Lubricant shear thinning behavior correlated with variation of radius of gyration via molecular dynamics simulations

Abstract

The shear thinning of a lubricant significantly affects lubrication film generation at high shear rates. The critical shear rate, defined at the onset of shear thinning, marks the transition of lubricant behaviors. It is challenging to capture the entire shear-thinning curve by means of molecular dynamics (MD) simulations owing to the low signal-to-noise ratio or long calculation time at comparatively low shear rates (10-10 s), which is likely coincident with the shear rates of interest for lubrication applications. This paper proposes an approach that correlates the shear-thinning phenomenon with the change in the molecular conformation characterized by the radius of gyration of the molecule. Such a correlation should be feasible to capture the major mechanism of shear thinning for small- to moderate-sized non-spherical molecules, which is shear-induced molecular alignment. The idea is demonstrated by analyzing the critical shear rate for squalane (CH) and 1-decene trimer (CH); it is then implemented to study the behaviors of different molecular weight poly-α-olefin (PAO) structures. Time-temperature-pressure superpositioning (TTPS) is demonstrated and it helps further extend the ranges of the temperature and pressure for shear-thinning behavior analyses. The research leads to a relationship between molecular weight and critical shear rate for PAO structures, and the results are compared with those from the Einstein-Debye equation.

Shear heating, flow, and friction of confined molecular fluids at high pressure

“…To verify the force-field for the fluids used here, it was first ensured that simulated densities were close to the experimental value at T = 313 K and P = 0.1 MPa using the bulk isothermalisobaric (NPT) procedure described in ref. 46 The density of both fluids, ρ f ; 0.78 g cm −3 for PAO and 0.85 g cm −3 for DCMP were within 4 % of the experimental values of 0.80 g cm −3 for PAO 30 and 0.89 g cm −3 for DCMP. 6 However, UA force-fields have been shown to significantly under-predict the η of long n-alkanes.…”

“…[24][25][26] Shear localization has also been observed in NEMD simulations of many other fluids such as model glasses 27 and entangled polymer melts. 28 Bulk NEMD simulations of lubricant molecules have shed light on their nonlinear rheological behavior under EHL conditions; [29][30][31] however, shear localization cannot be captured in such simulations since a linear velocity profile is imposed on the system. 23,28 Recently, the shear localization and friction of realistic lubricant and traction fluid molecules have also been investigated with confined NEMD simulations under EHL conditions.…”

“…Two fluid molecules were studied; hydrogenated 1-decene trimer, the major component of 4 cSt at 100 • C poly-alphaolefin (PAO)4 lubricant, 30 and 2,4-dicyclohexyl-2-methylpentane (DCMP), a traction fluid marketed as Santotrac 2000 (formerly Santotrac 40). 21,32 Both are well-characterized fluids and their molecular structures are shown in Fig.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Liu et al [129] proposed an approach to correlate shear thinning with the change in the molecular conformation, characterised by the radius of gyration of the molecule from NEMD simulations using a UA force-field [117]. This approach was tested by analysing the critical shear rate for squalane and 1-decene trimer, and then extended to study the behaviours of different molecular weight PAO structures.…”

Nonempirical Free Volume Viscosity Model for Alkane Lubricants under Severe Pressures

“…The viscosity model can be directly implemented in density-based Reynolds-solvers [35] and will contribute to a new cutting-edge simulation tool for tribological applications. The proposed shape parameters also opens new possibilities to quantify the role of molecular structure on rheology in anisotropic situations, such as shear thinning [36,37] or in nanometer-thin boundary lubrication films [38]. Our approach might also be useful for other soft materials, such as self assembled membranes [17,18], polymer-solvent systems [19] or adsorbates in nanopores [39].…”