Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N24: Focus Session: Friction, Fracture, and Deformation IV |
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Sponsoring Units: GSNP DMP Chair: Udo Schwarz, Yale University Room: LACC 411 |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N24.00001: Probing electronic friction in patterned silicon pn junctions Jeong Y. Park, D. F. Ogletree , P. A. Thiel, Miquel Salmeron Phononic and electronic excitations are two of the fundamental processes that give rise to friction forces between sliding bodies in close proximity. We demonstrate that electronic contributions in friction can be quite significant, and even tunable for semiconducting samples, where the free carrier density in the vicinity of the contact can be modified electronically. In our experiments one of the solids is a Si(100) sample patterned with p and n regions that provide large differences in the density of charge carriers. The pattern consists of an array of 2 $\mu $m wide strips of highly doped p-type material in a nearly intrinsic n-type substrate. The other solid is a conductive Atomic Force Microscope tip. By varying the bias between tip and Si sample, charge depletion or strong accumulation could be induced in the n and p regions, which produces significant differences in the friction force. We attribute the increase in friction force following charge accumulation to energy dissipation by electrons. This result demonstrates not only the importance of electronic contributions to friction, but also the capability to electronically control friction with potential applications to nanoscale devices with moving parts. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N24.00002: Simultaneous lateral force and STM imaging of Si (111) -7x7 surface using sub-Angstrom oscillation amplitude AFM Mehrdad Atabak, Goksel Durkaya, H. Ozgur Ozer, Ahmet Oral Lateral forces play an important role in friction studies as well as atomic manipulation. We present the design and performance of an nc-AFM which is capable of measuring lateral forces simultaneously with tunneling current. The microscope employs a sensitive fiber interferometer for high resolution force measurements. Home-made Tungsten cantilevers with typical stiffness of about 150 N/m is dithered in lateral directions respect to the sample with sub-{\AA}ngstrom oscillation amplitudes (A$_{0 }$=0.25 {\AA}) at a frequency, well below the resonance frequency and the changes in lateral oscillation amplitudes are recorded using a lock-in amplifier. In addition, the microscope can simultaneously be operated as STM. By changing the tunneling current and bringing the tip closer to the surface, we investigate the lateral forces during STM imaging. The lateral force images will be presented as a function of tunnel current (relative tip-sample distance) on Si(111) (7$\times $7) surface. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N24.00003: Energy loss from repulsive contact to non-contact Ernst Meyer, Enrico Gnecco, Laurent Nony, Lars Zimmerli, Sabine Maier, Simon Rast, Urs Gysin, Patric Ruff, Roland Bennewitz Force microscopy experiments under ultrahigh-vacuum conditions are performed at separations from repulsive contact up to separations of 200nm. Energy loss at rather large separations is primarily related to the application of electrostatic fields. The relationship of adsorbates and non-contact friction is investigated. The transition to the repulsive contact is studied by the use of torsional oscillations. In the regime of repulsive contact, the important role of instabilities is confirmed. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 9:12AM |
N24.00004: Molecular Level Investigations of Interfacial Friction of Polymer Brush Surfaces Invited Speaker: The development of synthetic polymer lubricants to mimic joint lubrication within the human body will be presented. Unlike most industrial applications involving oils and greases, lubrication of these joints is accomplished in an aqueous environment. Fundamentally, water is a poor lubricant in most settings due to the weak pressure dependence of its viscosity, yet the contacting surfaces of skeletal joints function with low friction throughout a lifetime. Motivated by the molecular structure of materials making up joint surfaces, interfacial friction between polymer brush surfaces under aqueous environments has been probed with an array of molecularly sensitive surface analytical techniques including atomic force microscopy. The brush surfaces, comprised of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG), have been generated through the spontaneous adsorption of polymer from solution onto oxide substrates and sodium borosilicate surfaces (AFM tip). The character of the polymer films has been investigated in-situ with the quartz crystal microbalance (QCM) and atomic force microscope (AFM) and ex-situ with ellipsometry and X-ray photoelectron spectroscopy (XPS). The interfacial friction measurements have been carried out on polymer-coated substrates with bare or polymer-coated, microsphere-attached tips in over a range of solution conditions. It was found that the adsorption of polymer on oxides strikingly reduced the interfacial friction, resulting in ultra-low friction under certain conditions. By using a series of PLL-g-PEG polymers differing from each other in PEG side-chain length and grafting ratio, we observed that frictional properties of polymer-coated interfaces strongly depend on the architecture of PLL-g-PEG. Polymer-film formation and the influence of polymer architecture will be reviewed while the role of solvent and manifestation of ultra-low friction will be discussed in detail. