24th International Colloquium Tribology - January 2024 175 Effect of Polar Additives on the Slip and Bulk Shear of Hydrocarbon Oils Seyedmajid Mehrnia 1* , Maximilian Kuhr 1 , Peter F. Pelz 1 1 Chair of Fluid Systems, Technical University of Darmstadt, Darmstadt, Germany * Corresponding author: seyedmajid.mehrnia@tu-darmstadt.de 1. Introduction Different liquid lubricants are utilized on sliding surfaces and rotating parts to reduce the unfavorable consequences of friction. Despite this, most researchers are moving to decrease the usage of lubricant fluids because of their undesirable influence on the environment. Polar additives are interested in developing research considerations. This is because of their specifics, such as improving viscosity and reducing friction. In this paper, the wall slip and bulk shear of a polar lubricant additive blended with the non-polar hydrocarbon oil, i.e., Polyalphaolephin (PAO) base oil, at various temperatures in interaction with metal surfaces will be investigated. The polar molecule used here is Poly Alkyl Methacrylate (PAMA), a Viscosity Index (VI) improver lubricant additive. Molecular Dynamics (MD) simulation is employed as a tool for capturing the liquid-metal interaction. Figure 1 shows calculated dynamic viscosity in different apparent shear rates for three forms of 1-Decane molecule of PAO oil by MD simulation. In this simulation, the lubricant is confined to a stationary and moving wall. There is a Couette flow between the walls due to wall movement. The of 1-Decane tetramer molecule with various branches are used in this simulation. A hybrid force field consisting of different potential energy functions was employed in this simulation. The potential energy functions calculate the force for each atom, depending on the position of the other atoms. Newton’s laws define how those forces will influence the atoms‘ movements. The potential energy depends on the angle, bond, dihedral, and improper of the atoms. Force field application is representing the time evolution of angle, stretching, and torsion rotation of the bonds, besides the non-bonded interactions within atoms [1-3]. 1.1 Simulation method All molecules models are designed using Avogadro software, then assembled and optimized by the Packmol software. The model utilized for the liquid molecules is a full atom-style model that includes bond stretching, bending, and torsion. The MD equations of motion were integrated using the velocity-Verlet algorithm with an integration time-step of 1.0-fs for the selected force field. The MD modeling in this research was performed by the LAMMPS molecular dynamics simulator, an open-source code [4]. Updating velocity and positions of atoms in the group each time step was performed by a fixed NVE integration. This generates a system trajectory compatible with the microcanonical ensemble. At the start of the simulation, the system was relaxed to equilibrium in 0.05-ns. Hence, the system achieved a thermodynamic stability state. Through all simulations, the temperature is uniformly controlled by a Langevin thermostat. The main outcome of this research is calculation of wall slip and the effect of polar additives on viscosity and friction in different temperatures. Figure 1: Schematic structure of PAMA-L C 2 O2 (CH2)7 (CH3)4 1.2 Slip and bulk shear The concept of apparent and real measures has proven to be practical in rheology and is hence used here also in the context of MD tribology. If we assume a fluid film between two parallel walls having the distance h sliding relative to each other with velocity U the apparent shear rate is γ ̇ app -=-U⁄ h, the apparent shear stress is τ app = m γ ̇ app . For any stationary Couette flow, the real shear stress τ(z) = τ w is constant across the channel height 0 ≤ z ≤ h and equal to the wall shear stress τ w . For small Reynolds number, the shear stress is dominated by molecular, i.e. viscous forces and the result of the constant shear stress is a constant field of shear rate τ =-τ w -=-m γ ̇ =-cons. [1]. In the context of polar additive molecules, their polar functional groups exhibit an affinity for the fluid molecules. This interaction can give rise to a thin layer of fluid molecules in proximity to the surface, commonly referred to as the boundary layer or adsorbed layer. The boundary layer possesses distinct characteristics when compared to the bulk fluid, leading to alterations in viscosity and flow behavior. In this specific scenario, slip values were exceedingly small, approaching zero. This occurs when the fluid molecules strongly bind with the polar molecules, causing them to adhere closely to the surface. Such minimal slip results in heightened friction and increased resistance to flow, leading to reduced flow rates and elevated pressure drop. Figures 2 and 3 depict the impact of polarity on the black-marked PAO 6 near the metal wall. 176 24th International Colloquium Tribology - January 2024 Effect of Polar Additives on the Slip and Bulk Shear of Hydrocarbon Oils Figure 2: Side view of a 3D simulation box depicting a black marked PAO 6 molecule along a metal wall. Figure 3: Side view of a 3D simulation box depicting a black marked PAO 6 molecule along a metal wall in interaction with PAMA molecules. The viscosity of fluids typically decreases significantly as temperature rises, which can be a significant issue for lubricants that often operate across a range of temperatures. The role of a Viscosity Index (VI) improver is to enhance the viscosity of a low-viscosity base fluid to achieve a desirable viscosity level at high temperatures while avoiding excessive viscosity increase at low temperatures. VI improvers are employed to modify the relationship between viscosity and temperature in lubricants. Figure 4 illustrates the impact of polar additives on viscosity as temperature increases. Figure 4: Calculated apparent viscosity in different temperature. 1.3 Conclusion In simulations involving confined lubricants, various methods can be employed to evaluate slip. One such approach involves determining slip velocity by analyzing the velocity profile of lubricant molecules near the solid surface. The findings from this analysis suggest that slip values are minimal, mainly due to the presence of polar atoms at the wall surfaces. Calculating the viscosity of confined polar lubricant additives mixed with PAO oil between metal surfaces in MD simulations necessitates considering the interactions among the lubricant additives, PAO oil, and the metal surfaces. To achieve this, a shear gradient is applied, the stress tensor is computed, and stress autocorrelation is analyzed to estimate viscosity. The resulting viscosity values demonstrate that polar molecules exert a significant influence on viscosity, primarily as a result of coil expansion. The concept of coil expansion proposes that a polymer initially maintains a coiled conformation at lower temperatures and then expands as solubility increases at higher temperatures, leading to elevated viscosity. Furthermore, the interaction between polar molecules and carbon groups contributes to the heightened viscosity. References [1] Pelz P. F., Corneli T., Mehrnia S., Kuhr M. (2022): Temperature-dependent wall slip of Newtonian lubricants. Journal of Fluid Mechanics, 948, A8. [2] Mehrnia S., Pelz P. F. (2021): Slip length of branched hydrocarbon oils confined between iron surfaces. Journal of Molecular Liquids, 336, 116589. [3] Mehrnia S., Pelz P. F. (2022): Tribological design by Molecular Dynamics simulation - The influence of molecular structure on wall slip and bulk shear. Chemical Engineering & Technology, 202200448. [4] Plimpton S. (1995): Fast parallel algorithms for shortrange molecular dynamics. Journal of Computational Physics, 117, 1-19.