Resonance in Atomic-Scale Sliding Friction

Friction represents a major energy dissipation mode, yet the atomistic mechanism of how friction converts mechanical motion into heat remains elusive. It has been suggested that excess phonons are mainly excited at the washboard frequency, the fundamental frequency at which relative motion excites the interface atoms, and the subsequent thermalization of these nonequilibrium phonons completes the energy dissipation process. Through combined atomic force microscopy measurements and atomistic modeling, here we show that the nonlinear interactions between a sliding tip and the substrate can generate excess phonons at not only the washboard frequency but also its harmonics. These nonequilibrium phonons can induce resonant vibration of the tip and lead to multiple peaks in the friction force as the tip sliding velocity ramps up. These observations disclose previously unrecognized energy dissipation channels associated with tip vibration and provide insights into engineering friction force through adjusting the resonant frequency of the tip-substrate system.

Total internal reflection Raman spectroscopy of poly(alpha-olefin) oils in a lubricated contact

A novel total internal reflection (TIR) Raman tribometer has been used to explore the physiochemical changes associated with shear-thinning in synthetic base oil.

Stability of the parallel layer during alkane spreading and the domain structures of the standing-up layer

The spreading of liquid alkanes over surfaces plays an important role in applications such as lubrication, painting, and printing. To make significant advances in these fields, it is essential to increase our understanding of the interactions between alkanes and surfaces. Long-chain alkanes form two typical adsorption structures on a surface--the parallel phase and the standing-up phase. The most thermodynamically stable structure is the parallel phase, in which the alkane molecules lie flat on the surface. If the temperature is slightly below the bulk melting point, then alkanes form a thermodynamically stable standing-up phase on top of an existing parallel layer. At lower temperatures, the standing-up phase becomes metastable. Using atomic force microscopy, we have found that the standing-up alkane layer consists of multiple domains, indicating that the standing-up layer forms through a multinucleation process during the liquid-solid transition on the surface. If, however, the temperature is above the melting point, then we have found that the standing-up layer shrinks to a droplet and leaves a residue on its original position. During the spreading of an alkane droplet, the parallel layer forms on the substrate surface surrounding the droplet by adsorption from the vapor, which precedes the arrival of the liquid. There has been uncertainty, however, as to whether the parallel layer moves with the liquid alkane or remains stationary during spreading. In this study, we used the residue left on the parallel layer as a landmark to monitor the movement of the parallel layer during the spreading of an alkane droplet. Using this landmark, we found that the parallel layer remained stationary on the substrate, indicating that the liquid alkane spreads on a stationary parallel layer surface. Therefore, this study reveals that the surface properties of the parallel layer--not the surface properties of the substrate--control the spreading and wetting of a liquid alkane.

Micromechanics of friction: effects of nanometre-scale roughness

Nanometre-scale roughness on a solid surface has significant effects on friction, since intersurface forces operate predominantly within a nanometre-scale gap distance in frictional contact. To study the effects of nanometre-scale roughness, two novel atomic force microscope friction experiments were conducted, each using a gold surface sliding against a flat mica surface as the representative friction system. In one of the experiments, a pillar-shaped single nano-asperity of gold was used to measure the molecular-level frictional behaviour. The adhesive friction stress was measured to be 264 MPa and the molecular friction factor 0.0108 for a direct gold–mica contact. The nano-asperity was flattened in contact, although its hardness at this length scale is estimated to be 3.68 GPa. It was found that such a high pressure could be reached with the help of condensed water capillary forces. In the second experiment, a micrometre-scale asperity with nanometre-scale roughness exhibited a single-asperity-like response of friction. However, the apparent frictional stress, 40.5 MPa, fell well below the Hurtado–Kim model prediction of 208–245 MPa. In addition, the multiple nano-asperities were flattened during the frictional process, exhibiting load- and slip-history-dependent frictional behaviour.

