Swelling/shrinkage induced by shear in narrow pores

International audienceIn this work, using molecular dynamics simulations, we have explored the swelling/shrinkage of a simple slit nano-pore immerged in a Lennard-Jones liquid reservoir induced by shear. It is shown that the pore can swell or shrink when the solid walls are displaced in a direction parallel to the fluid-solid interface. This is due to the fact that the normal pressure of the confined fluid appreciably varies with the relative structural ordering between the solid (crystalline) surfaces for a given pore size. Thus, when the solid walls are moved at a constant velocity, the instantaneous pore size oscillates with time and yields, on average, a shear-induced swelling/shrinka

On the use of a friction model in a Volume of Fluid solver for the simulation of dynamic contact lines

We consider the implementation of a friction contact angle model in a Navier-Stokes VoF-CSF solver for the simulation of moving contact lines at the nano-scale. A liquid-liquid interface confined in a Couette flow generated by two solid walls moving at the same velocity in opposite directions is considered to discuss the relevance of the friction model. The simulations are compared with a reference case obtained using MD simulations by Qian et al. [46]. We show that the Navier Stokes simulations are able to reproduce the MD simulations for both the interface shape and the velocity field. The appropriate contact line friction is found to be grid convergent and of the same order as the friction measured in MD simulations. A detailed investigation of the interface shape has revealed an auto-similar linear profile in the center of the channel. Close to the wall the interface shape follows the classical Log evolution given by the Cox relation despite the wall confinement

Modeling of Simple Fluids Confined in Slit Nanopores (Transport and Poromechanics)

Ce travail vise à étudier les propriétés de transport et le comportement poromécaniquede fluides simples confinés dans des nanopores lamellaires par le biais de simulationsmoléculaires. Pour ce faire, nous avons proposé différents schémas de simulations de ladynamique moléculaire dans des ensembles adaptés aux propriétés étudiées (diffusion demasse, viscosité, force de friction, gonflement ). Il a été note que les propriétés de transportde fluides fortement inhomogènes variaient fortement dans la direction perpendiculaire auxmurs solides. Nous avons alors proposé une approche non-locale permettant de déterminerquantitativement la viscosité locale de fluides inhomogènes à partir du profil de densité etapplicable pour des sphères dures, molles et le fluide de Lennard-Jones. Il a été égalementmontré qu un fluide de Lennard-Jones fortement confiné pouvait avoir un comportementviscoplastique (et rhéofluidifiant) si un ordre structurel était induit dans le fluide par laposition relative des murs solides. Enfin, nous avons montré qu une modification importantede la pression de solvatation du fluide confiné peut être induite par cisaillement ce qui peutinduire un gonflement dynamique d un nanopore lamellaire.This work aims at investigating the transport properties and the poromechanics of simple spherical fluids confined in slit nanopores through molecular simulations. To do so, we have proposed different schemes to perform molecular dynamics simulations in ensembles adequate to deal with the properties we were looking after (mass diffusion, shear viscosity,friction force, swelling ). The transport properties of strongly inhomogeneous fluids were found to be varying with space perpendicularly to the solid walls. We have then proposed a non-local approach to determine quantitatively the local shear viscosity of such inhomogeneous fluids from the density profile applicable from the Hard-Sphere to the Lennard-Jones fluids. In addition, it has been shown that highly confined Lennard-Jones fluid may exhibit a visco-plastic (+ shear thinning) behavior when a strong structural order is induced in the whole confined fluid because of the relative position of the solid walls. Finally, it was demonstrated that shear induced modifications of the solvation pressure of a confined fluid may exist that leads to a dynamic swelling when a slit micropore is sheared.PAU-BU Sciences (644452103) / SudocSudocFranceF

Thermodynamic and Transport properties of fluids: towards a single LJ-SAFT like molecular model valid for n-alkanes ?

International audienceApart from the low density conditions, there is still a lack of a molecular based approach able to provide both equilibrium and transport properties using the same molecular parameters in all fluid regimes (gas, liquid and supercritical). This is even more critical when dealing with poly-atomic fluids. This lack is largely due to the fact that a comprehensive theory is still not available for evaluating the transport properties in dense fluids in terms of a realistic molecular model, i.e. molecular structure and interaction potentials. However, from the equilibrium properties side, the combination of the Lennard-Jones Chain (LJC) molecular model with the Statistical Associating Fluid Theory (SAFT) is able to represent very well the thermodynamic properties of a great variety of fluids [1]. Furthermore, combined with Density Functional or Density Gradient Theory, these approaches are capable to yield very good results on interfacial properties for the same systems [2]. Additionally, based on Molecular Dynamics simulations results, has been proposed recently semi-empirical accurate correlation to describe viscosity [3] and thermal conductivity [4] of the LJC fluid model over a wide range of thermodynamic states. So, in this work, we have used the LJC molecular model combined with LJ-SAFT and transport properties correlation on normal alkanes (methane, n-butane, n-heptane and n-decane) along the vapour/liquid coexistence line. The idea was to test if, with a single set of molecular parameters for each n-alkane, this model was able to provide a reasonable estimate of both thermodynamic and transport properties. Good results have been obtained for methane (monomer) and n-butane (dimer) except for thermal conductivity in the gas state. For n-heptane (trimer) and n-decane (quadrimer) it has been found that both viscosity and thermal conductivity are not always well estimated using this approach. It will be shown that these trends are fully related to the bad modelling of the internal degrees of freedom when using the LJC model. Furthermore using molecular dynamics simulation it will be shown that, for viscosity, a single additional "rigidity" parameter allows to strongly improve all the results. [1] S. P. Tan, H. Adidharma, M. Radosz, Ind. Eng. Chem. Res., 47 (2008), 8063-8082. [2] P. Paricaud, A. Galindo, G. Jackson, Fluid Phase Equilib 194-197 (2002), 87-96. [3] G. Galliero, C. Boned, Phys. Rev. E, 79 (2009) 021201. [4] G. Galliero, C. Boned, Phys. Rev. E, 80 (2009) 061202

