The frictional phenomena appear in various systems. Their scales spread to extraordinary wide range. The frictional systems of large scale extend to landslide, glacier and earthquake, while those of small scale reach to sub-nanometer. There are universal behaviors in friction which are independent from the scale of the system. Typical examples are the maximum static frictional force, which is the threshold strength of the external force to cause the translational motion, and the kinetic frictional force, which results from energy dissipation accompanied by the caused motion. Besides these, stick-slip phenomena, memory effect of static frictional force, velocity dependence of kinetic frictional force and so on appear in various systems with wide range of scale. There are common mechanisms in these universal phenomena. Here we review frictional phenomena of various systems from a unified point of view.
We systematically discuss frictional properties occurring in atomic-force microscope (AFM) tip on graphite system, the graphite/C60/graphite system, and the C60 intercalated graphite system. Several possible mechanism to induced superlubricity of C60 intercalated graphite system are proposed and discussed. It can be expected that the superlubricity is induced by internal sliding between close-packed C60 monolayers and graphite layers. Our results propose one of the simple guidelines of designing practical superlubric system—reduction of the contact area between intercalated C60 and graphite sheet to the point-like contact. We anticipate our novel lubrication system to be a startpoint for developing more practical superlubricant using intercalated graphite, which will contribute to solving the energy and environmental problems.
The nano-friction of 4He films adsorbed on graphite substrate has been reviewed. From experiments of the quartz-crystal microbalance (QCM), the following properties of the sliding motion of films and the friction have been revealed: (1) At two-atom and three-atom thick films, the frictional force of the boundary of the first and second atomic layers is much smaller than that of the boundary between the film and the substrate. (2) At two-atom and three-atom thick films, the frictional force remains metastable at low temperatures after switching of the driving force, before relaxing to the value. (3) At four-atom thick films, the solid atomic layer underneath the superfluid layer stops the sliding motion on the oscillating substrate. From these observations, it is concluded that a non-uniform sliding of the second atomic layer occurs with respect to the the first atomic layer, and the frictional force of films is related to the resistance force for the motion of this local density change.
The authors discuss the molecular motion of surface-immobilized molecular machines. The cyclodextrin necklace and double-decker phthalocyanine complexes were used as examples of the translational motion and rotational motion of molecules, respectively. For the translational motion of cyclodextrins (CyDs), the relationship between the intermolecular interaction of CyDs composed of the cyclodextrin necklace and the interaction between CyDs and substrates was an important factor in determining the molecular mobility. For the rotational motion of double-decker phthalocyanine complexes, the combination of molecules composed of double-deckers as well as the free space around a top ligand was important issues in controlling the molecular motion of immobilized double-decker complexes on solid surfaces.
The surface of ice and snow is slippery. This slipperiness property of ice surface has been usually explained by a lubricating layer of water film, which exists in the surface. Ice crystal has a melting layer on the surface even at low temperatures below the bulk melting point. The properties of ice surface have attracted scientific interests for more than a century, ever since 1859 when Faraday predicted the existence of a melting layer on ice surface. This paper reviews the dynamics of ice surface and the effects of the dynamics on the formation process of the melting layer in ice surface.
Frictional behaviors between mica surfaces have been investigated with the Surface Force Apparatus under various relative vapor pressures (RVP) of both water and cyclohexane. The dependence of frictional forces on RVP, particularly in the low RVP range, has been studied because the embryo of liquefaction and layering of molecules occurs in these conditions. The results of the measurements of kinetic shear stress as a function of sliding velocity at different RVP reveal the role of liquid condensed around the contact zone. A mechanism based on capillary condensation is proposed to explain the role of RVP on friction by adsorption of liquid layers on free mica surfaces and at contact.
A new approach to systematically understand the physics of friction at the microscopic level is introduced, where the collective dynamics of quantum condensate of solid is used as a model system of friction at the solid interface. This approach is free from problems of wear, and makes reproducible experiments possible in a well controlled manner. In particular, we focus on the dynamics of driven vortices in superconductors, and investigated the velocity dependence of kinetic friction and the waiting-time dependence of the maximum static friction. Based on the experimental data, we discuss the condition for the validity of the Amontons-Coulomb's law, and propose a simple criterion for the issue.