We performed a series of molecular dynamics simulations investigating the static and dynamic properties of polymer melts confined between planar solid surfaces. The solid–melt interface was found to be very narrow (approximately two segment diameters) and independent of chain length. Inside the interface the segment density profile was oscillatory, the bond orientation altered between directions parallel and normal to the solid surface, and the chain ends accumulated very close to the wall (in the absence of strong wall–segment attraction). The oscillations of the segment density profile were weaker and were dampened faster than those of a simple fluid density profile next to the same solid surface. This reflected the reduced ability of sequences of connected segments (chains) to layer themselves against a solid surface because of restrictions on their configurations imposed by the chain connectivity requirement. This effect made the solid–melt interface even narrower than that of a simple fluid. Only the chain portions lying inside the interface had their shape affected by the wall. Chain statistical segments inside the interface assumed orientations parallel to the wall. In the absence of wall–segment attraction, the size of the statistical segments inside the interface was unaffected. This situation resulted in an apparent decrease of the radius of gyration normal to the wall an apparent increase of the radius of gyration parallel to the wall and spatial independence of the total radius of gyration. The wall effect was gradually diminished and chains assumed their bulk dimensions when their center-of-mass was so far from the solid surface that no portions of the chain could reach the interface (i.e., at a distance comparable to the bulk radius of gyration). The microscopic dynamics of chain portions inside the interface were strongly anisotropic. The mobility increased in the direction parallel to the wall and decreased normal to the wall. This fact was caused by the angular asymmetry of the segment–segment collisions inside the interface, i.e., by the same mechanism that induces the segment layering. The total mobility inside the neutral wall–melt interface was identical with that in the bulk reflecting the fact that the average segment density inside the interface had essentially the bulk value. The presence of strong wall–segment attraction increased the average interfacial density above the bulk value and lowered the mobility of the interfacial chain portions in all directions. The mean-square displacement of the chain center-of-mass during a certain time interval was affected by the solid only if the chain had a portion of itself inside the interface for a fraction of this time interval. The longest relaxation time of the chains, a property that cannnot be localized properly on a length scale smaller than the interfacial width, exhibited a weak and strongly diminishing with chain length spatial dependence.
A classical molecular dynamics study of recombination reactions in a microporous solidThe method of nonequilibrium molecular dynamics is used to study the viscosity and flow properties of strongly inhomogeneous liquids, a particular case of which is a liquid confined in ~ micr.opore only a few molecular diameters wide. Fluid inhomogeneity is introduced by lmposmg an external potential that in one case simulates flat solid walls and in the other case causes density peaks in·the middle of a thin liquid film. For comparison a homogeneous fluid is also simulated. In both types of inhomogeneous fluid, the shear stress and effective viscosity are smaller than in the homogeneous fluid. The density profiles and the diffusivities in the micropore were found to be independent of flow, even at the extremely high rates, 10 10 _10 11 S-I ofthe simulation. The Green-Kubo relation is found to be valid for the diffusivity under t~e fl~,,: st1.~died .. We propose a local average density model (LADM) of viscosity and dlffuslVlty, m whlch the local transport coefficients are those of homogeneous fluid at a mean dens~ty ?btained by avera~ng the local density over a molecular volume. LADM predicts qUalltatlvely correct veloclty profiles, effective viscosities, and shear stresses using only equilibrium density profiles and molecular diameters. Recently Vanderlick and Davis generalized Enskog's theory of diffusivity to strongly inhomogeneous fluids. Their theoretical pore diffusion coefficient is also in good agreement with simulation results and LADM.
A recently introduced model is used to study several flows in fluids with large density variations over distances comparable to their molecular dimensions (strongly inhomogeneous fluids). According to our model, the local average density model (LADM), local viscosity coefficients can be assigned at each point in a strongly inhomogeneous fluid and the stress tensor retains its Newtonian form provided that the properly defined local viscosities are used. The model has been previously shown to agree with the results of molecular dynamics simulations on diffusion and flow properties in plane Couette flow. Application of this model requires determination of the molecular density profiles in the flow region. Using a successful closure for the pair distribution function, we solve the Yvon–Born–Green (YBG) equation of fluid structure in order to determine the density profiles of a fluid confined between planar micropore walls only a few molecular diameters apart. The fluid confinement produces a strongly inhomogeneous structure. Subsequently we apply LADM to set up the fluid mechanical equations for Couette flow, Poiseuille flow, and squeezing flow between parallel plates. With the use of the YBG theoretical density profiles we solve the flow equations and predict velocity profiles, stress distributions, and effective viscosities. The dependence of these quantities on the fluid inhomogeneity is described. The effective viscosity of strongly inhomogeneous fluids is found to be quite sensitive to the nature of the flow. Our squeezing flow analysis provides a first explanation of recent experimental findings on the effective viscosity of simple fluids confined in very narrow spaces.
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Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed and is due to the inability of fluid layers to undergo the gliding motion of planar flow. This effect is partially responsible for the strong viscosity increases observed experimentally in thin films that still maintain their fluidity.
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