7032
Langmuir 2007, 23, 7032-7038
Influence of Cyclohexane Vapor on Stick-Slip Friction between Mica Surfaces Satomi Ohnishi,*,†,‡ Daisaku Kaneko,†,§ Jian Ping Gong,§ Yoshihito Osada,§ A. M. Stewart,† and Vassili V. Yaminsky†,‡,| Research School of Physical Sciences and Engineering, Australian National UniVersity, Canberra, A.C.T. 0200, Australia, Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes, SA 5095, Australia, DiVision of Biological Sciences, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan, and AdVanced Technologies Center, Moscow, Russia ReceiVed NoVember 8, 2006. In Final Form: February 9, 2007 Stick-slip friction between mica surfaces under cyclohexane vapor has been investigated with the Surface Force Apparatus. The dynamic shear stress decreased from 60 to 10 MPa with increasing relative vapor pressure (rvp) from 5% to 50%. Between a rvp of 50% and 80%, the shear stress remained at ∼10 MPa, with a slight decrease on increasing the rvp. At a rvp greater than 80%, the values of shear stress were below 5 MPa. The stick-slip behavior was observed in the rvp range of 20% to saturation. When the rvp reached 20%, stick-slip appeared but faded out with sliding time. At a rvp greater than 50%, the stick-slip pattern was stable without fading. By taking into account the size of the meniscus formed by capillary condensation of the liquid around the contact area and the Laplace pressure, the dependence of shear stress and the stick-slip modulation on rvp suggests that the origin of the stick-slip observed in cyclohexane vapor is as follows: At a rvp greater than 50%, where stable sick-slip is observed, the stick-slip caused by the cyclohexane layering in the contact area is of essentially the same origin as that observed with mica surfaces sliding in bulk cyclohexane liquid. As with the bulk liquid experiment, decreasing the layer thickness (or the number of the layers) between the surfaces increases the shear stress at the onset of slip. In the vapor phase experiments, the stick-slip is enhanced by the increase of the negative Laplace pressure in the capillary condensed liquid, thereby forcing the surfaces toward each other more strongly with decreasing rvp. In the rvp range between 20% and 50%, where the fading stick-slip is observed, the condensate liquid seeps into the contact area under the influence of the applied tangential force and thus triggers the slip motion. Due to the small condensation volume, the liquid condensed around the contact area is exhausted in the process of repeating stick-slip. As the slip length is limited to the meniscus size, the stick-slip amplitude becomes smaller, and eventually the surfaces start sliding without stick-slip.
Introduction We have previously investigated the humidity dependence of interfacial friction between molecularly smooth mica surfaces and showed that capillary condensation and adsorption of water vapor on the mica surfaces noticeably affect the shear stress and the associated frictional features.1 We also reported that dynamic friction exhibited stick-slip behavior while the humidity was decreasing.1 Since the saturated vapor pressure of many organic solvents (ethanol, chloroform, and so forth) is higher than that of water, condensation and adsorption of the organic liquids proceed more rapidly than in the case of water.2,3 Therefore, one might expect a stronger influence of condensation and adsorption on the friction of the surfaces in vapors of these liquids. In this paper, we report a study of how the frictional behavior of mica surfaces in cyclohexane vapor depends on the relative vapor pressure (rvp). Friction between mica surfaces in bulk cyclohexane liquid has already been studied by several researchers,4-6 and detailed results regarding the role of layering of cyclohexane molecules * To whom correspondence should be addressed. Telephone: +61 8 8302 3493. Fax: +61 8 8302 3755. E-mail:
[email protected]. † Australian National University. ‡ University of South Australia. § Hokkaido University. | Advanced Technologies Center. (1) Ohnishi, S.; Stewart, A. M. Langmuir 2002, 18, 6140. (2) Ohnishi, S.; Yaminsky, V. V. Langmuir 2002, 18, 5644. (3) Kohonen, M. M.; Maeda, N.; Christenson, H. K. Phys. ReV. Lett. 1999, 82, 4667. (4) Homola, A. M.; Israelachvili, J. N.; Gee, M. L.; McGuiggan, P. M. J. Tribol. 1989, 111, 675.
