Lubricant, computer simulation

However, the assumption of molecule orientation normal to the surface is not convincing enough for this author, and it does not consist well with the results of the molecular d5mamics simulations for the alkane confined between solid walls. An example in Fig. 3 shows that the chain molecules near the wall are found mostly lying parallel, instead of normal, to the wall [6]. This means that the attractions between lubricant molecules and solid wall may readily exceed the inter-molecule forces that are supposed to hold the molecules in the normal direction. Results in Fig. 3 were obtained from simulations for liquid alkane with nonpolar molecules, but similar phenomenon was observed in computer simulations for the functional lubricant PFPE (per-fluoropolyether) adsorbed on a solid substrate [7], confirming that molecules near a solid wall lie parallel to the surface. 

The present author has performed computer simulations to examine the transition of pressure distributions and shear response from a hydrodynamic to boundary lubrication. Figure 4(a) shows an example of a smooth elastic sphere in contact with a rigid plane, the EHL pressure calculated at a very low rolling speed coincides perfectly with the... 

It has been proposed recently [28] that static friction may result from the molecules of a third medium, such as adsorbed monolayers or liquid lubricant confined between the surfaces. The confined molecules can easily adjust or rearrange themselves to form localized structures that are conformal to both adjacent surfaces, so that they stay at the energy minimum. A finite lateral force is required to initiate motion because the energy barrier created by the substrate-medium system has to be overcome, which gives rise to a static friction depending on the interfacial substances. The model is consistent with the results of computer simulations [29], meanwhile it successfully explains the sensitivity of friction to surface film or contamination. 

Press, Boca Raton, FL, 2001, pp. 717—770. Computer Simulations of Friction, Lubrication,... 

S. Izumisawa and M. S. Jhon, Stability analysis and computer simulation of nanoscale lubricant films with chain-end functional groups, J. Appl. Phys. 91(10), 7583-7585 (2002). 

The computer simulation involves a f 1 nite-d1fference solution of the field equations which describe this problem which Involves coupling of fluid mechanics with pressure and temperature fields. The lubrication approximation is utilized to eliminate the momentum equation in the radial direction. This simplification is reasonable for the flow fields considered in this study since the following criteria cire met (21) ... 

Since the middle of the 1990s, another computation method, direct simulation Monte Carlo (DSMC), has been employed in analysis of ultra-thin film gas lubrication problems [13-15]. DSMC is a particle-based simulation scheme suitable to treat rarefied gas flow problems. It was introduced by Bird [16] in the 1970s. It has been proven that a DSMC solution is an equivalent solution of the Boltzmann equation, and the method has been effectively used to solve gas flow problems in aerospace engineering. However, a disadvantageous feature of DSMC is heavy time consumption in computing, compared with the approach by solving the slip-flow or F-K models. This limits its application to two- or three-dimensional gas flow problems in microscale. In the... 

The greatest limitation of QC methods is computational expense. This expense restricts system sizes to a few hundred atoms at most, and hence, it is not possible to examine highly elaborate systems with walls that are several atomic layers thick separated by several lubricant atoms or molecules. Furthermore, the expense of first-principles calculations imposes significant limitations on the time scales that can be examined in MD simulations, which may lead to shear rates that are orders of magnitude greater than those encountered in experiments. One should be aware of these inherent differences between first-principles simulations and experiments when interpreting calculated results. 

Finally, with the advent of high speed computers, parallel architecture, and the popularity of extended system methods, NEMD has seen yet another resurgence. Researchers are beginning to access the resources that allow one to compute the physical properties of real lubricants. The aim of this chapter is to provide the theoretical and technical foundations to enable simulators to model realistic systems under nonequilibrium conditions. 

Mendonca ACF, Padua AAH, Malfreyt P (2013) Nonequilibrium molecular simulations of new ionic lubricants at metallic surfaces prediction of the friction. J Chem Theory Comput 9(3) 1600-1610. doi 10.1021/ct3008827... 

Well-defined flow characteristics in a microfluidic device can also reveal new aspects of mass transfer. For example, it was possible to elucidate the relative rates of the mass transfer at the bubble cups and at the lubricating fluid interface between the gas bubble and the solid wall (see the sidebar figure). Van Eaten and Krishna (2004) have demonstrated through computational flnid dynamics (CFD) simulations that the mass transfer rates at the hemispherical babble cups and the lubricating liquid interfaces of the rising Taylor gas flows contribute differently to the rates of the mass transfer. 

A major challenge is how to handle the large number of isomers of hydrocarbons that are possible in lubricant range materials. These range from a billion at C30 to a billion billion at C50. Few of these have relevance as lubricants and judicious selection of representative isomers and groups of isomers is needed. Experimental validation of computed results is also problematic since there are few pure isomers available in this range. Nevertheless, the simulations done so far have been very helpful in guiding synthesis. 

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