Shear behavior of squalane and tetracosane under extreme confinement. III. Effect of confinement on viscosity

Abstract

This study uses nonequilibrium molecular dynamics simulation to explore the rheology of confined liquid alkanes. Two alkanes that differ in molecular structural complexity are examined: tetracosane ( C 24 H 50 ), which is a linear alkane, and squalane ( C 30 H 62 ), which has six symmetrically placed methyl branches along a 24 carbon backbone. These model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. This paper, the third of a three part series, compares the viscosities of the confined fluids to those of the bulk fluids. The alkanes are described by a well-documented potential model that has been shown to reproduce bulk experimental viscosity and phase equilibriameasurements. Details of the simulation method, and structural information can be found in the preceding two papers of this series. The measured strain rates in these simulations range between 10 8 and 10 11 s −1 , which is typical of a number of practical applications. The confined fluids undergo extensive shear thinning, showing a power-law behavior. 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 (10–10 5 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. The viscosity is found to be independent of the wall spacing. The calculated power-law exponents are similar to experimentally observed values. We also note that the exponent increases with increasing density of the fluid.