Biochemical composition of the superficial layer of articular cartilage

To gain more information on the mechanism of lubrication in articular joints, the superficial layer of bovine articular cartilage was mechanically removed in a sheet of ice that formed on freezing the cartilage. Freeze-dried samples contained low concentrations of chondroitin sulphate and protein. Analysis of the protein by SDS PAGE showed that the composition of the sample was comparable to that of synovial fluid (SF). Attenuated total reflection infrared (ATR-IR) spectroscopy of the dried residue indicated that the sample contained mostly hyaluronan. Moreover, ATR-IR spectroscopy of the upper layer of the superficial layer, adsorbed onto silicon, showed the presence of phospholipids. A gel could be formed by mixing hyaluronan and phosphatidylcholine in water with mechanical properties similar to those of the superficial layer on cartilage. Much like the superficial layer of natural cartilage, the surface of this gel became hydrophobic on drying out. Thus, it is proposed that the superficial layer forms from hyaluronan and phospholipids, which associate by hydrophobic interactions between the alkyl chains of the phospholipids and the hydrophobic faces of the disaccharide units in hyaluronan. This layer is permeable to material from the SF and the cartilage, as shown by the presence of SF proteins and chondroitin sulphate. As the cartilage dries out after removal from the joint, the phospholipids migrate towards the surface of the superficial layer to reduce the surface tension. It is also proposed that the highly efficient lubrication in articular joints can, at least in part, be attributed to the ability of the superficial layer to adsorb and hold water on the cartilage surface, thus creating a highly viscous boundary protection.

Probing the local structure and mechanical response of nanostructures using force modulation and nanofabrication

Nanostructures of self-assembled monolayers (SAMs) are designed and produced using coadsorption and nanografting techniques. Because the structures of these artificially engineered domains are predesigned and well-characterized, a systematic investigation is possible to study the mechanical responses to force modulation under atomic force microscope tips. Force modulation imaging reveals characteristic contrast sensitivity to changes in molecular-level packing, molecule chain lengths, domain boundaries, and surface chemical functionalities in SAMs. By means of actively tuning the driving frequency, the resonances at the tip-surface contact are selectively activated. Therefore, specific surface features, such as the edges of the domains and nanostructures or desired chemical functionalities, can be selectively enhanced in the amplitude images. These observations provide a new and active approach in materials characterization and the study of nanotribology using atomic force microscopy.

Macroscopic diagnostics of microscopic friction phenomena

We show that the static friction force which must be overcome to render a sticking contact sliding is reduced if an external torque is also exerted. As a test system we study a planar disk lying on a horizontal flat surface. We perform experiments and compare with analytical results to find that the coupling between static friction force and torque is nontrivial: It is not determined by the Coulomb friction laws alone, instead it depends on the microscopic details of friction. Hence, we conclude that the macroscopic experiment presented here reveals details about the microscopic processes lying behind friction.

Lateral force microscopy profiles for amorphous potentials

In this work, the lateral force profiles of the scanning force microscope tip on an amorphous surface were simulated with the use of an independent oscillator model. The correlation between the lateral force profiles and the surface potential were studied as a function of the tip-surface normal force and relative scanning velocity. It is shown that the microscope resolution is governed by the quotient between the average potential interaction energy and the average elastic energy stored before the jumps. We show that there is an optimal velocity with which the scanning tip better senses the surface potential and we present its scaling laws.

Friction traced to the single atom

Friction is caused by dissipative lateral forces that act between macroscopic objects. An improved understanding of friction is therefore expected from measurements of dissipative lateral forces acting between individual atoms. Here we establish atomic resolution of both conservative and dissipative forces by lateral force microscopy, presenting the resolution of atomic defects. The interaction between a single-tip atom that is oscillated parallel to an Si(111)-(7 x 7) surface is measured. A dissipation energy of up to 4 eV per oscillation cycle is found. The dissipation is explained by a "plucking action of one atom on to the other" as described by G. A. Tomlinson in 1929 [Tomlinson, G. A. (1929) Phil. Mag. 7, 905-939].

Examination of line crossings by atomic force microscopy

Until now, the most widely used methods for the forensic examination of line crossings in documents were optical and electron microscopy. The combination of both techniques allows one in most cases to establish the sequence of lines. The recent development of scanning probe microscopy [1] gives an opportunity to complement or even replace the classical instruments used in this field. Scanning probe microscopes have been designed to study surfaces at high magnification. The aim of this study was to verify if their most popular member, the atomic force microscope (AFM) [2], can be applied to line crossing problems. The results show for the first time that AFM images present the same qualitative information obtained by scanning electron microscope (SEM) images and, consequently, allow the determination of the line crossing sequence under ambient conditions without vacuum and conductive coating of specimens.