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N24.00005: Tribological properties of self-assembled monolayers in humid environments Christian D. Lorenz, Edmund B. Webb III, Michael E. Chandross, Mark J. Stevens, Gary S. Grest Microelectromechanical systems (MEMS) are a rapidly growing area of technology. Due to the large surface area to volume ratio in MEMS, surface forces including friction and adhesion are tribological limitations that affect their performance. Self-assembled monolayer (SAM) coatings, which have high hydrophobicity, low surface energies and compact packing structures, are good canditates for MEMS lubrication. Large scale molecular dynamics simulations are used to study the frictional and adhesive behavior of hydrocarbon and fluorocarbon SAMs coatings on amorphous silica in the presence of water. The systems consist of SAMS with a chain length of 11 carbons physisorbed to the amorphous silica substrate with the water placed between the SAMs and the substrate. Humidity has no observed effect on the maximum adhesion of either of the SAMs. The coefficient of friction decreases with increasing water, which is in agreement with what is observed experimentally. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N24.00006: Tip-based simulations of nanotribology of self-assembled monolayers Michael Chandross, Gary S. Grest, Mark J. Stevens Friction and adhesion simulations are generally performed with opposing flat-plate geometries, ignoring the effects of load-dependent contact areas arising from curved probe tips. While some previous tip/substrate simulations do exist, they tend to either use multi-timestep approximations or unrealistically sharp tips. We present the results of true dynamical nanotribological simulations of alkylsilane self-assembled monolayers (SAMs) with realistic tip/substrate geometries. Tips matching experimental dimensions ($\sim$~30 nm radius of curvature) were cut out of an amorphous silica substrate (a-SiO$_2$) and either coated with SAMs or annealed for uncoated tips. The adhesion and friction of the tip in contact with a SAM-coated amorphous a-SiO$_2$ substrate were studied with massively parallel molecular dynamics simulations. The effects of load-dependent contact areas are compared to previous simulations with flat plate geometries, and to atomic force microscopy measurments. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N24.00007: Understanding the frictional response of organic monolayer coatings using Atomic Force Microscopy Erin E. Flater, Alex D. Corwin, Maarten P. de Boer, Robert W. Carpick Friction and wear are yet to be fundamentally understood, yet they can be major limiting factors for applications including microelectromechanical systems (MEMS). We use atomic force microscopy to determine frictional constitutive relations for nanoscale contacts designed to represent the asperities in MEMS. Quantitative measurements of friction and contact stiffness are performed using SiO$_{2}$- and organic monolayer-functionalized tips on organic monolayer-functionalized silicon. Using octadecyltrichrolosilane, octadecene, and perfluorinated monolayers, we find that friction depends on the type of molecule, its packing density, and the surface attachment chemistry. We also find that fluorination increases friction, as in MEMS, and that molecular transfer to the SiO$_{2}$ tip causes large variation in the measurements. With monolayer-coated tips, this variation, as well as the overall friction and adhesion, are significantly reduced. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N24.00008: Capillary condensation and Ice formation at room temperature in nano-friction experiments K.B. Jinesh, J.W.M. Frenken We report several direct observations of manifestations of capillary condensation in atomic-scale friction experiments. We have used a dedicated high-resolution friction force microscope to investigate the forces between a tungsten tip and a graphite surface under ambient conditions at a range of relative humidities. The velocity dependence of the friction shows a variety of new effects. We observe high friction and pronounced stick-slip instabilities with periods differing from those on graphite at very low scan velocities and moderate humidities. On the other hand, we see smooth sliding with strongly velocity dependent friction at higher humidities. We show that all aspects of the observed behavior can be interpreted in terms of capillary condensation of water, melting-freezing transitions and visco-elastic effects. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N24.00009: Nanoscopic friction as a probe of local phase transitions Elisa Riedo, Robert Szoszkiewicz Water gas-liquid phase transitions have been investigated by measuring nanoscale friction forces between an atomic force microscope tip and a glass surface while varying the relative humidity, the scanning velocity and the temperature. We observe that it is possible to obtain the same friction versus velocity curves by fixing the sample temperature and varying the buffer humidity or by fixing the buffer humidity and varying the sample temperature. This behavior can be understood by introducing the concept of local humidity at the glass surface, which depends on the temperature. By using the well known macroscopic relationship between relative humidity and temperature we can fully explain our experimental results. This finding suggests that the water gas-liquid phase diagram is the same at the macroscopic scale as well as at the nanoscopic scale at a solid-gas interface. Furthermore, friction data for varying the scanning velocity provide mean nucleation times for capillary bridges formation. These times were found to alter from 3.5 ms up to 0.6 ms for temperatures ranging from 299 K up to 332 K. Natural logarithms of nucleation times plotted against inverse of experimental temperatures produce an Arrhenius plot and give a nucleation energy of 7.8$\times$10$^{-20}$ J for a nano-sized capillary bridge formation, in agreement with recent theoretical models. Our study provides the first direct experimental evidence of the thermally activated condensation of capillary bridges at the nanoscale. