Rheology of confined liquid alkanes by nonequilibrium molecular dynamics

Lubrication between materials at the molecular level is of particular concern in many applications, and the choice of a lubricant for a given application depends on its rheological properties.

In this work, nonequilibrium molecular dynamics simulation of the rheology of confined films is used to explore the microscopic properties and response of model lubricants under shear. The rheological behavior of two alkanes that differ in molecular structural complexity is examined: tetracosane (C₂₄H_{50/sub>), which is a linear alkane, and squalane (C30H62/sub>), which has six symmetrically placed methyl branches along a 24 carbon backbone. These fluid alkanes are described by a well-documented potential model that has been shown to reproduce bulk experimental viscosity and phase equilibria measurements. The model lubricants are confined between walls that have short chains tethered to them, which are described by the same potential model as for the fluid. The wall-fluid interactions are represented by a model potential function, with the wall having no structure, this approach is justified because the tethered chains screen the wall details. Shear flow is generated by moving the walls at constant velocity, and various properties are calculated after attainment of steady state. Heat generated by viscous dissipation is removed by thermostatting the first two atoms of the tethered molecules at 300 K, which allows a temperature profile to develop across the width of the lubricant layer.}

The measured strain rates in these simulations range between 10^{8 - 1011 s-1 which is typical of a number of practical applications. Under these conditions, both tetracosane and squalane exhibit extensive shear-thinning and marked interfacial slip. The architectural difference between the two molecules leads to differences in the conformational and rheological properties. In particular, tetracosane tends to adopt extended configurations compared to squalane, which is manifested by a lower degree of slip for tetracosane. Additionally, when the wall spacing is an integral multiple of the molecular diameter, tetracosane shows the formation of very well-defined layers of fully extended molecules but without crystallization or a glass transition.}

Comparison of results for the confined fluid to those for the bulk fluid reveal that, for the conditions examined, there is no difference between the bulk and confined viscosities for these alkanes. This observation is in contrast to experimental results at much lower strain rates (10 - 10⁵ s^-1), which indicate the viscosities of the confined fluid to be much larger than the bulk viscosities. In making the comparison, we have carefully accounted for slip at the wall, and have performed simulations of the bulk fluid at the same conditions of strain rate, temperature, and pressure as for the corresponding confined fluid.