Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J39: Focus Session: Friction and Contact |
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Sponsoring Units: DMP GSNP Chair: Mark Robbins, Johns Hopkins University Room: Morial Convention Center 231 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J39.00001: A rigorous approach to the contact mechanics of rough, elastic solids Martin Muser The basic ideas of a statistical field theory is presented, which allows one to calculate the displacement field and the pressure distribution $\Pr(p)$ in a contact formed by an elastic body and a rigid counter face of arbitrary topography. The theory is a cumulant expansion, which contains Persson's contact mechanics theory as the leading-order term. The cumulant approach provides a framework with which corrections can now be systematically derived. Comparison is made to numerical data for surfaces that interact via exponentially repulsive forces. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J39.00002: Molecular dynamics study of contact mechanics: contact area and interfacial separation from small to full contact Chunyan Yang, Bo Persson We report a molecular dynamics study of the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. We study the contact area and the interfacial separation from small contact (low load) to full contact (high load). For small load the contact area varies linearly with the load and the interfacial separation depends logarithmically on the load [1-4]. For high load the contact area approaches to the nominal contact area (i.e., complete contact), and the interfacial separation approaches to zero. The present results may be very important for soft solids, e.g., rubber, or for very smooth surfaces, where complete contact can be reached at moderate high loads without plastic deformation of the solids. \newline References: \newline [1] C. Yang and B.N.J. Persson, arXiv:0710.0276, (to appear in Phys. Rev. Lett.) \newline [2] B.N.J. Persson, Phys. Rev. Lett. 99, 125502 (2007) \newline [3] L. Pei, S. Hyun, J.F. Molinari and M.O. Robbins, J. Mech. Phys. Sol. 53, 2385 (2005) \newline [4] M. Benz, K.J. Rosenberg, E.J. Kramer and J.N. Israelachvili, J. Phy. Chem. B.110, 11884 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J39.00003: Atomistic effects on friction and contact area in single and multi asperity contacts Binquan Luan, Mark Robbins Contact and friction are universal phenomena in our daily life. Theoretical studies of macroscopic contact and friction are usually based on continuum theories such as Hertz theory and Amontons's laws. Recent advances in nanotechnology have stimulated research into friction at the nanometer scale where new phenomena emerge. Contact and friction in single- and multi-asperity contacts with nanometer dimensions were studied using molecular dynamics simulations (MD) and a hybrid method. The hybrid method retains a full atomistic treatment near contacts and replaces more distant regions with a more efficient finite element description. Our results demonstrate that atomic-scale changes in surface structure produce huge changes in friction and contact area and substantial deviations from the predictions of continuum theories. Unanticipated surface plasticity is observed near peaks on crystalline surfaces. In the case of multiasperity amorphous systems, the rate of local plastic deformation near the surface is directly related to the frictional dissipation of energy. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J39.00004: Local contact stress measurements at a rough interface Julien Scheibert, Alexis Prevost, Eytan Katzav, Mokhtar Adda-Bedia, Georges Debr\'egeas, Joel Frelat An original MEMS-based force sensing device has been designed. It allows for spatially resolved measurements of both normal and tangential stress fields at the base of an elastomeric film in contact with a rigid substrate [1]. Model contact geometries involving a rough, nominally flat film pressed against smooth spherical and cylindrical glass substrates have been studied, in two different regimes, normal indentation and steady sliding. The measured stress profiles have been compared to calculations which assume a smooth contact obeying Amontons-Coulomb's friction law. For the normal indentation a Finite Elements method was used, whereas for the sliding regime a semi-analytical model was developed. These direct comparisons showed that our device was accurate enough to discriminate between dry and lubricated contact conditions and evidenced load-dependent deviations from Amontons-Coulomb's profiles. These deviations are qualitatively interpreted by taking into account the finite compliance of the contacting micro-asperities population. [1] J. Scheibert\textit{ et al}., arXiv:0711.1117v1 [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J39.00005: Friction induced displacement and stress fields within contacts with elastomers Antoine Chateauminois, Christian Fretigny Friction is known to be associated with strongly heterogeneous stress and displacement fields within the contact zone. However, experimental approaches are often based on the measurement of friction forces (or mean shear stress), which makes difficult a detailed analysis of interface dynamics within sliding contacts. We have developed a new methodology for the determination of the interface shear stress distribution within macroscopic sliding contacts. It is based on an in situ measurement of the displacement field induced at the surface of highly deformable solids such as elastomers. An inversion of this field using contact mechanics models then provides the interface shear stress distribution. The experiments were carried out using two different contact configurations. The first one involves the linear sliding of a glass sphere on the elastomer substrate. The second one corresponds to an original torsional contact configuration which minimizes bulk viscoelastic dissipation during steady state sliding. Experimental distributions of frictional shear stress will be discussed in the light of theoretical models assuming either a constant interface shear stress (Tabor's model) or a local Coulomb's friction law. