Search

The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly effect friction and wear. This is the case for rubber sliding on road surfaces at speeds above 1 mm s(-1). We derive equations which describe the frictional heating for solids with arbitrary thermal properties. The theory is applied to rubber friction on road surfaces and we take into account that the frictional energy is partly produced inside the rubber due to the internal friction of rubber and in a thin (nanometer) interfacial layer at the rubber-road contact region. The heat transfer between the rubber and the road surface is described by a heat transfer coefficient which depends on the sliding speed. Numerical results are presented and compared to experimental data. We find that frictional heating results in a kinetic friction force which depends on the orientation of the sliding block, thus violating one of the two basic Leonardo da Vinci 'laws' of friction.

We present accurate numerical results for the friction force and the contact area for a viscoelastic solid (rubber) in sliding contact with hard, randomly rough substrates. The rough surfaces are self-affine fractal with roughness over several decades in length scales. We calculate the contribution to the friction from the pulsating deformations induced by the substrate asperities. We also calculate how the area of real contact, A(v, p), depends on the sliding speed v and on the nominal contact pressure p, and we show how the contact area for any sliding speed can be obtained from a universal master curve A(p). The numerical results are found to be in good agreement with the predictions of an analytical contact mechanics theory.

The properties of butane (C4H10) lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider the linear n-butane and the branched iso-butane. For the linear molecule, well defined molecular layers develop in the lubricant film when the width is of the order of a few atomic diameters. The branched iso-butane forms more disordered structures which permit it to stay liquid-like at smaller surface separations. During squeezing the solvation forces show oscillations corresponding to the width of a molecule. At low speeds (< 0.1 m/s) the last layers of iso-butane are squeezed out before those of n-butane. Since the (interfacial) squeezing velocity in most practical applications is very low when the lubricant layer has molecular thickness, one expects n-butane to be a better boundary lubricant than iso-butane. N-butane possessing a slightly lower viscosity at high pressures, our result refutes the view that squeeze out should be harder for higher viscosities, on the other hand our results are consistent with wear experiments in which n-butane were shown to protect steel surfaces better than iso-butane., 7 pages, 10 figures, format revtex. Submitted to J. Chem. Phys

We study the dependence of the heat transfer between two semi-infinite solids on the dielectric properties of the bodies. We show that the heat transfer at short separation between the solids may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the heat transfer is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. We study the dependence of the heat flux between two metal surfaces on the electron concentration using the non-local optic dielectric approach, and co mpare with the results obtained within local optic approximation., 5 pages, 3 figures

When two solids are squeezed together they will in general not make atomic contact everywhere within the nominal (or apparent) contact area. This fact has huge practical implications and must be considered in many technological applications. In this paper I briefly review basic theories of contact mechanics. I consider in detail a recently developed contact mechanics theory. I derive boundary conditions for the stress probability distribution function for elastic, elastoplastic and adhesive contact between solids and present numerical results illustrating some aspects of the theory. I analyze contact problems for very smooth polymer (PMMA) and Pyrex glass surfaces prepared by cooling liquids of glassy materials from above the glass transition temperature. I show that the surface roughness which results from the frozen capillary waves can have a large influence on the contact between the solids. The analysis suggest a new explanation for puzzling experimental results [L. Bureau, T. Baumberger and C. Caroli, arXiv:cond-mat/0510232] about the dependence of the frictional shear stress on the load for contact between a glassy polymer lens and flat substrates. I discuss the possibility of testing the theory using numerical methods, e.g., finite element calculations., Comment: Review paper, 29 pages, 31 pictures

We consider the effect of an external bias voltage and the spatial variation of the surface potential, on the damping of cantilever vibrations. The electrostatic friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the surface of the tip by the bias voltage and spatial variation of the surface potential. A similar effect arises when the tip is oscillating in the electrostatic field created by charged defects in a dielectric substrate. The electrostatic friction is compared with the van der Waals friction originating from the fluctuating electromagnetic field due to quantum and thermal fluctuation of the current density inside the bodies. We show that the electrostatic and van der Waals friction can be greatly enhanced if on the surfaces of the sample and the tip there are two-dimension (2D) systems, e.g. a 2D-electron system or incommensurate layers of adsorbed ions exhibiting acoustic vibrations. We show that the damping of the cantilever vibrations due to the electrostatic friction may be of similar magnitude as the damping observed in recent experiments of Stipe \textit{et al} [B.C.Stipe, H.J.Mamin, T.D.Stowe, T.W.Kenny, and D.Rugar, Phys.Rev. Lett.% \textbf{87}, 0982001]. We also show that at short separation the van der Waals friction may be large enough to be measured experimentally., Comment: 11 pages, 2 figures

