Non-Equilibrium Molecular Dynamics Simulations of Confined Fluids in Contact with the Bulk


1. Introduction:

Understanding the atomic processes occurring at the interface of two dry or wet materials when they are brought together or moved with respect to one another is central to many technological problems, including adhesion, lubrication, friction, and wear.1-6 Although our understanding of static interfaces has advanced considerably, very little is known about dynamic interfaces at the molecular level. The classical physics of the continuum has historically provided most of the theoretical and computational tools for engineers. However, technology is now reaching to nanoscale dimensions where the continuum picture is no longer valid. Modern molecular simulation and microscopic experimental techniques provide excellent opportunities to tackle these problems. On the theoretical front, several molecular dynamics (MD) simulations have been carried out to study properties of thin films under shear. 7-22 On the experimental front, the surface-force apparatus (SFA) is commonly employed to study both static and dynamic properties of molecularly thin films sandwiched between two molecularly smooth surfaces.

A non-equilibrium molecular dynamics (MD) simulation study is reported of the structural and rheological properties of confined n-decane between two Au(111) surfaces in contact with its bulk under constant normal loads or constant heights. In the constant-load MD simulations, it was observed that fluid molecules were squeezed out of the pore continuously in a single simulation upon compression, whereas fluid molecules in the bulk were soaked into the pore when applied normal load was released. Pore separation depends on bulk pressure under the same normal load and approaches a steady value as normal load increases. In the constant-height simulations, density, velocity, and orientational profiles of the confined film were accumulated along the Z (perpendicular to the walls) and Y (parallel to the walls and finite due to the bulk) directions. These distributions are not uniform not only along the Z direction but also along the Y direction, particularly for weak fluid-wall interactions. The shear-thinning behavior and ‘slip’ boundary conditions were also studied in this work. Even though the shear thinning behavior was reported by several studies before, the number of particles was fixed and the bulk condition was unknown in those simulations. The simulation geometry employed in this work is closer to that of surface force apparatus (SFA) experiments and of engine lubricating systems where confined liquid is in contact with its bulk.

 

2. Methodology:

Illustration of the simulation box. Fluid particles are filled in both confined and bulk regions. Both walls are kept at either constant height or constant normal load while the bulk pressure is kept constant for all simulations. The top and bottom walls move in opposite directions along X-axis at constant velocity U*. Z and Y directions are finite due to confined walls and bulk fluids, respectively, while X direction is infinite through periodic boundary condition. The pore size was either kept constant in constant-height simulations or varied with the specified normal loads in constant-load simulations. During simulations, fluid molecules in both confined and bulk regions can exchange. When the system (confined fluid + its bulk) reaches steady-state, there is a flow along the X direction, but not the Y and Z directions if shear is applied along the X direction. Using this strategy one could overcome the difficulty of inserting or deleting molecules, especially for longer molecule chains.

To maintain the bulk pressure in MD (both constant-load and constant-height) simulations, we assume that there is a virtual piston contacting with the bulk on each side of the simulation cell. We added the following equation of motion for the piston:

 

3. Properties of confined fluids (not uniform in X and Y directions):

Density profiles in 2D

Orientation profiles in 2D

 

Velocity profiles and shear viscosity

 

4. Animation of 'squeezed out' confined fluids:

Click here to download the movie file [9.1 MB]

338 n-decane molecules confined by Au(111) 

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January 21, 2002.