Tribological Design by Molecular Dynamics Simulation: Influence of the Molecular Structure on Wall Slip and Bulk Shear

Abstract

The tribological properties of a complex branched‐hydrocarbon oil under shear in a gap between smooth iron atom surfaces were studied by large‐scale molecular dynamics (MD) simulation. The liquid was a nonpolar lubricant, i.e. a polyalphaolefin (PAO) oil mixture of 1‐decane dimer, trimer, and tetramer molecules. The rheological characteristics of the lubricant, including the shear stress and viscosity as well as the relaxation time of the liquid molecule, were calculated. The results show that, as the number of branches of the liquid molecule increases, the shear stresses and the wall slip increase. However, for a mixture fluid containing three different branched molecules, the wall slip decreases in comparison to a liquid consisting of only one kind of branched molecule.

Tribological Design by Molecular Dynamics Simulation - The Influence of Polar Additives on Wall Slip and Bulk Shear

“…An increase in the number of molecule branches and molecular mass leads to an elevated relaxation time. Additionally, it's noteworthy that higher temperatures result in a higher critical shear rate and, consequently, a shorter relaxation time [2,3]. Figure 4 is shown calculated viscosity profile of PAO 6 oil The fluid's shear viscosity was determined by calculating the ratio of shear stress to the apparent shear rate 𝜇 = 𝜏/ 𝛾̇ [2].…”

“…The MD simulations were carried out using the open-source LAMMPS molecular dynamics simulator, and a constant NVE integration method was applied to update the velocity and positions of atoms at each time step. The initial macroscopic variables-number of atoms (N), system volume (V), and system energy (E)-were held constant to align with the microcanonical ensemble, ensuring a system trajectory consistent with this ensemble [2,3].…”

“…where the interaction parameters for unlike pairs are calculated through the Lorentz-Berthelot combination rules (Eq. 3 and 4) [3]. Non-bonded LJ potential parameters are tabulated in Table 1.…”

“…Through MD simulations, it investigated the structural and dynamic properties of the lubricant film in the presence of the polar additives, aiming to comprehend their influence on bulk shear and develop insights into optimizing lubricant formulations. In the investigation of non-polar molecules in lubrication, several studies have explored the phenomenon of wall slip and bulk shear with hydrocarbon oils interacting with metal surfaces [1][2][3][4][5]. For instance, a slip model was proposed for linear hydrocarbon lubricants confined between metal atom surfaces, considering specific surface parameters and shear viscosity [4].…”