We simplify the formalism of Polder and Van Hove [Phys.Rev.B {\bf 4}, 3303(1971)], which was developed to calculate the heat transfer between macroscopic and nanoscale bodies of arbitrary shape, dispersive and adsorptive dielectric properties. In the non-retarded limit, at small distances between the bodies, the problem is reduced to the solution of an electrostatic problem. We apply the formalism to the study of the heat transfer between: (a) two parallel semi-infinite bodies, (b) a semi-infinite body and a spherical body, and (c) that two spherical bodies. We consider the dependence of the heat transfer on the temperature $T$, the shape and the separation $d$. We determine when retardation effects become important., Comment: 11 pages, 5 figures

We use a general theory of the fluctuating electromagnetic field to calculate the friction force acting on a small neutral particle, e.g., a physisorbed molecule, or a nanoscale object with arbitrary dispersive and absorptive dielectric properties, moving near a metal surface. We consider the dependence of the electromagnetic friction on the temperature $T$, the separation $d$, and discuss the role of screening, non-local and retardation effects. We find that for high resistivity materials, the dissipative van der Waals interaction can be an important mechanism of vibrational energy relaxation of physisorbed molecules, and friction for microscopic solids. Several controversial topics related to electromagnetic dissipative shear stress is considered. The problem of local heating of the surface by an STM tip is also briefly commented on., Comment: 11 pages, No figures

Liquids play an important role in adhesion and sliding friction. They behave as lubricants in human bodies especially in the joints. However, in many biological attachment systems they acts like adhesives, e.g. facilitating insects to move on ceilings or vertical walls. Here we use molecular dynamics to study how liquids confined at the nanoscale influence the adhesion between solid bodies with smooth and rough surfaces. We show that a monolayer of liquid may strongly affect the adhesion., 5 pages, 9 color figures. Some figures are in Postscript Level 3 format. Minimal changes with respect to the previous version. Added doi and reference to the published article also inside the paper

We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces. We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic shear stress has a bell-shaped f orm, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness on rubber sliding friction., Comment: 14 pages, 16 figures

We show that the radiative heat transfer between two solid surfaces at short separation may increase by many order of magnitude when the surfaces are covered by adsorbates. In this case the heat transfer is determined by resonant photon tunneling between adsorbate vibrational modes. We propose an experiment to check the theory., Comment: 4 pages, No figures

We study the van der Waals friction between two flat metal surfaces in relative motion. For good conductors we find that normal relative motion gives a much larger friction than for parallel relative motion. The friction may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes., Comment: Published in PRL

Free surfaces of liquids exhibit thermally excited (capillary) surface waves. We show that the surface roughness which results from capillary waves when a glassy material is cooled below the glass transition temperature can have a large influence on the contact mechanics between the solids. The theory suggest a new explanation for puzzling experimental results [L. Bureau, T. Baumberger and C. Caroli, arXiv:cond-mat/0510232] about the dependence of the frictional shear stress on the load for contact between a glassy polymer lens and flat substrates. It also lend support for a recently developed contact mechanics theory., 4 pages, 2 figures

We present a new mechanism for the chemical contribution to surface-enhanced Raman scattering (SERS). The theory considers the modulation of the polarizability of a metal nanocluster or a flat metal surface by the vibrational motion of an adsorbed molecule. The modulated polarization of the substrate coupled with the incident light will contribute to the Raman scattering enhancement. We show that for a metal cluster and for a flat metal surface this new chemical contribution may enhance the Raman scattering intensity by a factor of approximately 102 and approximately 104, respectively. The new SERS process is determined by the electric field parallel to the surface of the metal substrate at the molecular binding site.

We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this “low-speed” regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a “hot-crack” propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold.

We discuss how the excitation of high-frequency modes in adsorbed molecules may result in motion (e.g., rotation, translation, or dissociation) of the molecules. Our study is based on rate equations and considers one- and two-vibrational excitation processes, corresponding to linear and quadratic dependences of the reaction rate on the tunneling current in the case the scanning tunneling microscopy is used to excite the vibrations (inelastic tunneling). From the results reported in this paper it should be possible to obtain intramolecular transition rates directly from the experimental data, and gain some understanding on how these important quantities depend on the modes involved and on the substrate.

