Pressure dependence of confined liquid behavior subjected to boundary-driven shear

Multiscale simulation of fluids: coupling molecular and continuum

Computer simulation is an important tool for scientific progress, especially when lab experiments are either extremely costly and difficult or lack the required resolution. However, all of the simulation methods come with limitations. In molecular dynamics (MD) simulation, the length and time scales that can be captured are limited, while computational fluid dynamics (CFD) methods are built on a range of assumptions, from the continuum hypothesis itself, to a variety of closure assumptions. To address these issues, the coupling of different methodologies provides a way to retain the best of both methods. Here, we provide a perspective on multiscale simulation based on the coupling of MD and CFD with each a distinct part of the same simulation domain. This style of coupling allows molecular detail to be present only where it is needed, so CFD can model larger scales than possible with MD alone. We present a unified perspective of the literature, showing the links between the two main types of coupling, state and flux, and discuss the varying assumptions in their use. A unique challenge in such coupled simulation is obtaining averages and constraining local parts of a molecular simulation. We highlight that incorrect localisation has resulted in an error in the literature. We then finish with some applications, focused on the simulation of fluids. Thus, we hope to motivate further research in this exciting area with applications across the spectrum of scientific disciplines.

Slip and stress from low shear rate nonequilibrium molecular dynamics: The transient-time correlation function technique

We derive the transient-time correlation function (TTCF) expression for the computation of phase variables of inhomogenous confined atomistic fluids undergoing boundary-driven planar shear (Couette) flow at constant pressure. Using nonequilibrium molecular dynamics simulations, we then apply the TTCF formalism to the computation of the shear stress and the slip velocity for atomistic fluids at realistic low shear rates, in systems under constant pressure and constant volume. We show that, compared to direct averaging of multiple trajectories, the TTCF method dramatically improves the accuracy of the results at low shear rates and that it is suitable to investigate the tribology and rheology of atomistically detailed confined fluids at realistic flow rates.

Effects of shear flow on the structure and dynamics of ionic liquids in a metallic nanoconfinement

It has been shown that a weak shear can induce crystallisation in a disordered, glassy state. In this study, we use molecular dynamics simulations in order to investigate the out-of-equilibrium properties of [BMIM][BF₄] confined between metal slabs. In particular, we want to understand the extent to which the shear flow modifies the interfacial properties. In particular, the questions we address here are (i) is the shear able to promote the crystalline phase in [BMIM][BF₄]? (ii) Can, as a consequence of shear flow, a solid-like layer develop at the interface with a metallic surface? (iii) What are the tribological properties of nanoconfined [BMIM][BF₄]? We find that the system behaves quite differently from the ideal linear Couette flow. Indeed, the portion of fluid closer to the shearing slabs behaves as a disordered, solid-like layer, which, under the investigated conditions extends to a few nanometres. The linear velocity regime is only recovered in the central region of the ionic liquid slab. The formation of such a solid-like glassy rather than crystalline layer is in agreement with recent mechanical impedance measurements performed on nano-confined ionic liquids.

Entropy Scaling of Viscosity - II: Predictive Scheme for Normal Alkanes

In this work, a residual entropy value 6/10 of the way between the critical point and a value of -2/3 of Boltzmann's constant is shown to collapse the scaled viscosity for the family of normal alkanes. Based on this approach, a nearly universal correlation is proposed that can reproduce 95% of the experimental data for normal alkanes within ±18% (without removal of clearly erroneous data). This universal correlation has no new fluid-specific empirical parameters and is based on experimentally accessible values. This collapse is shown to be valid to a residual entropy half way between the critical point and the triple point, beyond which the macroscopically-scaled viscosity has a super-exponential dependence on residual entropy, terminating at the triple point. A key outcome of this study is a better understanding of entropy scaling for fluids with intramolecular degrees of freedom. A study of the transport and thermodynamic properties at the triple point rounds out the analysis.

The Influence of Lubrication and the Solid-Fluid Interaction on Thermodynamic Properties in a Nanoscopic Scratching Process

Liquid lubricants play an important role in contact processes; for example, they reduce friction and cool the contact zone. To gain better understanding of the influence of lubrication on the nanoscale, both dry and lubricated scratching processes in a model system are compared in the present work using molecular dynamics simulations. The entire range between total dewetting and total wetting is investigated by tuning the solid-fluid interaction energy. The investigated scratching process consists of three sequential movements: A cylindrical indenter penetrates an initially flat substrate, then scratches in the lateral direction, and is finally retracted out of the contact with the substrate. The indenter is fully submersed in the fluid in the lubricated cases. The substrate, the indenter, and the fluid are described by suitably parametrized Lennard-Jones model potentials. The presence of the lubricant is found to have a significant influence on the friction and on the energy balance of the process. The thermodynamic properties of the lubricant are evaluated in detail. A correlation of the simulation results for the profiles of the temperature, density, and pressure of the fluid in the vicinity of the chip is developed. The work done by the indenter is found to mainly dissipate and thereby heat up the substrate and eventually the fluid. Only a minor part of the work causes plastic deformation of the substrate.

