Triple-point coexistence properties of the lennard-jones system

The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation.Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [Gattinoni, Maćkowiak, Heyes, Brańka and Dini, Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼ 0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the Central Localization high wall speed region of the phase diagram.Stick-Slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential between the wall atoms, but increases when the attractive part of the potential between wall atoms and confined molecules is made larger.2

Section: Liquid (L)mentioning

confidence: 99%

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

Maćkowiak

Heyes

Dini

et al. 2016

“…A completely independent check useful for solid-fluid coexistence lines is the Ladd-Woodcock direct coexistence technique, 31 consisting in running a simulation at the predicted coexistence temperature and pressure of a system composed of equilibrated configurations of the two coexisting phases. If the predicted coexistence point is correct, the two phases will stay at equilibrium indefinitely, while if the parameters are not correct one phase will prevail.…”

Section: Consistency Checksmentioning

confidence: 99%

Phase diagram of a tetrahedral patchy particle model for different interaction ranges

Romano

Sanz²,

Sciortino

2010

126

173

We evaluate the phase diagram of the Kern-Frenkel patchy model with four interaction sites for four different values of the radial interaction range ͑all in the single-bond-per-patch regime͒ keeping the area of the interaction patches fixed. Four stable crystal phases are investigated, namely diamond cubic ͑DC͒, bcc, fcc, and plastic fcc. The DC is favored at low temperatures and pressures, while the bcc is favored at low temperatures and intermediate to high pressures. At low temperatures and very high pressures an ordered fcc phase is found, while-as expected-at high temperatures, the only stable crystal is a plastic fcc phase. We find a rich phase diagram with several re-entrant coexistence lines, which can be brought in the equilibrium phase diagram by a proper choice of the range. We also show that the gas-liquid phase separation becomes metastable as the range narrows, and it takes place in a region of the phase diagram where the low density diamond crystal is the thermodynamically stable phase.

“…It was with this idea in mind that Ladd and Woodcock devised in the late 70s the direct coexistence method, in which the liquid-solid coexistence properties are estimated from the simulation of the two phases in coexistence. [4][5][6] Though the first applications of the method were not very successful, probably due to the small system sizes and short lengths of the simulations considered at the time, the method is now becoming increasingly popular. Some recent applications include the study of fluidsolid coexistence in simple fluids, 7-12 metals, [13][14][15][16] silicon, 17 ionic systems, 18 hard dumbells, 19 nitromethane, 20 and water.…”

Section: Introductionmentioning

confidence: 99%

Determination of the melting point of hard spheres from direct coexistence simulation methods

Noya

Vega

Miguel

2008

107

We consider the computation of the coexistence pressure of the liquid-solid transition of a system of hard spheres from direct simulation of the inhomogeneous system formed from liquid and solid phases separated by an interface. Monte Carlo simulations of the interfacial system are performed in three different ensembles. In a first approach, a series of simulations is carried out in the isothermal-isobaric ensemble, where the solid is allowed to relax to its equilibrium crystalline structure, thus avoiding the appearance of artificial stress in the system. Here, the total volume of the system fluctuates due to changes in the three dimensions of the simulation box. In a second approach, we consider simulations of the inhomogeneous system in an isothermal-isobaric ensemble where the normal pressure, as well as the area of the ͑planar͒ fluid-solid interface, are kept constant. Now, the total volume of the system fluctuates due to changes in the longitudinal dimension of the simulation box. In both approaches, the coexistence pressure is estimated by monitoring the evolution of the density along several simulations carried out at different pressures. Both routes are seen to provide consistent values of the fluid-solid coexistence pressure, p = 11.54͑4͒k B T / 3 , which indicates that the error introduced by the use of the standard constant-pressure ensemble for this particular problem is small, provided the systems are sufficiently large. An additional simulation of the interfacial system is conducted in a canonical ensemble where the dimensions of the simulation box are allowed to change subject to the constraint that the total volume is kept fixed. In this approach, the coexistence pressure corresponds to the normal component of the pressure tensor, which can be computed as an appropriate ensemble average in a single simulation. This route yields a value of p = 11.54͑4͒k B T / 3 . We conclude that the results obtained for the coexistence pressure from direct simulations of the liquid and solid phases in coexistence using different ensembles are mutually consistent and are in excellent agreement with the values obtained from free energy calculations.