We study the non-contact friction between an atomic force microscope tip and a metal substrate in the presence of bias voltage. The friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the tip surface by the bias voltage. We show that the friction can be enhanced by many orders of magnitude if the ads orbate layer can support acoustic vibrations. The theory predicts the magnitude and the distance dependence of friction in a good agreement with recent puzzling non-contact friction experiment \cite{Stipe}. We demonstrate that even an isolated adsorbate can produce high enough friction to be measured experimentally., Published in PRL

Loss of braking power and rubber skidding on a wet road is still an open physics problem, since neither the hydrodynamical effects nor the loss of surface adhesion that are sometimes blamed really manage to explain the 20-30% observed loss of low speed tire-road friction. Here we advance a novel mechanism based on sealing of water-filled substrate pools by the rubber. The sealed-in water effectively smoothens the substrate, thus reducing the viscoelastic dissipation in bulk rubber induced by surface asperities, well established as a major friction contribution. Starting with the measured spectrum of asperities one can calculate the water-smoothened spectrum and from that the predicted friction reduction, which is of the right magnitude. The theory is directly supported by fresh tire-asphalt friction data., Comment: 5 pages, 4 figures. Published on Nature Materials (November 7th 2004)

We study crack propagation in a viscoelastic solid. Using simple arguments, we derive equations for the velocity dependence of the crack-tip radius, a (v) , and for the energy per unit area to propagate the crack, G (v) . For a viscoelastic modulus E (omega) which increases as omega(1-s) (0< s< 1) in the transition region between the rubbery region and the glassy region, we find that a (v) approximately G (v) approximately v(alpha) with alpha= (1-s) / (2-s) . The theory is in good agreement with experiment.

We consider dewetting at soft interfaces, e.g., a rubber ball squeezed in a fluid against a flat hard substrate. We show that the applied squeezing pressure, which was not taken into account in earlier studies of the dewetting transition, may have a big effect on the squeeze-out dynamics.

The dynamics of squeeze-out of thin liquid films between two solids is perhaps the most central topic in tribology. It is directly relevant for wear and indirectly involved in many other important processes, e.g., adhesion and friction. In this review we present a broad overview of the basic principles of squeeze-out, and present a number of applications to adhesion, friction and wear. We first describe the squeezing of 'thick' liquid films (thickness larger than {\sim } 100 A), which can be described using the Navier?Stokes equations of hydrodynamics, and present experimental illustrations for soft solids (rubber) and hard solids (mica). Next we consider molecularly thin liquid films. Here the squeeze-out occurs in a quantized manner involving a monolayer at each step. We discuss the nature of the nucleation of n\rightarrow n-1 monolayer squeeze-out, where n is the number of trapped monolayers. We consider in detail the nature of the spreading which follows the nucleation and show that the boundary line may exhibit instabilities. Sometimes the squeeze-out is incomplete, resulting in trapped islands. These islands may be pinned, or else they drift slowly to the periphery of the contact area where they get squeezed out through narrow liquid channels. We consider also dewetting at soft interfaces and present an application to the adhesion of soft objects on wet substrates. Finally, we present molecular dynamics and kinetic Monte Carlo simulation results on various aspects of squeeze-out for liquid-like and solid-like lubrication films, and discuss the implications for wear.

The dewetting transition of thin liquid films (approximately 100 nm) at soft viscoelastic interfaces is analyzed theoretically. It is shown that viscoelastic losses in the soft material can drastically increase the time to complete the dewetting. Thus, the influence of the thinning of the liquid film, due to the hydrodynamic drainage caused by the external applied pressure, has to be considered too. The squeezing pressure coupled with the hydrodynamic drainage may slow down the dewetting to almost zero growth rate of the dry zone; in this case a trapped rim of fluid should be observed.

In this paper we discuss the adhesion of a thin elastic plate to a randomly rough hard substrate. It is shown that at small magnification(long length scales) the plate, because of its higher compliance, is able to adhere in apparent full contact to the long wavelength corrugation of the underlying surface. That is, at length scales longer than the plate thickness, the gain in the adhesion energy upon the contact with the substrate overcomes the repulsive elastic energy produced by the elastic deformations, and the plate is able to fill out the large cavities of the rigid substrate. This produces a larger area of contact and an enhanced capability to adhere to a rough surface in comparison to the semi-infinite elastic solid case. However, at large enough magnification (small length scales) the plate behaves as a semi-infinite solid, and, depending on the roughness statistical properties, the area of true atomic contact may be much smaller than the nominal contact area.

