Friction at the Atomic Scale

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One way to examine the atomic-scale motions that are responsible for friction is to do perform simulations of friction on computers. These simulations are called molecular dynamics simulations and they involve tracking the motion of atoms and molecules as a function of time. The force on an atom governs its motion and is calculated using Newton's equations of motion. The atoms are moved forward in time by solving a set of coupled differential equations. The simulations can be analyzed by making pictures of the atom positions as a function of time. An example of such an animation of a molecular dynamics simulation is shown below.

To view the animation of the molecular dynamics simulation you will need to  download the QuickTime  plugin for your Web Browser.
 

The molecular dynamics simulation depicts an ethane molecule (green and yellow atoms)  trapped between two hydrogen-terminated diamond surfaces. The upper diamond surface slides at a constant velocity from left to right in the movie. As the sliding progresses, a hydrogen atom from the ethane (yellow atom) is sheared from the molecule. The remaining molecule fragment is an ethyl group. The free hydrogen atom remains at the interface until it abstracts a hydrogen atom from the lower diamond surface (red atom) to form a hydrogen molecule. The sliding motion brings the ethyl group in close proximity to the radical on the lower diamond surface. A chemical bond is formed. The result  is a chemically changed diamond surface. 

View the animation of the molecular dynamics simulation

 

Cool Images of Molecular Dynamics Simulations

J. A. Harrison & S. S. Perry, "Friction in the Presence of Molecular Lubricants and Solid/Hard Coatings", MRS Bulletin, 23 (1998) 27.  (Fundamentals of Friction issue, see cover for picture of our simulations    BMP Format  )

 Click here  for a list of references that discuss this molecular dynamics movie and related topics.

Disclaimer:

"Material contained herein is made available for the purpose of peer review and discussion and does not necessarily reflect the views of the Department of the Navy or the Department of Defense."