Hai Hoang

Shear viscosity of inhomogeneous fluids

Using molecular dynamics simulations on inhomogeneous fluids, we have studied the effects of strong density inhomogeneities of varying wavelengths on the shear viscosity computed locally. For dense fluids, the local average density model combined with an adequate weight function yields a good description of the viscosity profiles obtained by simulations. However, for low density inhomogeneous fluids, the local average density model is unable to describe correctly the viscosity profiles obtained by simulations. It is shown that this weakness can be overcome by taking into account the density inhomogeneity in the local translational contribution to the viscosity using a density gradient like approach.

Grand canonical-like molecular dynamics simulations: Application to anisotropic mass diffusion in a nanoporous medium

In this work, we describe two grand canonical-like molecular dynamics approaches to investigate mass diffusion phenomenon of a simple Lennard-Jones fluid confined between solid surfaces and in direct contact with reservoirs. In the first method, the density is used as the control variable in the reservoir whereas it is the pressure in the second method. Both methods provide consistent results, however, the constant density approach is the most efficient with respect to the computational time and implementation. Then, employing the constant density approach, we have studied the transient behavior of the diffusion process associated with the migration of one fluid into another one confined between parallel solid walls. Results have shown that the evolution of molar fraction of the invading fluid follows roughly a 1D diffusion model when the solid phase is weakly or moderately adsorbent with a characteristic time increasing when the pore width decreases. However, when the adsorption is high and the pore width small (i.e., below ten molecular sizes), the apparent mass diffusion in the adsorbed layer is reduced compared to that in the center of the slit pore. Hence, this mass diffusion process becomes a two-dimension phenomenon that must take into account an effective mass diffusion coefficient varying locally.

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.Microgravity simulators: improving oil field assessmentsTo support oil and gas exploration, researchers sent hydrocarbon mixtures into space to obtain accurate data on how each component behaves. The group—led by Guillaume Galliero from the University of Pau and Pays de l’Adour, France—wanted to study the effect of temperature on the movement of individual hydrocarbons in mixtures under typical reservoir conditions. Eliminating the effects of gravity allowed them to collect more accurate data than has previously been obtained. The team showed that thermodiffusion has a large impact on the distribution of hydrocarbon reservoirs under the ground. They state that thermodiffusion should therefore be considered in computer models that assess analytical data collected at potential underground reservoirs. This would allow oil and gas companies to more accurately predict the suitability of the hydrocarbons at potential drilling sites.

Thermodynamic scaling of the shear viscosity of Mie n-6 fluids and their binary mixtures

In this work, we have evaluated the applicability of the so-called thermodynamic scaling and the isomorph frame to describe the shear viscosity of Mie n-6 fluids of varying repulsive exponents (n = 8, 12, 18, 24, and 36). Furthermore, the effectiveness of the thermodynamic scaling to deal with binary mixtures of Mie n-6 fluids has been explored as well. To generate the viscosity database of these fluids, extensive non-equilibrium molecular dynamics simulations have been performed for various thermodynamic conditions. Then, a systematic approach has been used to determine the gamma exponent value (γ) characteristic of the thermodynamic scaling approach for each system. In addition, the applicability of the isomorph theory with a density dependent gamma has been confirmed in pure fluids. In both pure fluids and mixtures, it has been found that the thermodynamic scaling with a constant gamma is sufficient to correlate the viscosity data on a large range of thermodynamic conditions covering liquid and supercritical states as long as the density is not too high. Interestingly, it has been obtained that, in pure fluids, the value of γ is directly proportional to the repulsive exponent of the Mie potential. Finally, it has been found that the value of γ in mixtures can be deduced from those of the pure component using a simple logarithmic mixing rule.

Local shear viscosity of strongly inhomogeneous dense fluids: from the hard-sphere to the Lennard-Jones fluids

This work aims at providing a tractable approach to model the local shear viscosity of strongly inhomogeneous dense fluids composed of spherical molecules, in which the density variations occur on molecular distance. The proposed scheme, which relies on the local density average model, has been applied to the quasi-hard-sphere, the Week-Chandler-Andersen and the Lennard-Jones fluids. A weight function has been developed to deal with the hard-sphere fluid given the specificities of momentum exchange. To extend the approach to the smoothly repulsive potential, we have taken into account that the non-local contributions to the viscosity due to the interactions of particles separated by a given distance are temperature dependent. Then, using a simple perturbation scheme, the approach is extended to the Lennard-Jones fluids. It is shown that the viscosity profiles of inhomogeneous dense fluids deduced from this approach are consistent with those directly computed by non-equilibrium molecular dynamics simulations.

Shear behavior of a confined thin film: influence of the molecular dynamics scheme employed.

In this work, we have considered and compared two molecular dynamics schemes widely used when studying a thin fluid film confined between solid surfaces and undergoing boundary shear. In the first approach, the non-equilibrium simulations are performed on a confined fluid explicitly connected to bulk reservoirs. In the second one, non-equilibrium simulations are carried out on the confined fluid only, in which the average density is deduced from a prior simulation in the grand canonical ensemble. We have found that the apparent properties (average density and effective viscosity) of a strongly confined Lennard-Jones liquid are significantly different using one scheme or the other when the solid surfaces induce a strong structure in the whole fluid, i.e., for small separations between the solid surfaces. Furthermore, the shear velocity dependence of the friction force has been found to be as well very sensitive to the approach chosen and can be well understood in terms of the fluid structure, which can even lead to a visco-plastic behavior of the fluid in some cases. Finally, it is shown that the first scheme is the only one usable to explore the history-dependence of the friction force as observed in experiments.

Swelling/Shrinkage Induced by Shear in Narrow Pores

In 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/shrinkage.

Shear Viscosity of Inhomogeneous Hard-Sphere Fluids

Using molecular dynamics on Hard-Sphere-like fluids subject to an external sinusoidal field inducing density inhomogeneities and undergoing a bi-periodical shear flow, we have studied the local viscosity of the inhomogeneous fluid. It has been shown that for a slowly varying density profile the local average density model combined with the well-known models proposed in the density function theory yields a good description of the viscosity profile obtained by molecular simulation. However, for a rapidly varying density profile these models are unable to describe correctly the viscosity profile obtained by molecular simulations. So, to overcome the weakness of these models we have proposed a simple model that takes into account the effect of the angle formed by the colliding molecules and the direction of the flow.