in the frictional properties accompanied by stick-slip have been reported.6,7 Considering these results of the bulk liquid experiments, we discuss the differences between the observed friction patterns in cyclohexane vapor and those in liquid cyclohexane. Experimental Section Measurements. Friction force measurements were carried out with the Surface Force Apparatus (SFA, Mark IV, Australian National University) with the friction attachment, which has been described in detail previously.1,4,8 Some measurements on adsorbed film thickness between mica surfaces and the height of capillary condensation were carried out with the SFA Mark Π (Ian Wark Research Institute, University of South Australia).9 Thin mica sheets for interferometry were prepared by cleaving and silvering the back sides of the mica surfaces and then gluing them with symdiphenylcarbazide (Aldrich Chemical Co.) onto supporting silica disks installed in the usual geometrical configuration of the SFA. The lower surface is mounted horizontally on a double cantilever spring of 250 N/m stiffness. The other end of the spring is connected to a base which can be moved vertically; in this way, the normal load can be controlled, and the pull-off force can be measured. The upper mica surface is attached via another cantilever, a double stainless steel spring of 1000 N/m stiffness, to a horizontal translation stage driven by a motor at a constant speed of 0.2 µm/s except for (5) Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N.; Homola, A. M. J. Chem. Phys. 1990, 93, 1895. (6) Kumacheva, E.; Klein, J. J. Chem. Phys. 1998, 108, 7010. (7) Tamura, H.; Yoshida, M.; Kusakabe, K.; Young-Mo, C.; Miura, R.; Kubo, M.; Teraishi, K.; Chatterjee, A.; Miyamoto, A. Langmuir 1999, 15, 7816. (8) Parker, J. L.; Christenson, H. K.; Ninham, B. W. ReV. Sci. Instrum. 1989, 60, 3135. (9) Briscoe, W. H.; Horn, R. G. Langmuir 2002, 18, 3945.
10.1021/la0632732 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
Stick-Slip Friction between Mica Surfaces
Langmuir, Vol. 23, No. 13, 2007 7033
Figure 1. Typical shear stress-displacement curves for mica surfaces at different relative vapor pressures (rvps) of cyclohexane vapor, with a sliding velocity of 0.2 µm/s. Fpull-off shows the measured pull-off force normalized by the mean radius of the surface curvature. Freshly cleaved mica surfaces were used for measurements in each condition. The surfaces used in the measurements were obtained from a single cleavage of the mica. measurements with a changing sliding velocity as shown in Figure 5. Four strain gauges, each of 350 Ω resistance, are glued to the beams of the friction spring and are electrically connected in the form of a Wheatstone bridge. The bridge is driven by a voltage source at a frequency of 260 Hz, and the out-of-balance voltage is detected with a PAR 5210 lock-in amplifier (EC&G Princeton Applied Research). Any shear force that results from friction between the surfaces gives rise to a bending of the spring attached to the translation stage and thence to an out-of-balance electrical signal from the bridge which is proportional to the force. The horizontal position of the translation stage is measured with an electrical encoder, and the data are recorded as frictional force against stage position. Data are collected with a computer-simulated x-y recorder.10 The fringe pattern arising from the interference of white light between the silvered mica surfaces was continuously monitored, with the setup ensuring that the pattern remains stationary (with stick-slip motion in some cases) in the field of view while the upper surface moves. In this way, the radius (typically 25 µm) of the contact spot between the surfaces could be measured at any time, and any change or damage to the mica could be identified immediately. The shear stress acting during the sliding was calculated by dividing the measured frictional forces by the measured contact area, using the contact radius observed by interferometry. All measurements, unless particularly described, were carried out on mica surfaces at zero external normal load at a temperature of 25 ( 0.2 °C. The relative vapor pressure of cyclohexane in the SFA chamber was changed by vaporizing measured amounts of cyclohexane into the sealed chamber. The inside of the chamber was dried by purging nitrogen, dried beforehand with calcium hydride (CaH2, Fluka AG) and phosphorus pentoxide (P2O5, Aldrich). High rvps of cyclohexane in the SFA chamber were monitored by measuring the condensation height detected with the interference fringes,11 while low rvps were estimated from the introduced amount of cyclohexane. Errors in the rvp determination are (5-10% as confirmed with the interference fringes measurements at higher rvp. Materials. Cleaved sheets of 2-4 µm thickness Brown muscovite mica (Brown Mica Co., Sydney Australia) were prepared in a laminar flow cabinet (humidity, 30-40%; temperature, 23 ( 2 °C) in the standard manner12 and then stored adhesion-sealed in a desiccator (10) Stewart, A. M. Meas. Sci. Technol. 2000, 11, 298. (11) Curry, J. E.; Christenson, H. K. Langmuir 1996, 12, 5729.
under a vacuum of 10-2 Torr with silica gel. All the mica surfaces were used for measurements within 2 weeks of preparation. The mica surfaces used in a particular series of experiments were cut out of the same cleavage mica sheet. New pairs of mica surfaces were used for measurements at each rvp of cyclohexane. The reversibility of the vapor effects on frictional behavior was examined by the in situ changing of the rvp of cyclohexane. Cyclohexane (Fluka, analytical grade) was stored to remove trace water, with 4A molecular sieves under dry filtered nitrogen.
