Impact of Molecular Architecture on the High-Pressure Rheology of Hydrocarbon Fluids

Abstract

Molecular dynamics simulations are conducted on three C18 poly-α-olefin isomers under extreme conditions typical of traction fluids or lubricants under elastohydrodynamic lubrication conditions. The viscosity, self-diffusivity, and rotational relaxation times of the molecules are computed at pressures ranging from atmospheric to as high as 1 GPa. The dynamics of all three isomers are slowed as pressure increases, but a highly branched isomer shows a more dramatic reduction in mobility with pressure than does a linear or singly branched isomer. In particular, the viscosity of the highly branched molecule exhibits a much larger increase with pressure than does the viscosity of the other isomers, indicating that the highly branched molecule should exhibit more favorable traction properties than the other isomers. An explanation for the differences in dynamic properties between the isomers is given in terms of a reduction in liquid void volume coupled with the greater backbone stiffness of the highly branched molecule. A free volume analysis is conducted and shown to provide a better means of correlating the pressure dependence of diffusivity and viscosity than commonly used engineering models.

Stereoisomeric effects on dynamic viscosity versus pressure and temperature for the system cis-+trans-decalin

“…Generally, when a fluid is brought under pressure (compressed), the flexibility and mobility of the molecules are reduced, since the distance and space between the molecules become shorter, resulting in a reduction of the fluid mobility and an increase in the viscosity. Kioupis and Maginn [25] explained the fact that the viscosity increases with pressure as a result of a reduction in the liquid void volume coupled with the molecular structure, resulting in a lower motion of the molecules, because the motion is related either to molecules jumping or forcing adjacent molecules into these voids. Therefore, when the number of voids decreases with increasing pressure, complex molecules with a low flexibility will have difficulties of making these jumps or forcing other molecules into these voids, resulting in the trapping of the molecules and a higher viscosity.…”

Molecular dynamics characterization of thin film viscosity for EHL simulation

“…The phenomenon of shear thinning has been studied extensively both experimentally [4,30] and via molecular simulation [2,[5][6][7]21,26]. Although the mechanism behind shear thinning on different length scales may not be the same, it has been found to follow consistent, length scale-independent behavior using the time temperature superposition principle [31].…”

“…Therefore, the effect of shear thinning is often significant. Many research efforts have focused on characterization of viscosity as a function of shear rate [4][5][6][7]. Typically, the shear rate is varied by modulating the wall speed and maintaining constant film thickness.…”

Applications of13C NMR to predict low temperature viscosity performance of base oils

“…8 Structure-property correlations are extremely useful in identifying areas of potential base fluid improvement which will help meet the increasingly severe performance demands being placed on formulated lubricants.…”