We present molecular dynamics results for the interaction between two solid elastic walls during pull-off for systems with and without octane (C8H18) lubricant. We used two types of substrate—flat and corrugated—and varied the lubricant coverage from ;1/8 to ; 4M L~monolayers! of octane. For the flat substrate without lubricant the maximum adhesion was found to be approximately three times larger than for the system with the corrugated substrate. As a function of the octane coverage ~for the corrugated substrate! the pull-off force first increases as the coverage increases from 0 to ;1 ML, and then decreases as the coverage is increased beyond monolayer coverage. It is shown that at low octane coverage, the octane molecules located in the substrate corrugation wells during squeezing are pulled out of the wells during pull-off, forming a network of nanocapillary bridges around the substrate nanoasperities, thus increasing the adhesion between two surfaces. For greater lubricant coverages a single capillary bridge is formed. The adhesion force saturates for lubricant coverages greater than 3 ML. For the flat substrate, during pull-off we observe discontinuous, thermally activated changes in the number n of lubricant layers ( n21!n layering transitions!, whereas for the corrugated substrate these transitions are ‘‘averaged’’ by the substrate surface roughness. © 2004 American Institute of Physics. @DOI: 10.1063/1.1806814#

We study the time-dependent contact area as a viscoelastic solid is squeezed against a randomly rough substrate. Using a recently developed contact mechanics theory we study how the contact area depends on time and on the magnification zeta. Numerical results are presented for self-affine fractal surfaces, and applications to tack, rubber friction, and sealing are given.

We calculate the van der Waals friction between two semi-infinite solids in normal relative motion and find a drastic difference in comparison with the parallel relative motion. The case of the good conductors is investigated in details both within the local optic approximation, and using a non-local optic dielectric approach. We show that the friction may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. The theory is compared to atomic force microscope experimental data., Published in PRB

We present a simple model which illustrates the nature of the contact between an elastic solid and a hard surface with cosine-corrugation profile. In the continuum limit, the contact mechanics depends only on two dimensionless parameters, namely the ratio between the height and wavelength of the substrate corrugation, and the ratio between a surface energy and an elastic energy. The theory shows that the complete contact state is always a local energy minima (in the zero temperature limit), but for large enough surface roughness the global minima correspond to a partial contact state. We show that at nonzero temperature, the contribution to the free energy from the vibrational entropy is very important, and favors the detached state. Computer simulations results are also presented where we study more complicated roughness geometries and the influence of temperature on the adhesion. Simulation results agrees well with the analytical predictions. (C) 2003 American Institute of Physics.

We consider the dynamics of squeeze-out of a molecularly thin confined two-dimensional (2D) liquidlike layer. The squeeze-out is described by a generalized 2D Navier-Stokes equation which is solved exactly for the limiting case where the squeeze-out nucleates at the center of the contact area, and where the (perpendicular) three-dimensional pressure profile is Hertzian. We also present numerical results for the case where the nucleation is off-center. The theoretical results are in good agreement with recent experimental data by two of us for octamethylcyclotetrasiloxane. In light of our theoretical model calculations, we also discuss the spatially resolved diffusion experiments of Mukhopadhyay [Phys. Rev. Lett. 89, 136103 (2002)]. Here, we obtain a puzzling disagreement between theory and experiment which requires more investigation. (C) 2003 American Institute of Physics.

I study adhesion relevant to biological systems, e.g., flies, crickets and lizards, where the adhesive microstructures consist of arrays of thin fibers. The effective elastic modulus of the fiber arrays can be very small which is of fundamental importance for adhesion on smooth and rough substrates. I study how the adhesion depend on the substrate roughness amplitude and apply the theoretical results to lizards. (C) 2003 American Institute of Physics.

The properties of alkane lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and the elastic properties of the solid surfaces. We consider linear alkane molecules of different chain lengths, C3H8, C4H10, C8H18, C9H20, C10H22, C12H26 and C14H30 confined between smooth gold surfaces. In most cases we observe well defined molecular layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. We find that with increasing alkane chain length, the transition from n to n-1 layers occurs at higher pressure, as expected based on the increasing wettability ~or spreading pressure with increasing chain length. Thus, the longer alkanes are better boundary lubricants than the shorter ones, and this should result in less wear. We obtain good correlation between our theoretical results and wear experiments.