Thermodiffusion in model nanofluids by molecular dynamics simulations

In this work, a new algorithm is proposed to compute single particle (infinite dilution) thermodiffusion using Non-Equilibrium Molecular Dynamics simulations through the estimation of the thermophoretic force that applies on a solute particle. This scheme is shown to provide consistent results for simple Lennard-Jones fluids and for model nanofluids (spherical non-metallic nanoparticles + Lennard-Jones fluid) where it appears that thermodiffusion amplitude, as well as thermal conductivity, decrease with nanoparticles concentration. Then, in nanofluids in the liquid state, by changing the nature of the nanoparticle (size, mass and internal stiffness) and of the solvent (quality and viscosity) various trends are exhibited. In all cases the single particle thermodiffusion is positive, i.e. the nanoparticle tends to migrate toward the cold area. The single particle thermal diffusion 2 coefficient is shown to be independent of the size of the nanoparticle (diameter of 0.8 to 4 nm), whereas it increases with the quality of the solvent and is inversely proportional to the viscosity of the fluid. In addition, this coefficient is shown to be independent of the mass of the nanoparticle and to increase with the stiffness of the nanoparticle internal bonds. Besides, for these configurations, the mass diffusion coefficient behavior appears to be consistent with a Stokes-Einstein like law

Thermodiffusion in multicomponent n-alkane mixtures

Compositional grading within a mixture has a strong impact on the evaluation of the pre-exploitation distribution of hydrocarbons in underground layers and sediments. Thermodiffusion, which leads to a partial diffusive separation of species in a mixture due to the geothermal gradient, is thought to play an important role in determining the distribution of species in a reservoir. However, despite recent progress, thermodiffusion is still difficult to measure and model in multicomponent mixtures. In this work, we report on experimental investigations of the thermodiffusion of multicomponent n-alkane mixtures at pressure above 30 MPa. The experiments have been conducted in space onboard the Shi Jian 10 spacecraft so as to isolate the studied phenomena from convection. For the two exploitable cells, containing a ternary liquid mixture and a condensate gas, measurements have shown that the lightest and heaviest species had a tendency to migrate, relatively to the rest of the species, to the hot and cold region, respectively. These trends have been confirmed by molecular dynamics simulations. The measured condensate gas data have been used to quantify the influence of thermodiffusion on the initial fluid distribution of an idealised one dimension reservoir. The results obtained indicate that thermodiffusion tends to noticeably counteract the influence of gravitational segregation on the vertical distribution of species, which could result in an unstable fluid column. This confirms that, in oil and gas reservoirs, the availability of thermodiffusion data for multicomponent mixtures is crucial for a correct evaluation of the initial state fluid distribution

Thermodiffusion of the tetrahydronaphthalene and dodecane mixture under high pressure and in porous medium

A thermodiffusion cell is used in order to perform Soret experiments on binary mixtures at high pressure and in the presence of a porous medium. The cell is validated at atmospheric pressure with toluene/hexane and the tetrahydronaphthalene/dodecane mixtures. The mass separation follows a diffusive behaviour when the cell is filled with a porous medium. At least three times the relaxation time is needed to have a good estimation of the Soret coefficients. From the transient state of the mass separation and using accepted values of the diffusion coefficient, the tortuosity of the porous medium was evaluated, too. Finally, experiments at high pressure were performed with the tetrahydronaphthalene/dodecane system. In these experiments, decreases of the Soret coefficient and of the tortuosity of the porous medium were measured as a function of the pressure

Reference Correlation of the Viscosity of Squalane from 273 to 373 K at 0.1 MPa

International audienceThe paper presents a new reference correlation for the viscosity of squalane at 0.1 MPa. The correlation should be valuable as it is the first to cover a moderately high viscosity range, from 3 to 118 mPa s. It is based on new viscosity measurements carried out for this work, as well as other critically evaluated experimental viscosity data from the literature. The correlation is valid from 273 to 373 K at 0.1 MPa. The average absolute percentage deviation of the fit is 0.67, and the expanded uncertainty, with a coverage factor k = 2, is 1.5%