Atomic scale(molecular dynamic) simulationa

Atomic Scale Simulation 1

Advanced Tribology

Presentation on: Atomic Scale Simulation

By: Mr. Mekete Mulualem

Monday, May 1, 2023

Monday, May 1, 2023 Atomic Scale Simulation 2

Content • Introduction • Atomic Scale Simulation • Molecular Dynamic(MD) Simulation• Advantages of MD• Limitations of the MD technique• Application of ASS• Conclusion • Reference


Introduction

Most theoretical approaches to contact problems on the macro-scale rely on the continuum elasticity. But

continuum mechanics is not fully applicable as the characteristics dimensions of the contact between material bodies are reduced. Also interfacial effects become more

prominent.


Adhesion, capillary forces and other factors can be ignored in most macroscopic machines but often dominate the behavior at nanometer scales. here, we will discuss about theoretical method atomic scale simulation, especially on molecular dynamic simulation. …

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Atomic Scale Simulation

Well established engineering tools for designing macroscopic mechanical devices break down as dimensions decrease to nanometer scales.

For nanometer scale material bodies, adhesion, capillary forces and other factors has been identified as the main cause of failure in MEMS/NEMS. Concurrently, with the development and use of innovative experiment tools like SFA, STM and AFM/FFM..


Theoretical methods include analytical methods and molecular dynamics (MD) simulation. Analytically understanding of the nature of interatomic interaction in materials and computer-based modeling of complex systems have led to the development of MD simulation in recent years.

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Oil and water separation by molecular dynamic simulation(video)


Molecular dynamics (MD) is a computer simulation technique that allows one to predict the time evolution of a system of interacting particles (atoms, molecules, etc.).

MD have been used to understand the material removal mechanism, effects of tool geometry, temperature, and process parameters such as cutting speed and cutting force.

Molecular Dynamic(MD) Simulation


MD is a computer simulation method for studying the physical movements of atoms and molecules. The atoms and molecules are allowed to the interact for a fixed period of time, giving a view of dynamic evolution of the system.In order to do MDS;First, for a system of interest, one has to specify: A set of initial conditions(initial positions & velocities of all particles in the system) Interaction potential for deriving the forces among all the particles.

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Second, the evolution of the system in time can be followed by solving a set of classical equations of motion for all particles in the system.

If the particles of interest are atoms, and if there are a total of Nat of them in the system, the force acting on the ith atom at a given time can be obtained from the inter atomic potential U(r1,r2, r3, …, rNat) that, in general, is a function of the positions of all the atoms:

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Once the initial conditions and the interaction potential are defined, the equations of motion can be solved numerically. The results of the solution are the positions and velocities of all the atoms as a function of time,

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There are four basic behaviors of atoms or molecules that an atomic scale simulation has to phenomenologically produce: Atoms experience a short-ranged attraction. When atoms and/or molecules are far apart, they barely interact. When atoms and/or molecules come very close to one another, they strongly repel. Atoms and molecules are in a never-ending motion.


All these critical behaviors are produced by numerically solving Newton’s equations of motion for a system of atoms and molecules, which interact with each other through reasonable, inter-atomic and inter-molecular potentials.

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Advantages of MD

The only input in the model – description of interatomic or intermolecular interaction

No assumptions are made about the processes or mechanism to be investigated

Provides a detailed molecular/atomic-level information


Generally, MD is a deterministic technique; given initial positions and velocities, the evolution of the system in time is, in principle, completely determined (in practice, accumulation of integration and computational errors would introduce some uncertainty into the MD output).


Example 1: Collision of a droplet with a substrate (by Yasushi Katsumi, UVa)


Limitations of the MD technique1. Classical description of interatomic interaction

Electrons are not present explicitly; they are introduced through the potential energy surface that is a function of atomic positions only (Born-Oppenheimer approximation). The potential energy surface, in turn, is approximated by an analytic function that gives the potential energy U as a function of coordinates. Forces are obtained as the gradient of a potential energy function,


2. Time- and length-scale limitationsTime-scale: The maximum timestep of integration in MD simulation is defined by the fastest motion in the system. Vibration frequencies in a molecular system a period of ~10 fs. Optical phonon frequencies are period of ~100 fs. Therefore, a typical timestep in MD simulation is on the order of a femtosecond.

Using modern computers it is possible to calculate 106–108 timestep. Therefore we can only simulate processes that occur within 1 – 100 ns. This is a serious limitation for many problems.


Length-scale:The size of the computational cell is limited by the number of atoms that can be included in the simulation, typically 10^4–10^8. This corresponds to the size of the computational cell on the order of tens of nm. Any structural features of interest and spatial correlation lengths in the simulation should be smaller than the size of the computational cell.



Current applications of the MD simulation techniqueChemistry and Biochemistry: molecular structures, reactions, drug design, vibration relaxation and energy transfer…

Statistical Mechanics and Physics: theory of liquids, correlated many-body motion, properties of statistical ensembles,..

Materials Science: point, linear, and planar defects in crystals and their interactions, …


Conclusion MD simulation studies have been conducted to study adhesion, wear and indentation processes. MD simulations of cutting and chip formation, crack propagation in glass and ceramic materials, interfacial liquid junctions and confined films have also been reported. The MD simulation results correlate well with the experimental observations at nanometer-scale.


References 1. G. C. Maitland, M. Rigby, E. B. Smith, and W. A.

Wakeham. Intermolecular for ces: their origin and determination. Clarendon Press, Oxford, 1981.

2. C. G. Gray and K. E. Gubbins. Theory of molecularfluids. 1. Fundamentals. Claren-don Press, Oxford, 1984.

3. Gwidon w. stachowiak, Andrew w. batchelor. Engineering tribology, Butterworth-heinemann, third ed,2005


Thank you for your attention !!!

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Atomic scale(molecular dynamic) simulationa