The Ge(111) surface undergoes a high-temperature phase transition at Tc = 1050 K, only 160 K below the bulk melting temperature, Tm = 1210 K. Conflicting pictures have been offered for the state of the surface betweenTc and Tm. In this note we suggest that high-resolution electron energy loss spectroscopy may give new important information about the nature of the transition and in particular should be able to prove if incomplete surface melting has occurred, leading to a thin metallic surface layer, as suggested by recent theoretical work.

When rubber slides on a hard, rough substrate, the surface asperities of the substrate exert oscillating forces on the rubber surface leading to energy ``dissipation'' via the internal friction of the rubber. In this paper we extend an earlier published theory [B.N.J. Persson, J. Chem. Phys. $115,$ 3840 (2001)] to nonstationary sliding, and present a discussion of how the area of real contact and the friction force depend on the nature of the substrate surface roughness and on the history of the sliding motion. We consider in detail the case when the substrate surface has a self-affine fractal structure.

We demonstrate electron-stimulated migration for carbon monoxide (CO) molecules adsorbed on the Pd(110) surface, which is initiated by the excitation of a high-frequency (HF) vibrational mode (C–O stretching mode) with inelastic tunneling electrons from the tip of scanning tunneling microscopy. The hopping phenomenon, however, cannot be detected for CO/Cu(110), even though the hopping barrier is lower than in the CO/Pd(110) case. A theoretical model, which is based on the anharmonic coupling between low-frequency modes (the hindered-translational mode related to the lateral hopping) and the HF mode combined with electron-hole pair excitation, can explain why the hopping of CO is observed on Pd(110) but not on Cu(110).

The properties of Xe, CH4 and C16H34 lubricant confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider both smooth surfaces, and surfaces with short-scale roughness. In most cases we observe well defined molecular layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters, but in some cases atomic scale roughness inhibit the formation of these layers, and the lubricant exhibit liquid-like properties. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. We observe that the layering transition tends to nucleate in disordered or imperfect regions in the lubrication film. We also present and discuss results of sliding dynamics for Xe and C16H34 lubrication films. (C) 2002 American Institute of Physics.

We study the influence of surface roughness on the adhesion of elastic solids. Most real surfaces have roughness on many different length scales, and this fact is taken into account in our analysis. We consider in detail the case when the surface roughness can be described as a self affine fractal, and show that when the fractal dimension D_f >2.5, the adhesion force may vanish, or be at least strongly reduced. We consider the block-substrate pull-off force as a function of roughness, and find a partial detachment transition preceding a full detachment one. The theory is in good qualitative agreement with experimental data., 15 pages, 16 figures. Submitted to J. Chem. Phys

I consider a semi-infinite elastic solid sliding on a flat hard substrate. I present a linear instability analysis to determine when the steady sliding motion becomes unstable with respect to infinitesimal perturbations. I consider a general case where the interfacial frictional shear stress depends not only on the sliding velocity but also on a state variable. I show that when the pressure in the contact area between the solids is constant, no linear instability occurs if the kinetic friction coefficient increases monotonically with the sliding velocity, $d{\ensuremath{\mu}}_{\mathrm{k}}{/dv}_{0}g0.$ However, when the pressure at the interface varies spatially, elastic instabilities may also occur when $d{\ensuremath{\mu}}_{\mathrm{k}}{/dv}_{0}g0.$ I discuss the physical origin of this effect, and suggest that these instabilities may be precursors of the Schallamach waves.

The dynamics of the expulsion of the last liquid monolayer of molecules confined between two surfaces (measured recently for the first time) has been analyzed by solving the two-dimensional Navier-Stokes equation combined with kinetic Monte Carlo simulations. Instabilities in the boundary line of the expelled film were observed. We show that the instabilities produce a rough boundary for all length scales above a critical value and a smooth boundary for shorter lengths. The squeezing out of all but a few trapped islands of liquid is shown to be the result of the pressure gradient in the contact area.

I have developed a theory of contact mechanics between randomly rough surfaces. The solids are assumed to deform elastically when the stress sigma is below the yield stress sigma(Y), and plastically when sigma reaches sigma(Y). I study the dependence of the (apparent) area of contact on the magnification. I show that in most cases the area of real contact A is proportional to the load. If the rough surface is self-affine fractal (Hurst exponent H) the whole way up to the lateral size L of the nominal contact area, then (assuming no plastic deformation) A approximately L(H).