Results General Features of the Stick-Slip Observed under Cyclohexane Vapor. Figure 1 shows the frictional behavior of mica surfaces at different values of relative vapor pressure of cyclohexane. Here, we define the yield shear stress maximum observed before the surfaces start sliding (the first peaks in Figure 1) as the static shear stress, and the shear stress observed while the surfaces are sliding as the dynamic shear stress. With stickslip motions, we take the smaller value of shear stress (observed at the foot of the peaks) as the dynamic shear stress. The pattern observed at a rvp lower than 10% was similar to that in water vapor with a relative humidity below 15%.1 At a rvp lower than 5%, the mica surfaces did not slide, and they were scratched1,13 after the static shear stress reached ∼100 MPa. At 5% and up to ∼20% rvp, the surfaces slid continuously with a shear stress of 30-60 MPa after the static shear stress reached a slightly greater value than the dynamic shear stress. In this vapor condition, the shear stress decreases much more on increasing the rvp than that at a rvp higher than 40% (Figure 2). The pull-off force (normalized by the radius of curvature) of the mica surfaces measured after sliding was 960 ( 50 mN/m at ∼10% rvp, which is comparable to that in water vapor with a relative humidity lower than 15%. (As shown earlier,1 pull-off forces measured with no prior sliding after contacting the surfaces can be different according to the different conditions of vapor adsorption in the contact). (12) Ohnishi, S.; Hato, M.; Tamada, K.; Christenson, H. K. Langmuir 1999, 15, 3312. (13) Bailey, A. I.; Courtney-Pratt, J. S. Proc. R. Soc. London, Ser. A 1955, 227A, 500.
7034 Langmuir, Vol. 23, No. 13, 2007
Figure 2. Dependence of dynamic shear stress on the relative vapor pressure (rvp) of cyclohexane. The mica surfaces were obtained from one mica block. The inset shows a plot of dynamic shear stress against capillary force. Capillary forces were calculated from eq 2. The parameters for the calculations were taken from the interference fringe images observed at each condition.
At a rvp of ∼30%, the surfaces were sliding at a shear stress of 15-30 MPa. Sick-slips were occasionally observed at the beginning of the sliding, but the oscillation “faded” with sliding time (the slip distance decreased for repeating slips). It was observed that the shear stress started increasing when the stickslips disappeared. At a rvp higher than 50%, the surfaces slid while possessing a stable stick-slip motion without fading. The shear stress at a rvp between 50% and 80% was ∼10 MPa in most cases and was less dependent on the rvp. At a rvp higher than 80%, the shear stress decreased to 1-2 MPa while retaining stick-slip behavior. The after-sliding pull-off force for mica surfaces decreased to 300 mN/m, which corresponds to the capillary force of cyclohexane (F/R ) 4πγcyclohexane). When the height of the liquid condensate around the contact area grew to over 30 nm, the shear stress further decreased to less than 0.1 MPa. Stable stick-slip with further reduced amplitude was also observed under saturated vapor. Dependence of Dynamic Shear Stress on Relative Vapor Pressure. The dependence of the dynamic shear stress on the rvp of cyclohexane is shown in Figure 2. The shear stress in sliding decreases as the rvp increases. The cyclohexane molecules on the mica surfaces effectively work as a lubricant. The character of the stick-slip also varies on changing the rvp. According to the observed frictional behavior, the rvp scale can be subdivided into four regions, namely, a wear region (at 0% rvp), a no stickslip region, a fading stick-slip region, and a stable stick-slip region. The no stick-slip region, where continuous sliding without stick-slip is observed, extends up to a rvp of ∼20%. The fading stick-slip region, a transitional state from the continuous, wearfree sliding to full stick-slip, occurs in the region of 20% < rvp < 50%. It appears that the shear stress in this region decreases linearly with increasing rvp. The stick-slip pattern stabilizes at a rvp higher than 50%. The shear stress tends to decrease stepwise: 10 MPa at 50-80%, 1-2 MPa at 80-95%, and
