Atomic scale friction motion

Friction can now be probed at the atomic scale by means of atomic force microscopy (AFM) (see Section VIII-2) and the surface forces apparatus (see Section VI-4) these approaches are leading to new interpretations of friction [1,1 a,lb]. The subject of friction and its related aspects are known as tribology, the study of surfaces in relative motion, from the Greek root tribos meaning mbbing. 

Tribology is the branch of science and engineering of surfaces in relative motion. Included are issues of friction, wear, and lubrication of surfaces. Modem technology has enabled the study of these characteristics in a number of different ways. These studies have given rise to a new branch atomic-scale tribology. This branch deals with issues and processes from atomic/molecular scale to microscale. These... 

Two other possible segmental motions not depicted in Fig. 6 are in distinctly different time realms. At the slow end of the time scale (greater than milliseconds for reasonable amplitudes) are the Rouse-Zimm normal coordinate modes that result from the collective behavior of units of atoms (beads) along the chain pulling one another and acted on by Brownian forces, solvent frictional resistance, and other parts of the chain (Berne and Pecora, 1976). Librational motions, generally accorded to be of the order of 10 s , may also result from the thermally induced displacements of groups of atoms such wobbling motions have been proposed as important factors in the NMR relaxation of proteins (Howarth, 1979). 

When the friction coefficient is set to zero, HyperChem performs regular molecular dynamics, and one should use a time step that is appropriate for a molecular dynamics run. With larger values of the friction coefficient, larger time steps can be used. This is because the solution to the Langevin equation in effect separates the motions of the atoms into two time scales the short-time (fast) motions, like bond stretches, which are approximated, and longtime (slow) motions, such as torsional motions, which are accurately evaluated. As one increases the friction coefficient, the short-time motions become more approximate, and thus it is less important to have a small timestep. 

Molecular-scale imaging in FFM [46, 81], PFM-AFM [82], noncontact atomic force microscopy (nc-AFM) [83-88] and anisotropy in friction and molecular stick-shp motion [89] have been discussed. Other important issues in CFM are chiral discrimination [90-92] and calibration of AFM cantilever spring constants [67,93, 94] and tip radii [39, 95]. 

J. J. Bikerman No polymer surface is smooth on atomic or molecular scale. However, the friction of polymers (at not too small pressures) usually is determined not by surface roughness but by deformation of the bodies in tangential motion if this deformation is extensive, surface roughness ceases to be significant. 

To understand the principles at which biological systems operate, detailed studies on ultrastructure, material properties, force range, and motion pattern during locomotion are necessary. Such studies have become possible in the past several years due to new developments (1) in microscopical visualization techniques (atomic force microscopy, freezing and environmental scanning electron microscopy), (2) in characterisation of mechanical properties of biological materials and structures in situ and in vivo (measurements of stiffness, hardness, adhesion, friction) at local and global scales, and (3) in computer simulations. 

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