Fluid alkanes in confined geometries

Abstract

A comparative study of confined fluid films composed of three different alkanes has been carried out using molecular dynamic simulation techniques. The films were confined in thin slit pores, only a few molecular diameters thick, and the substances studied were n-butane, n-decane, and 5-butyl-nonane. The properties of the film were obtained in equilibrium conditions and under shear. All the studied films show a strong layering of the distribution of methylene subunits. Chains at the solid boundaries align with the walls and show a tendency to stretch. The diffusion parallel to the solid walls is found to be higher in the proximity of the walls than in the inner part of the pore. The molecular motion normal to the confining walls can be described as noncorrelated molecular transitions between the contact layer and the inner part of the pore. Shear flow was induced in the film by moving the solid walls. The resulting velocity profiles across the pore were computed as well as the viscosity of the films. The viscosities of the confined fluids in the three cases appear to be the same as those of the bulk, within the uncertainty of the results. No significant influence of the shear flow on the inter- or intramolecular was found.

Mechanical properties and local mobility of atactic-polystyrene films under constant-shear deformation

“…The form of the second potential was originally introduced in Ref. 35, U 2 (x, y, z) = Ae −αz B cos 2 π a x + C cos 2 π a y + D cos 2 π a (x + y) . (5) In this equation α = 5 nm −1 , A = 0.3 kcal/mol, B = C = 50, and C = 1; z denotes the distance from the monomer to the substrate and x, y correspond to the lateral directions, where periodicity is built in.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [711]. Exponentially decaying oscillatory solvation forces across confined liquids were first predicted by computer simulations and theory [711][712][713][714][715][716][717][718][719][720][721][722][723]. Experimental proof came a few years afterwards [724][725][726][727].…”

Structure and solvation forces in confined films of alkanes

“…In our simulations, we used a 9-3 wall-potential, which corresponds to the summation of the mean field atomsolid 10-4 potentials from different lattice planes of a solid consisting of Lennard-Jones particles [31]. The values of the parameters are taken from [32], where a simple 9-3 site-wall potential is introduced for a surface with some texture. However, in our calculations we ignore the corrugation of the surfaces and we used the smooth potential.…”