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

Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.

Elastohydrodynamic lubricant flow with nanoparticle tracking

Lubricants operating in elastohydrodynamic (EHD) contacts exhibit local variations in rheological properties when the contact pressure rises.

Molecular dynamics investigation of the influence of the shape of the cation on the structure and lubrication properties of ionic liquids

A theoretical study of the influence of the geometry of the cation on the response of ionic liquids to confinement and mechanical strain is presented. The specific friction is low and the friction force decreases with tail size. The low hysteretic losses during the linear cyclic motion suggest strong slip inside the gap.

Independence between friction and velocity distribution in fluids subjected to severe shearing and confinement

Lubricated friction at high shear and high enough pressure becomes saturated, independently of the velocity profile in the lubricant thickness.

On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction

Atomistic simulations and tribology experiments uncover the effect of molecular structure on the flow and friction behaviour of confined films under extreme conditions.

Scaling law for crystal nucleation time in glasses

Due to high viscosity, glassy systems evolve slowly to the ordered state. Results of molecular dynamics simulation reveal that the structural ordering in glasses becomes observable over "experimental" (finite) time-scale for the range of phase diagram with high values of pressure. We show that the structural ordering in glasses at such conditions is initiated through the nucleation mechanism, and the mechanism spreads to the states at extremely deep levels of supercooling. We find that the scaled values of the nucleation time, τ1 (average waiting time of the first nucleus with the critical size), in glassy systems as a function of the reduced temperature, T˜, are collapsed onto a single line reproducible by the power-law dependence. This scaling is supported by the simulation results for the model glassy systems for a wide range of temperatures as well as by the experimental data for the stoichiometric glasses at the temperatures near the glass transition.

Boundary-controlled barostats for slab geometries in molecular dynamics simulations

Molecular dynamics simulation barostat schemes are derived for achieving a given normal pressure for a thin liquid or solid layer confined between two parallel walls. This work builds on the boundary-controlled barostat scheme of Lupkowski and van Swol [J. Chem. Phys. 93, 737 (1990)]. Two classes of barostat are explored, one in which the external load is applied to a virtual regular lattice to which the wall atoms are bound using a tethering potential. The other type of barostat applies the external force directly to the wall atoms, which are not tethered. The extent to which the wall separation distribution is Gaussian is shown to be an effective measure of the quality of the barostat. The first class of barostat can suffer from anomalous dynamical signatures, even resonances, which are sensitive to the effective mass of the virtual lattice, whose value lacks any rigorous definition. The second type of barostat performs much better under equilibrium and wall-sliding nonequilibrium conditions and in not being so prone to resonance instabilities in the wall separation and does not require so many largely arbitrary parameters. The results of exploratory simulations which characterize the dynamical response of the model systems for both dry and wet or lubricated systems using the different barostats are presented. The barostats which have an inherent damping mechanism, such as the ones analogous to a damped harmonic oscillator, reduce the occurrence of large fluctuations and resonances in the separation between the two walls, and they also achieve a new target pressure more quickly. Near a nonequilibrium phase boundary the attributes of the barostat can have a marked influence on the observed behavior.

Traction and nonequilibrium phase behavior of confined sheared liquids at high pressure

Nonequilibrium molecular dynamics simulations of confined model liquids under pressure and sheared by the relative sliding of the boundary walls have been carried out. The relationship between the time-dependent traction coefficient, μ(t), and the state of internal structure of the film is followed from commencement of shear for various control parameters, such as applied load, global shear rate, and solid-liquid atom interaction parameters. Phase diagrams, velocity and temperature profiles, and traction coefficient diagrams are analyzed for pure Lennard-Jones (LJ) liquids and a binary LJ mixture. A single component LJ liquid is found to form semicrystalline arrangements with high-traction coefficients, and stick-slip behavior is observed for high pressures and low-shear velocities, which is shown to involve periodic deformation and stress release of the wall atoms and slip in the solid-liquid boundary region. A binary mixture, which discourages crystallization, gives a more classical tribological response with the larger atoms preferentially adsorbing commensurate with the wall. The results obtained are analyzed in the context of tribology: the binary mixture behaves like a typical lubricant, whereas the monatomic system behaves like a traction fluid. It is discussed how this type of simulation can give insights on the tribological behavior of realistic systems.