Nanolubrication by ionic liquids: molecular dynamics simulations reveal an anomalous effective rheology

Abstract

This article describes molecular dynamics simulations of an ionic liquid (IL) confined between iron oxide surfaces under relatively high pressure and severe shearing, representative of a typical steel-steel lubricated contact. The simulations reveal the presence of hydrodynamic and thermal slip at the walls, despite the wetting nature of the fluid/wall interface. A crucial consequence of the temperature slip is the subsequent increase of the fluid temperature under shear, which modifies its effective rheology, resulting in saturation of the shear stress at high shear rates. Overall, this article provides a methodology for accurate modeling of tribological contacts lubricated by a nanometer-thick IL film. The results contribute to the debate on the saturation of the shear stress at high shear rates, and reveal the rich phenomenology arising in severe tribological conditions, departing from the traditional understanding of nanofluidic transport, mainly built in the linear response regime and standard thermodynamic conditions.

Lubrication at Extreme Conditions: A Discussion About the Limiting Shear Stress Concept

“…Shaded areas highlight transition regions between coexisting phases. Dots represent the occurrence of LSS deduced from MD simulations in the literature (green dots from [24] and red dot from [25]) (Color figure online) phase regimes in the film behavior, which are sketched in Fig. 2 in the general case and described hereafter.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…The method was initially tested for an L−J fluid near its triple point and yielded a viscosity in agreement with literature results [87]. The method has since been used to predict the viscosity behaviour of simple alkanes [88] as well as more complex multicomponent systems such as lipid bilayers [89], ionic liquids [90], and polymer viscosity modifier additives [91,92]. R-NEMD simulations are simple to implement, versatile and can be used to study transport properties in bulk or confined systems.…”

“…A viscosityshear rate power law was identified for L−J fluids at different pressures and thicknesses [145]. Both the Carreau [90] and Eyring [27] shear thinning models have been successfully used to describe the viscosityshear rate behaviour of confined fluids from NEMD simulations.…”

Rheological behavior of multiwalled carbon nanotube-imidazolium tosylate ionic liquid dispersions

“…We have seen that temperature is a crucial factor to be taken into account for lubricants since arising severe tribological conditions, may result in saturation of the shear stress [43]. We have evaluated the influence of temperature on pure ionic liquid and on the diluted dispersions in figure 6.…”