Energy dissipation in non-isothermal molecular dynamics simulations of confined liquids under shear

Abstract

Energy is commonly dissipated in molecular dynamics simulations by using a thermostat. In non-isothermal shear simulations of confined liquids, the choice of the thermostat is very delicate. We show in this paper that under certain conditions, the use of classical thermostats can lead to an erroneous description of the dynamics in the confined system. This occurs when a critical shear rate is surpassed as the thermo-viscous effects become prominent. In this high-shear-high-dissipation regime, advanced dissipation methods including a novel one are introduced and compared. The MD results show that the physical modeling of both the accommodation of the surface temperature to liquid heating and the heat conduction through the confining solids is essential. The novel method offers several advantages on existing ones including computational efficiency and easiness of application for complex systems.

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

“…At low v s (1-5 m s −1 ), the simulations can be considered isothermal since there is a negligible increase in T f , which is within 10 K of the target value (350 K). 32,35 Generally, larger increases in T f occur; i) at higher v s , ii) at higher P, and iii) for DCMP compared to PAO. Possible reasons for these observations are discussed later in this section.…”

“…5 The thermostat acted only in the direction perpendicular to both sliding and compression (y). 32,35 The thermostat was applied only to the middle layer of slab atoms (orange boxes in Fig. 1), allowing a T gradient to develop within the fluid film due to shear heating.…”

“…Conversely, in this current NEMD study, particular attention will be paid towards non-isothermal conditions where significant shear heating occurs within the fluid film. 35 The resultant spatially resolved T profiles will be used to investigate the thermal conductivity of the molecular fluids, λ f , as they have been for LJ fluids. 24,25 This will allow the applicability of macroscale models for EHL film T rise to be tested at the molecular-scale.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…For example, the slip behaviour of thermostatted molecules confined between rigid walls disagrees with that observed experimentally and in simulations where only flexible wall atoms are thermostatted [57] (see Section 2.2.3). In confined NEMD simulations of tribological systems, stochastic thermostats (e.g., Langevin [45]) are generally more popular than the NH thermostat since they can remove heat more efficiently [58]. For the relatively thin films usually studied in confined NEMD simulations, the temperature rise, which is generally largest in the centre of the film, only becomes significant at high shear rates [58].…”

“…In confined NEMD simulations of tribological systems, stochastic thermostats (e.g., Langevin [45]) are generally more popular than the NH thermostat since they can remove heat more efficiently [58]. For the relatively thin films usually studied in confined NEMD simulations, the temperature rise, which is generally largest in the centre of the film, only becomes significant at high shear rates [58]. More specialised thermostatting strategies have also been developed, for example, thermostats can be applied to "virtual" wall atoms rather than the wall atoms themselves [59].…”