Friction anisotropy

Friction Anisotropy at Ni( 100)/( 100) Interfaces. Molecular Dynamics Studies. 

The stochastic model of ion transport in liquids emphasizes the role of fast-fluctuating forces arising from short (compared to the ion transition time), random interactions with many neighboring particles. Langevin s analysis of this model was reviewed by Buck [126] with a focus on aspects important for macroscopic transport theories, namely those based on the Nernst-Planck equation. However, from a microscopic point of view, application of the Fokker-Planck equation is more fruitful [127]. In particular, only the latter equation can account for local friction anisotropy in the interfacial region, and thereby provide a better understanding of the difference between the solution and interfacial ion transport. 

In contrast, for plastically deformed surfaces, friction anisotropy appears to be primarily attributable to the movement of atomic slip planes within the bulk of the metal, and not to commensurability at the sliding interface. Early experiments using diamond surfaces showed that friction anisotropy disappeared at low loads where no plastic deformation was evidenced. 

Friction anisotropy has also been seen between clean and monolayer-coated metal (100) single crystals in UHV, and persists when the surface is covered with as much as 20-40 monolayers of octane, beyond which no anisotropy is detectableJ ... 

In recent years, it has become evident that this standard treatment may be restricted in its applicability. Limitations of the standard treatment in the presence of anisotropic friction have been discussed by a number of authors (97-99,101-110). It has been shown that introduction of large anisotropy in the friction coefficients can lead to qualitatively new physics. The particle may prefer to avoid the saddle point rather than escape through it. The activation energy may become dependent on the friction anisotropy, which can lead to a lowering of the apparent activation energy. The extent of these effects depends both on the details of the potential energy surface as well as the friction anisotropy. 

Neither frictional anisotropy nor depth hysteresis was observed for sliding perpendicular to the column tilt axis. However, sliding with and against the column tilt axis results in a measurable friction anisotropy as well as depth hysteresis. In contrast, planar films do not show either frictional anisotropy or depth hysteresis [115]. 

Internal-friction anisotropy varied from about 3 to 20. Internal friction was always higher in the transverse direction, which tends to sample the matrix. Despite their wide elastic-modulus-anisotropy variation, the graphite-reinforced epoxies varied little in internal-friction anisotropy. 

Friction anisotropy in different sectors of lamellar crystals (154), which is thought to be related to the orientation of the folds at the fold surface, has also... 

Monolayer films of PCEA exhibit strong threefold friction anisotropy. Friction is highest when scanning perpendicular to the polymer backbone direction. We propose that this effect results from anisotropic film deformation modes. 

Using lateral force microscopy on Langmuir-Blodgett films, we found unexpected friction anisotropies and asymmetries in the frictional behavior. The data were found not to be related to the hexagonal packing of the molecules, but rather related to a small molecular tilt. 

For polymers, anisotropic friction 10) was observed on lamellar crystals II -13 as well as on extended-chain crystals obtained by friction transfer deposition 14). The anisotropic friction for lamellar crystals was explained by the occurrence of chain-folding oriented preferentially in planes parallel to the corresponding crystal face. For extended-chain crystals friction anisotropy observed parallel and perpendicular to the chain direction was interpreted by the interlocking asperity model. In this paper we give an overview regarding our nanotribological observations on oriented high density polyethylene (HDPE), poly(tetrafluoroethylene) (PTFE) and on crystals of polyethylene (PE) and transcrystallized poly(ethylene oxide) (PEO). 

For measurements in air, we observed a reduction in pull-off forces for -CF3 tips as compared to unmodified, or differently modified tips. The friction anisotropy observed on HDPE is summarized in Figure 5a. For the -CF3 terminated tip a smaller magnitude of friction is measured than that for a -COOH tip. The ratio of friction (F[perpendicular/parallel] 4) is lower than that for PTFE (vide supra). The friction anisotropy was measured for HDPE with unmodified tips in air as a function of relative scan angle (Figure 5b). Similar to PTFE (vide supra) the magnitude of the friction force was smallest for scans parallel to the polymer chain direction and increased with increasing relative scan angle. 

In the present study, solution grown lamellar crystals of PE on mica were studied by LFM in air, as well as in water. The friction anisotropy shown in the corresponding micrographs was well pronounced. The relative friction measured by LFM on PE crystals in different orientations, as well for the same crystal after... 

In the chain-parallel direction, only the HOPE showed a periodic stick slip behavior with a repeat distance of ca. 2.5 A. This distance is equal to the repeat unit of polyethylene (16, 20). For PTFE the LFM friction loops in our experiments did not reveal any stick slip behavior. Thus, in this case, we cannot determine the value of do in the chain-parallel direction. In this case we can, however, assume that the value of d is close to 0. Based on equation 1, the friction anisotropy is therefore expected to be larger for PTFE, than that for HDPE. For a semi-quantitative comparison of friction forces predicted by equation (1) on one hand and experimental results obtained on highly oriented polymer surfaces on the other hand, one should be... 

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