.13,14 On surfaces that interact weakly with 8CB, such as nylon, octadecyltriethoxysilane (OTS), cadmium arachidate14 treated mica, or at the air interface, the molecules align perpendicularly to the surface (homeotropic configuration). The oscillatory surface potential measured for 8CB14 shows a corresponding dependence on the surfaces used in SFA experiments. For OTS-coated mica, oscillation periods of 31-32 Å have been measured, corresponding to the length of homeotropic 8CB dimers, and have been observed to extend more than 600 smectic layers.15 In its planar configuration, 8CB shows several oscillations with a period of ∼10 Å, which is assumed to correspond to the diameter of the cigar-shaped 8CB dimer.16 A single orientational domain 3.4 mm in diameter has been reported for 8CB confined between mica surfaces.13 In a previous study by Ruths et al.,14 it was shown that a shear-induced single orientational domain is established in 8CB between untreated mica surfaces at separations up to ∼0.5 mm. They observed a smooth, “liquidlike” sliding down to a film thickness of 16-17 Å. In the present study we performed experiments using the same system with particular focus on the orientational properties of the film relative to the mica surfaces and to the sliding direction. As we shall see, the frictional behavior of the film is anisotropic and provides evidence of the strong relationship between the structure and orientation of molecular films and their lubricating properties.
Artsyukhovich et al.
Figure 2. Schematic depiction of our SFA setup. The upper surface is mounted on two piezo bimorphs for friction measurements. The lower surface is mounted on a double cantilever with a magnet to apply normal load.
Experimental Section To obtain atomically smooth contact areas, muscovite mica sheets ∼5 mm thick (Mica New York Corp., Ruby V-4) were glued (Shell, Epon 1004F) to quartz cylindrical lenses (20 mm radius of curvature) and placed in a crossed cylinder configuration. Since the two mica pieces were cut from the same sample, their relative crystallographic orientation was known and could be mounted parallel to each other to within 10°. The mica surfaces were further cleaved to
