Correlation between ordering and shear thinning in confined OMCTS liquids

Abstract

Despite the extensive research that has been conducted for decades on the behavior of confined liquids, detailed knowledge of this phenomenon, particularly in the mixed/boundary lubrication regime, remains limited. This can be attributed to several factors including the difficulty of direct experimental observations of the behavior of lubricant molecules under non-equilibrium conditions, the high computational cost of molecular simulations to reach steady state, and the low signal-to-noise ratio at extremely low shear rates corresponding to actual operating conditions. To this end, we studied the correlation between the structure formation and shear viscosity of octamethylcyclotetrasiloxane confined between two mica surfaces in a mixed/boundary lubrication regime. Three different surface separations corresponding to two-, three-, and five-layered structures were considered to analyze the effect of confinement. The orientational distributions with one specific peak for n=2 and two distributions, including a parallel orientation with the surface normal for n>2, were observed at rest. The confined liquids exhibited a distinct shear-thinning behavior independent of surface separations for a relatively low shear rate, \dot{\gamma}<10^{8} s^{-1}. However, the shear viscosities at \dot{\gamma}<10^{8} s^{-1} depended on the number of layered structures. Newtonian behavior was observed with a further increase in the shear rate. Furthermore, we found a strong correlation between the degree of molecular orientation and the shear viscosity of the confined liquids. The magnitude of the shear viscosity of the confined liquids can primarily be determined by the degree of molecular orientation, and shear-thinning originates from the vanishing of specific orientational distributions with increasing shear rate.

Combining Molecular Dynamics and Machine Learning to Analyze Shear Thinning for Alkane and Globular Lubricants in the Low Shear Regime

“…In our MD simulations, the density distributions show that the two-layered structures of the systems are retained at high and low shear (Figure 3a and b), similar to the cases of LJ fluids and other molecules 5,57 The multiple peaks of OMCTS indicate that it is oriented perpendicular to the wall, and the weaker peaks reflect the orientation variation at a high shear rate. 18 By contrast, the two clear peaks of PAO indicate that it is oriented parallel to the wall. On the other hand, the velocity profiles demonstrate that little slip occurred at the mica−lubricant interface at V = 1 m/s (Figure 3c and d).…”

“…These intramolecular configurations presumably affect the intermolecular movements and mobility of liquid films, which reasonably support previous work on the correlation between the viscosity and structures and alignments. 18,22,80 However, the origin that drives diffusion, such as molecular alignment, wall interactions, and momentum transfer to lubricants, is not addressed in this work. Our study can serve as a starting point for future investigations on the information contained in the movement of molecular centers.…”

“…18 Several theoretical models have been developed and validated against both experiments and NEMD simulations for bulk 16 and confined systems. 18 The Carreau model 19 describes changes in shear thinning depending on changes in molecular alignments, 20 which could promote molecular movements along the shear direction. 21 Based on in this model, correlations between shear thinning and shear-induced alignments, such as the radius of gyration 22 and molecular orientations, 18,23 have been found in both bulk and confined systems.…”

“…In these studies, the confined cases used thick oil films with a high sliding speed, and the system converged well. While the dependence of shear thinning on the wall slit width is widely known, systems with a narrow slit and wide plane have rarely been studied at very low shear rates because the sliding of walls over a sufficient distance in the simulations takes an extremely long time . Several theoretical models have been developed and validated against both experiments and NEMD simulations for bulk and confined systems .…”

“…While the dependence of shear thinning on the wall slit width is widely known, systems with a narrow slit and wide plane have rarely been studied at very low shear rates because the sliding of walls over a sufficient distance in the simulations takes an extremely long time . Several theoretical models have been developed and validated against both experiments and NEMD simulations for bulk and confined systems . The Carreau model describes changes in shear thinning depending on changes in molecular alignments, which could promote molecular movements along the shear direction .…”