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The Stick-Slip toolkit enables users to easily perform non-equilibrium molecular dynamics simulations of the atomic stick-slip that occurs between an atomic force microscope tip and substrate, and then analyze their simulation results quantitatively.
The following material, geometric, and simulation parameters can be specified through the toolkit interface.
- Material (tip-substrate): This is a drop down box containing possible combinations of tip and substrate materials. Five different material models are currently implemented in this version of the Stick-Slip toolkit: Platinum, gold, copper, silver and aluminum. The lattice type, lattice constant (units: Å), and atomic mass (units: amu) for each of these metals is:
Pt: FCC, 3.92, 195.08
Au: FCC, 4.08, 196.97
Cu: FCC, 3.61, 63.54
Al: FCC, 4.05, 26.98
Ag: FCC, 4.09, 107.9
From these materials, there are eleven possible tip-substrate combinations that can be simulated: Pt-Pt, Pt-Au, Au-Pt, Au-Au, Cu-Cu, Cu-Ag, Ag-Cu, Ag-Ag, Cu-Al, Al-Al, and Al-Cu.
- Tip size: This is a drop down box containing five possible tip sizes. As a size is selected from the drop down box, a bottom view of the tip will appear. The approximate contact area between tip and substrate for each of the five tip sizes constructed of the five possible tip material are as follows (units: nm2).
Pt: 0.24, 0.97, 2.17, 3.86, 6.03
Au: 0.26, 1.05, 2.35, 4.18, 6.54
Cu: 0.20, 0.82, 1.84, 3.28, 5.12
Al: 0.26, 1.03, 2.32, 4.12, 6.44
Ag: 0.26, 1.05, 2.36, 4.20, 6.57
- Tip rotation: The tip can be rotated to change the lattice misfit angle between the tip and substrate; 0 to 360 ͦ.
- Temperature: The temperature of the simulation; 1 to 300K
- Load: The normal load applied to the top of the tip; a positive value is upward away from the substrate, a negative value is downward towards the substrate.
- Timestep: Set the timestep size for subsequent molecular dynamics simulations. The larger the value you choose, the faster the simulation will be. However, the simulation precision will be decreased if the timestep is too large. You can choose any value between 0.002ps and 0.01ps. 0.002ps is recommended.
- Sliding Speed: The velocity of the tip moving laterally relative to the fixed substrate; 1 to 10 m/s.
- Simulation Time: Total simulation duration in picoseconds; 400 to 1600 ps.
Molecular dynamics simulations are run using lammps-07Jul09. The boundary conditions of the simulation box are periodic except in the direction normal to the interface which is “shrink-wrapped”. Both the tip and substrate are divided into two parts. The bottommost atomic layers of the substrate are fixed in place by zeroing their force. A constant lateral velocity is applied to the uppermost atomic layers of the tip which act as a rigid body but are allowed to move normal to the interface to respond to an applied load. All other atoms are unrestricted as the simulation is run in the NVT ensemble (constant number of atoms, volume, and temperature). The embedded atom model (EAM) is used to describe all atomic interactions.
To being a simulation, click the Simulate button on the right-hand side of the Stick-Slip toolkit window. A status bar at the bottom of the panel will provide information about the progress of the simulation. Once the simulation is complete, the following analysis options will be available from the drop down box labeled Results:
- Plot: friction force VS sliding distance: Friction (tangential) force as a function of sliding distance. Typically a stick-slip pattern can be observed.
- Plot: adhesion force VS sliding distance: Adhesion (normal) force between the tip and substrate as a function of sliding distance.
- Plot: average friction force VS rotation angle: The average friction force as a function of the tip rotation angle. A trend can be obtained by running multiple simulations with different rotation angles and all other parameters constant
- Plot: average friction force VS contacting area: The average friction force as a function of the contacting area between tip and substrate. A trend can be obtained by running multiple simulations with different tip sizes and all other parameters constant.
- Plot: average friction force VS load: The average friction force as a function of the normal load. A trend can be obtained by running multiple simulations with different loads and all other parameters constant.
- Plot: average friction force VS temperature:The average friction force as a function of the temperature. A trend can be obtained by running multiple simulations with different temperatures and all other parameters constant
- Simulation result file: log.lammps: Contains the contents of the log file generated when the LAMMPS lmp_linux command is run.
- Simulation result file: dump.file : Contains the contents of the dump file. You can download it and use VMD to visualize the whole process.
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