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Molecular Dynamics Simulation Studies of Surface Contacts

Molecular Dynamics Simulation Studies of Surface Contacts. C.W. Yong and W. Smith Daresbury Laboratory & K. Kendall, Birmingham. (DEM workshop). DL_POLY. general-purpose molecular dynamics simulation package, developed by Daresbury Laboratory under the auspices of CCP5.

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Molecular Dynamics Simulation Studies of Surface Contacts

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  1. Molecular Dynamics Simulation Studies of Surface Contacts C.W. Yong and W. Smith Daresbury Laboratory & K. Kendall, Birmingham (DEM workshop)

  2. DL_POLY • general-purpose molecular dynamics simulation package, developed by Daresbury Laboratory under the auspices of CCP5. • world-wide user base and applications in many areas of molecular simulations. • Written in Fortran (mostly F77, with some F90 features), DL_POLY consists of a collection program modules, which can be compiled on any UNIX platform, including the Beowulf’s system. Software is supplied as source code.

  3. DL_POLY_2 • Broadly based on the Replicated Data (RD) parallelisation strategy for distributed memory parallel computers • The RD strategy allows various types of complex procedures and force fields to be incorporated and adapted- neighbour list, Ewald summation etc. • Down side: high communication overhead, in poor performance scaling with large number of processors. • offset by the advances in hardware architecture and communications, simulating larger systems (10 000 atoms or more). DL_POLY_3 • Domain decomposition version of DL_POLY, for large scale simulations, capable to simulate systems of order 106 atoms) • Does not support all the features of DL_POLY.

  4. Applications • Liquids and solutions - bulk structures, interface properties • Ionic solids - ceramics, zeolites, effects of temperature and pressures on a-SiO2 and berlinite (a-AlPO4) • Molecular crystals • Amorphous materials - polymers, glasses • Solid interfaces - dissolution processes of calcite minerals, hydrated surfaces kaolin and silica, thin film supports and material implantation • Biomolecular systems - proteins, lipid membranes (solvation, dynamics, structures etc) W. Smith, C.W. Yong and P.M. Rodger, ‘DL_POLY: Application to Molecular Simulation’, Molecular Simulation28, 385 (2002)Licence, user manual: http://www.dl.ac.uk/TCS/Software/DL_POLY/

  5. Huge commercial importance Physics of powders poorly understood (Solids? Fluids?) Theoretical models difficult (multiple contacts, roughness, size and shape distributions) Major problems in: Transport Compaction Segregation Molecular Dynamics? Powder Technology

  6. Friction • Occurs whenever two moving bodies come into contact. • Friction is not a single phenomenon; many processes contribute to the dissipation of bulk kinetic energy. • Why is Fs ~ L (independent of surface area)? • Lack details regarding the underlying factors that contribute to frictional forces (exchange of atoms, defects, contaminants etc). • Important to investigate systematically the atomistic factors underlying friction and to establish links from these to macroscopically observed frictional phenomena.

  7. rcut Molecular Dynamics Basics Pair Potential: Lagrangian:

  8. The Equations of Motion Leapfrog algorithm

  9. The Molecular Dynamics Simulations • Consider two surfaces in close contact - important for understanding of adhesion and friction. • Assume point-to-surface contact. • Look at the forces arising when contact is made and broken. • Looked for the atomic mechanisms that underlie the observed force behaviour. • Extrapolate results to macroscopically observed adhesion and friction (as far as possible). • Code used: DL_POLY

  10. Computational Model (001)MgO, NaCl - (n x n) Fixed Back Plane (110)TiO2 - (2 x m) Probe Probeis advanced towards the surface andwithdrawn while the force on the back plane is monitored Probe is advanced in discrete steps, ensuring equilibration at all times Surface Slab free bath Fixed Back plane

  11. Contact Forces & Probe Dimensions (MgO) Yong et. al., J. Mater. Chem. 12, 593 (2002)

  12. Simulating Contact (MgO) (a) (b) (c)

  13. Configuration Energy 6x6 probe

  14. NaCl Contact Forces Yong et. al., J. Mater. Chem. 12, 2807 (2002)

  15. NaCl Contact `Anomaly’

  16. NaCl Probe Withdrawal

  17. Force and Free Energy

  18. Scaling Behaviour

  19. Contact Forces & Probe Dimension (TiO2) Yong et. al., J. Mater. Chem. 12, 2807 (2002)

  20. Simulating Contact (TiO2) (c) (a) (b)

  21. Narrow Probe Retraction

  22. Narrow Probe Retraction (a) (b)

  23. Broad Probe Retraction

  24. Broad Probe Retraction

  25. TiO2 Probe Withdrawal

  26. TiO2 probe Withdrawal

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