Structure and Shear Response in Nanometer‐Thick Films

Abstract

Simulations of the structure and dynamics of fluid films confined to a thickness of a few molecular diameters are described. Confining walls introduce layering and in-plane order in the adjacent fluid. The latter is essential to transfer of shear stress. As the film thickness is decreased, by increasing pressure or decreasing the number of molecular layers, the entire film may undergo a phase transition. Spherical molecules tend to crystallize, while short chain molecules enter a glassy state with strong local orientational and translational order. These phase transitions lead to dramatic changes in the response of the film to imposed shear velocities v. Spherical molecules show an abrupt transition from Newtonian response to a yield stress as they crystallize. Chain molecules exhibit a continuously growing regime of non-Newtonian behavior where the shear viscosity drops as v −2/3 at constant normal load. The same power law is found for a wide range of parameters, and extends to lower and lower velocities as a glass transition is approached. Once in the glassy state, 1 chain molecules exhibit a finite yield stress. Shear may occur either within the film or at the film/wall interface. Interfacial shear dominates when films become glassy and when the film viscosity is increased by increasing the chain length.

Molecular simulation and continuum mechanics study of simple fluids in non-isothermal planar couette flows

“…26 In most of our simulations, the sheared fluid is not thermostatted and only the confining walls are maintained at a specified temperature by coupling the motion of their atoms to a heat bath. 18 In order to investigate the effects of thermostatting on the viscosity of the fluid, a set of simulations was also performed by using a thermostatting scheme reported in literature 17,19 where, in addition to the walls, the fluid is also kept at a constant temperature by coupling the y component of the equation of motion of its atoms to a heat bath maintained at the desired temperature. ͑Note that we do not advocate the use of this particular type of thermostat; we only use it to compare our results to those published in the literature.͒ This is achieved by adding Langevin noise and frictional terms to the equation of motion in the y direction:…”

“…[11][12][13][14][15][16][17][18][19][20] These can be classified into two broad categories: homogeneous shear methods and boundary driven shear methods. Homogeneous shear methods 11 impart shear on a fluid by modifying the equations of motion and employing ''sliding brick'' periodic boundary conditions.…”

“…Later, working with pseudo-crystalline solid walls, Thompson and Robbins 17 showed that for Lennard-Jones fluids, the wall-induced structure in the direction parallel to the walls did play an important role in determining the flow boundary conditions. In addition to the boundary conditions, Thompson et al 18,19 also studied the shear response of confined fluids with simulations in which the component of the stress tensor normal to the walls was held fixed. Most of their results were for simple fluids, polymer chains composed of six beads, and for film thicknesses smaller than three sphere diameters.…”

“…In the boundary driven shear simulations of Thompson, et al, 19 Manias et al, 20 as well as in all homogeneous shear simulations, 11 a thermostatting mechanism was used to impose a constant temperature across the entire fluid film; the heat generated in the fluid by ''viscous heating'' was thus removed instantaneously. Such a feat cannot be realized experimentally, and it is therefore of interest to examine to what extent the results of isothermal simulations would deviate from reality.…”

On the question of whether lubricants fluidize in stick–slip friction

“…Such solid sliding is a major cause of frictional dissipation, and can persist even in the presence of lubricants (1). At a nanotribological level, surface force balance (SFB) measurements, supported by theory and computer simulations, have shown that when simple organic liquids are confined between atomically smooth, solid (mica) surfaces to films thinner than some six to eight molecular layers, they may become solid-like, and are often layered (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Subsequent sliding of the surfaces across such films when they are subjected to shear may then take place via stick-slip motion (15,16).…”