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J39.00006: Optical measurements of pressure and displacement fields at a rough interface Alexis Prevost, Julien Scheibert, Georges Debr\'egeas We report on optical measurements of both pressure and displacement fields at the interface between a rough, nominally flat transparent elastomeric film and a smooth spherical glass lens. The multi-contact interface is imaged by transmission and the pressure field is deduced from the spatial distribution of the transmitted light. The displacement field is obtained using Digital Image Correlation, allowing for a submicron resolution. For normal loading, the radial pressure profiles deviate from Hertz theory, as expected for a rough interface. A good quantitative agreement has been obtained within the statistical description of a rough sphere-on-plane contact by Greenwood and Tripp. When the interface is tangentially loaded below the macroscopic sliding threshold, analysis of the displacement field has shown a coexistence between an inner stuck region and an outer slipping annulus, as suggested by Catteneo and Mindlin. Quantitative comparison with this model yields a good overall agreement. However, small deviations are observed and can be related to the tangential compliance of the rough layer. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J39.00007: Beller Lectureship Talk: Crack-like processes govern the onset of frictional motion Invited Speaker: The dynamics of frictional slip have been studied for hundreds of years, yet many aspects of these everyday processes are not understood. One such aspect is the onset of slip. First described by Coulomb and Amontons as the transition from static to dynamic friction, the onset of frictional slip is central to fields as diverse as physics, tribology, the mechanics of earthquakes and fracture. We study the dynamics of how this transition takes place by performing real-time visualization of the \textit{true} contact area which forms the interface separating two blocks of like material. The results show that the onset of frictional motion is driven by the interplay of three different types of coherent crack-like fronts, which propagate along the interface, reducing the contact area as they progress. Two of these, whose propagation speeds are, respectively, slightly below and significantly above the shear wave velocity, appear to be related to known propagation modes of shear cracks. The third type of front does not correspond to known fracture modes. It propagates over an order of magnitude more slowly, and is the most efficient of the three modes in reducing contact area along the interface. We first show that, at applied stresses that are well below the (Coulomb-Amontons) threshold for the onset of frictional motion, significant precursor activity occurs along the interface. This activity is comprised of propagating (subsonic) shear cracks which arrest before traversing the entire interface. In their wake, these ``precursor'' cracks systematically transform the intial spatially uniform contact area along the interface to a highly nonuniform one. \textit{Only} at the transition to overall motion will these precursor cracks simultaneously excite, at their point of arrest, both the slow propagation modes and the intersonic ones. Until to this point, no overall frictional motion occurs. Frictional sliding only takes place when either the slow modes or additional shear cracks excited by the slow modes traverse the entire interface. These results suggest that to understand the transition to frictional motion, the dynamics of this entire chain of events must be taken into account. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J39.00008: Precursor events and the onset of frictional sliding Andras Libal, Mark Robbins The precursor events leading to steady state sliding friction are investigated using a simple two-dimensional model of a rectangular block on a flat surface. As in experiments[1], a succession of cracks nucleates from the rear of the block. Each propagates rapidly and then arrests after a distance that scales with the height at which the lateral force is applied to the block. The propagation distance grows with each successive crack until a steady sliding state is attained. The distribution of local shear stress at the interface can be obtained directly in our simulations. The relation between this stress distribution, the static friction, the normal load, and the nucleation and propagation of successive cracks will be discussed. [1] S.M. Rubinstein, G. Cohen and J. Fineberg, PRL 98, 226103 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J39.00009: The Duality of Nanoscale Friction: Amontons' Law vs. Superlubricity U. D. Schwarz, D. Dietzel, C. Ritter, T. Monninghoff, H. Fuchs, A. Schirmeisen One of the most fundamental questions in nanotribology is the contact area dependence of frictional forces on the nanometer scale. Unfortunately, conventional friction force microscopy techniques are limited for analyzing this problem due to the unknown and ill-defined tip-sample contact. This limitation can be circumvented by measuring the lateral force signal during the manipulation of nanoscale particles with a well-defined, clean contact to the substrate. In our study, the samples under investigation were metallic islands with diameters between 50-500 nm grown by thermal evaporation of antimony on highly oriented pyrolytic graphite (HOPG). Experiments that included the controlled manipulation of a large number of nanoparticles in ultrahigh vacuum show two distinct frictional states during particle sliding: While some particles show finite friction increasing linearly with interface area, thus reinforcing Amontons' law at the nanoscale, other particles assume a state of frictionless or `superlubric' sliding. Additional experiments revealed a similar result even in air, which can be explained by contamination effects of the interface that alter the frictional properties. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J39.00010: Dragging shadows causes real friction: sliding Moire' patterns A. Vanossi, C. Negri, N. Manini, G.E. Santoro, E. Tosatti Surface Moire' patterns are shadow-like modulations (kinks) which form at crystalline overlayers that are out of registry with their substrates. They were hardly considered in the context of friction so far; we here argue that they can be relevant. 1D model calculations suggest in fact that under the action of an external slider, the kinks are the real objects being rigidly dragged, as opposed to the real particles, which are not [1]. For a crystalline periodic slider, we predict peculiar phenomena on the fly caused by the pinning/depinning of the kink lattice to the slider, in full analogy with the well known real lattice static counterpart [2]. The frictional dissipation by a vibrating and/or sliding AFM probe should moreover be enhanced at the kinks, where atoms take poorly stable positions. Thus, AFM frictional maps [3] should reveal with much more contrast the Moire' patterns than topographic maps of the same patterns. This concept is demonstrated by means of a simple model, which also provides a guide to the key parameters determining the enhancement. [1] A. Vanossi et al., PRL 97, 056101 (2006). [2] A. Vanossi et al., PRL 99, 206101 (2007). [3] C. Loppacher et al., PRB 62, 13674 (2000). [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J39.00011: Contribution of Plowing to Nanotribology of Self-Assembled Monolayers Michael Chandross, Christian Lorenz, Gary Grest, Erin Flater, Robert Carpick Atomic force microscope experiments and molecular dynamics (MD) simulations on self-assembled monolayer (SAM) systems have demonstrated that the nanotribology of these systems may be dominated by a microscopic plowing mechanism. Due to relatively weak chain-to-chain interactions, compression only affects molecules directly under the probe tip, and not those outside the contact area. Under shear, the tip must plow into the molecules in front leading to frictional energy dissipation. We will present the results of coupled experiments and MD simulations of alkylsilane SAMs studying the plowing mechanism in detail. In particular, combinations of uncoated and SAM-coated substrates and tips are studied to probe the relationships between friction force and both contact area and applied load. As a SAM coating on the substrate (tip) is (is not) expected to result in plowing during shear, the contrast in these results, combined with detailed calculations using the MD results, will shed light on the complicated response of these systems. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J39.00012: Accelerated Molecular Dynamics Simulation of AFM Experiments Using the Bond-Boost Method Woo Kyun Kim, Michael Falk We apply an accelerated molecular dynamics (MD) methodology to simulate the friction between a silicon tip and a silicon surface under perfect vacuum conditions. These simulations model recent Atomic Force Microscope (AFM) experiments which observed stick-slip motion and a lateral force showing dependences on temperature and sliding velocity. Our AFM models consist of crystalline silicon with an oxidized layer. We achieved the decrease in the simulated sliding velocity by several orders of magnitude compared with conventional MD simulations using the bond-boost method. This method is based on Voter's hyper molecular dynamics scheme accelerating the process between slip events. The decrease in the sliding velocity makes it possible to simulate systems closer to the regime of the actual experiments. We compare the simulation results with the experimental data to elucidate the atomic level processes during sliding. We studied the effects of atomic mass transfer between the tip and the substrate on friction. Moreover, the dependence of friction on temperature and sliding velocity has been quantified, and compared with the modified Tomlinson model. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J39.00013: Measurement of lateral tip-sample forces in the attractive regime with picometer resolution in three dimensions B.J. Albers, T.C. Schwendemann, M.Z. Baykara, N. Pilet, U.D. Schwarz Three-dimensional (3D) dynamic force spectroscopy, i.e., the acquisition of frequency shift vs. distance curves in a dense raster over a surface in order to recover the true tip-sample interaction forces with high local resolution, has so far suffered from relatively low resolution, as long-term drift stability has been a problem. Nevertheless, its promise to deliver not only the normal forces with atomic resolution, but also the lateral forces as well as the energy dissipated during an individual oscillation cycle makes it interesting for high-resolution nanotribology. Using our recently completed home-built low temperature, ultrahigh vacuum NC-AFM, we were able to map the full 3D force field over highly oriented pyrolytic graphite, which was chosen due to its qualities as a solid lubricant. Lateral forces have been measured quantitatively in a grid with spacing better than 6 pm in all three directions and pN resolution. We will discuss the distance-dependence of the static lateral forces, their local distributions with regard to the underlying lattice, as well as influences of the tip shape. [Preview Abstract] |
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