Simulated Non-Newtonian Lubricant Behaviour under Extreme Conditions

Abstract

Equilibrium and non-equilibrium molecular dynamics techniques are used to simulate a molecular liquid (n-hexadecane) at extreme physical conditions (high temperatures, pressures and shear rates), typical of those encountered in many elastohydrodynamic lubrication applications, and to study the effects of pressure and temperature on the calculated rheological properties. Pressure and temperature effects are found to be more pronounced at low shear rates than at high shear rates, where the non-Newtonian behaviour is dominated by the strong external kinematic effects. The observed non-Newtonian behaviour of the pressure and temperature coeficients of viscosity and the variation of certain molecular geometric functions calculated to assist the microscopic understanding are combined to provide a possible explanation for the phenomenon of limiting shear stress under elastohydrodynamic lubrication conditions. Further implications for the behaoiour of lubricant-like molecules in practical applications are also discussed.

Material functions of liquid n-hexadecane under steady shear via nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects

“…[37][38][39] As shown in Fig. 1 over a wide shear range of 1 ϫ 10 9.5 −1ϫ 10 12.5 s −1 , both results are in accordance with those characteristics above of 1 and − 2 and are also in close agreement with the results of Chynoweth et al 49 In this present study, the shear rate is least limited at 1 ϫ 10 9.5 s −1 due to the fact that lower shear rates generate the lower signal-to-noise ratio producing large statistical errors. 10 Obviously, between 1 ϫ 10 10.0 and 1 ϫ 10 12.0 s −1 the 1 -␥ and − 2 -␥ curves are close to the linear line in the logarithmic plot.…”

The shear rheology of thin compressed liquid films

“…Examples from the litera ture are shown in F igs 8 and 9 from references [29] and [28] respectively. The Carreau parameters from simulations are listed in Table 1.…”

“…M olecular simulations also provide normal stress difference values [25,28] which are a measure of elastic effects that will be discussed later. Alth ough about three decades of shear rate separate the capabilities of rheometers and simulations, it is clear from the parameters in Table 1 that there is consistency between the techniques.…”

“…The power-law indices are similar and G is approximately rR T M for both techniques. In fact, Chenoweth et al [28] found that l is approximately equal to the terminal relaxation time for R ouse mechanics:…”

Nanocontraction flows of short-chain polyethylene via molecular dynamics simulations

“…In the present study, the modelling of the motions of molecules mentioned above is based on the set of realistic potential models of Chynoweth et al [25] and Chynoweth and Michopoulos [26] (CM), who fit optimal parameters of the vdW potential using experimental data on the heat enthalpy of vaporisation. Overall, the CM model predicted rheological properties of shear flows better than 'the transferable potential for phase equilibria (TraPPE)' model [27], which is an adaptation for calculating the vapour -liquid coexistence curve and the surface tension of n-alkane phase diagrams.…”