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N24.00010: Superconductivity Dependent Friction M. Highland, J. Krim In order to gain a fundamental understanding of friction, one must understand, at the molecular level, how the energy associated with the work to overcome friction is converted to heat. One of the simplest possible geometries in which friction can occur, and thus be studied, is that of a fluid or crystalline monolayer adsorbed on an atomically flat surface. This geometry is experimentally accessible to experiments with a Quartz Crystal Microbalance (QCM), to numerical simulation techniques, and to analytic theory. A prior QCM experiment [1] sought to explore the nature of electronic contributions to friction by measuring the friction associated with nitrogen monolayers sliding on Pb substrates, that had been exposed to air, as the temperature passed through the superconducting transition at 7.2K. The work inspired a number of subsequent theoretical and experimental efforts, which yielded contradictory results. We have repeated these measurements on Pb substrates that were prepared \textit{in situ }for nitrogen and water films. We have observed the functional form of the rapid but smooth change in friction between nitrogen slipping on Pb near T$_{c}$. We have also observed a wider temperature range of frictional effects as compared to bulk changes in resistivity. We present these results and compare them to previous observations of superconductivity-dependent friction. [1] A. Dayo, W. Alnasrallah and J. Krim, Phys. Rev. Lett. vol 80, 1690 (1998); Work funded by NSF. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N24.00011: Origin for Static Friction at Atomic Level Studied with Molecular Dynamics Simulations Qing Zhang, Yue Qi, Louis Hector, Tahir Cagin, William Goddard Static friction has been always an interesting topic because of its ubiquitous presence in the sliding. With Molecular Dynamics simulations, we studied the static friction behavior for commensurate and incommensurate Al$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ interfaces, and flat and rough Al/Al interfaces. It is found that incommensurate Al$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ interface has lower static friction than commensurate Al$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ interface and roughness on the surface increases the static friction drastically. The relation between interfacial adhesion and friction has been investigated. Simulation results reveal that the origin of static friction is to overcome the potential barriers at the interface along the sliding distance. Static friction is determined by the amplitude of potential barrier of interfacial interaction rather than the absolute value of interfacial adhesion. The relation of static friction and kinetic friction are also discussed. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N24.00012: Theory of Lubrication Due to Collective Pinning Jeffrey Sokoloff In collective pinning theory, the problem of two three dimensional solids in contact is at its critical dimension. This implies that when the disordered forces acting between the two solids at the interface are relatively strong, the force of static friction should be large, but at smaller values of these forces, the system switches over to a regime of weak static friction. This provides a mechanism for the reduction of friction in boundary lubrication. It was shown previously that small lubricant molecules reduce static friction by filling in atomic depth holes in the surface and thus allowing the force pushing the surfaces together to be supported by more points of contact, which can switch the interface from the strong to weak static friction regime. Here it will be shown that lubricant molecules which are large compared to atomic dimensions can also put the interface in the weak pinning limit because molecules attached to high points on the surfaces can be easily compressed, allowing the load to be spread over more points of contact, and hence putting the interface in the weak pinning regime. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N24.00013: A Model for Static and Dry Friction Christopher Daly, Jeffrey Sokoloff It will be shown that the Muser-Robbins (MR) model, consisting of mobile molecules trapped between two incommensurate crystalline solids, exhibits many of the qualitative features of friction between macroscopic solids, such as the result that the static friction is greater than the kinetic friction, stick-slip motion and a force of static friction which increases as a function of the time that the two solids are in contact and stationary. At zero temperature, the kinetic friction is highly sensitive to the direction of sliding, but this sensitivity decreases markedly as the temperature rises. At low temperatures (with the surfaces stationary for a relatively long time), the model gives a static friction approximately 3 times larger than the kinetic friction for sufficiently slow velocities, but this ratio decreases steadily as the temperature is increased. [Preview Abstract] |
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N24.00014: Lack of Pinning for Rigid Sliding Monolayers in Microbalance Itzhak Webman, Jeffrey Sokoloff Recent work on the dynamics of monolayers on a metallic substrate attached to a quartz oscillator has provided interesting data on kinetic friction at the microscopic level. Sliding of the film relative to the substrate is often observed even though theoretical estimates seem to predict that the extremely small inertial forces should not be sufficient to make the film slide. It is shown here that if the defect potentials have a range of a little more than an atomic spacing, the net forces on the film due to the defects are likely to be quite small due to cancellations of the forces exerted by the defect on the atoms in the range of its potential. Thus, the net pinning force on the film is much smaller than it would be if each defect only acted on one atom at a time. It will be shown that this reduction of the pinning force due to the defects is quite significant and is able to account for the fact that films adsorbed on the quartz crystal are able to slide, even under the weak inertial forces provided by the quartz crystal's oscillations. [Preview Abstract] |
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