The friction force equals the shear stress integrated over the area ΔA of real contact. Because of surface roughness, the area of real contact is usually much smaller than the apparent area of contact. In this section we discuss the physical processes which determine the area of real contact and present some experimental methods which have been used to estimate ΔA. In most practical applications, the diameter of the contact areas (junctions) are on the order of ~ 10 urn. However, the present drive towards microsystems, e.g., micromotors, has generated a great interest in the nature of nanoscale junctions. The physical processes which determine the formation and behavior of nanoscale junctions are quite different from those of microscale junctions. We consider first microscale junctions and then nanoscale junctions.

The dynamics of the interaction between molecules and metal surfaces is of fundamental importance in surface science, since these determine key physical and chemical properties —essential in, e.g., catalysis —such as energy transfer and molecular reactivity [1]. Surface chemical dynamics are determined by the time scales of various competing relaxation channels [2]: For a complete understanding of surface reactions, it is essential to know how different vibrational modes of the reactant are coupled to each other and to the substrate. Time-resolved surface vibrational infrared (IR) spectroscopy [3] offers a powerful means of obtaining such insights. Using picosecond (ps) techniques, information about the dynamics of energy exchange between adsorbed carbon monoxide (CO) and metal substrates has been obtained [4], by monitoring transient band shifts of the C— O vibration after optical excitation. The technique has further been used to observe vibrational energy relaxation [5] and reversible transient chemical transformations [6] at surfaces. These experiments were limited to temperatures below those at which desorption of the adsorbate occurs. However, when one is interested in surface reactions, the more relevant situation occurs at higher temperatures, where higher-lying vibrational modes get thermally occupied that play an important role in the surface chemistry. We present here femtosecond time-resolved vibrational sum-frequency generation (fs-SFG) spectra of CO molecules adsorbed on a Ru(001) surface, taken while a significant number (50%) of these molecules is desorbing due to fs laser excitation. With this fs-SFG method, snapshots of the (C— O) stretch vibration can be taken, under conditions where desorption is occurring (at lattice temperatures transiently exceeding the desorption temperature by over 500 K), shedding new light on the dynamics of the desorption process and the coupling of

A lubricant is used to lower the friction and reduce the wear between two sliding solid bodies. Here we consider only liquid lubricants although some solids, e.g., graphite and molybdenum disulfide, are also used as lubricants. As pointed out before, almost all surfaces are normally covered by a layer of grease, which may have a similar influence on the sliding friction as an added lubricant.

When rubber is slid on a hard, rough substrate, the surface asperities of the substrate exert oscillating forces on the rubber surface leading to energy “dissipation” via the internal friction of the rubber. We present a qualitative discussion of how the resulting friction force depends on the nature of the surface roughness and comment on the origin of the wear of sliding rubber surfaces.

In this chapter we consider the sliding of adsorbate layers on surfaces. This problem is of great interest in its own right but is also directly relevant for several sliding friction problems, as illustrated by the following two examples.

In this section we consider creep and other slow (thermally induced) relaxation processes which occur at low sliding velocity [11.1]. We first argue that the lubrication film at low sliding velocities has a granular structure, with pinned adsorbate domains accompanied by elastic stress domains in the block and substrate. At zero temperature, the stress domains form a “critical” state, with a continuous distribution P(σ) of local surface stresses σ extending to the critical stress σ a, necessary for fluidization of the pinned adsorbate structure. During sliding adsorbate domains will fluidize and re-freeze. During the time for which an adsorbate domain remains in a fluidized state, the local elastic stresses built up in the elastic bodies during “stick” will be released, partly by emission of elastic wave pulses (sound waves) and partly by shearing the lubrication fluid. The role of temperature-activated processes (relaxation and creep) will be studied and correlated with experimental observations. In particular, the model explains in a natural manner the logarithmic time dependence observed for various relaxation processes; this time dependence follows from the existence of a sharp step-like cut-off at σ = σ a in the distribution P(σ) of surface stresses. A simple experiment is suggested to test directly the theoretical predictions: By registering the elastic wave pulses emitted from the sliding junction, e.g., using a piezoelectric transducer attached to the elastic block, it should be possible to prove whether, during uniform sliding at low velocities, rapid fluidization and refreezing of adsorbate domains occur at the interface. Finally, we present a detailed discussion of plastic creep and relaxation in solids, which forms the basis for calculating how the area of real contact between two solids changes with the time of stationary contact. This topic is, of course, also of central importance in many engineering applications as is illustrated with a few examples.

Around 1940 Bowden and Tabor presented a simple theory for the origin of the sliding friction for clean surfaces [6.1]. They assumed that the friction force is the force required to shear cold-welded junctions formed between the solids.