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Rheological and structural studies of liquid decane, hexadecane, and tetracosane under planar elongational flow using nonequilibrium molecular-dynamics simulations Concentration effects on lubrication rheology for polymer solution in molecularly thin film using molecular dynamics J. Appl. Phys. 95, 8450 (2004); 10.1063/1.1751629 Molecular dynamics study of the nano-rheology of n-dodecane confined between planar surfaces Rheological, thermodynamic, and structural studies of linear and branched alkanes under shearWe have performed extensive equilibrium and nonequilibrium molecular dynamics ͑EMD and NEMD͒ simulations of three isomers of C 30 H 62 at temperatures of 311 and 372 K employing a united atom model. Using the rotational relaxation time calculated from the EMD simulation, the Rouse model predicts a zero-shear viscosity for n-triacontane within 16% of the value determined by NEMD. Compared to experiment, NEMD and the united atom model underpredict the kinematic viscosities of n-triacontane and 9-n-octyldocosane but accurately predict the values for squalane ͑within 15%͒. In addition, the predicted values of the kinematic viscosity index for both 9-n-octyldocosane and squalane are in quantitative agreement with experiment and represent the first such predictions by molecular simulation. This same general potential model and computational approach can be used to predict this important lubricant property for potential lubricants prior to their synthesis, offering the possibility of simulation-guided lubricant design.

Lubrication is a phenomenon of immense practical importance and fundamental scientific interest, and the automobile engines of the future are envisioned by the Partnership for a New Generation Vehicle will require the development of improved lubricants that perform well at higher operating temperatures and higher engine speeds. The rheological properties of liquid alkanes of intermediate molecular sizes (C20H42-C40H82) are among the most important properties in lubricant performance. Though realistic study of these systems by molecular simulation has previously been limited by both high computational costs and the lack of potential models accurate over a wide range of physical conditions, the advent of massively parallel supercomputers has now made such studies possible. As an illustration of the ability of molecular simulation as a useful tool for lubricant development, we present the first molecular-simulation-based calculation of the kinematic viscosity index of an alkane liquid, viz. 2,6,10,15, 19,23-hexamethytetracosane, commonly called squalane. In the mass range of interest, squalane is one of the few commercially available isoparaffins and has been the subject of previous studies by molecular simulation (Mondello and Grest 1995;Mundy et al., 1997).Though numerous properties of a lubricant are important to end-use applications, the viscosity is considered most significant. Mundy et al. (1996) investigated the variation of decane's viscosity with pressure, calculating its pressureviscosity coefficient by equilibrium molecular dynamics simulations. The kinematic viscosity index (VI) is another widelyused industrial characterization of automotive lubricants. It was proposed by Dean and Davis (1929) as an indication of an oil's viscosity-temperature characteristics in terms of its Saybolt viscosities at 311 K (100°F) and 372 K (210°F). Two series of reference lubricating-oil fractions ( H and L ) were used for comparison. Series H exhibited little change of viscosity with temperature while the viscosities of series L oils exhibited large variation with temperature. Series H and L represented, respectively, the best and worst oils available in 1929. Series H oils were assigned a VI of 100, series L a Correspondence concerning this article should bc addressed to P. T. Cummings. value of 0. The VI of an oil under test ( T ) was calculated from the equation L -Hwhere U is the kinematic viscosity at 311 K of the oil in question, L and H , respectively, are the kinematic viscosities at 311 K of the series L and H having the same kinematic viscosity at 372 K as the oil T. Thus, the higher the VI the less the viscosity of an oil is affected by temperature and, therefore, the better the oil. Subsequently, Hardiman and Nissan (1945) proposed revision to the VI system because it had reached the end of usefulness in its previous form. It had become necessary to give VI values to oils with better viscosity-temperature behavior than those represented by 100 on the VI scale. Hardiman and Nissan found that the VI system faile...