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Engineering, Mechanical Engineering, Materials Science

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Full-Text Articles in Mechanical Engineering

Thermal Activation In Atomic Friction: Revisiting The Theoretical Analysis, Yalin Dong, D. Perez, H. Gao, A. Martini Apr 2015

Thermal Activation In Atomic Friction: Revisiting The Theoretical Analysis, Yalin Dong, D. Perez, H. Gao, A. Martini

Dr. Yalin Dong

The effect of thermal activation on atomic-scale friction is often described in the framework of the Prandtl–Tomlinson model. Accurate use of this model relies on parameters that describe the shape of the corrugation potential β and the transition attempt frequency f0. We show that the commonly used form of β for a sinusoidal corrugation potential can lead to underestimation of friction, and that the attempt frequency is not, as is usually assumed, a constant value, but rather varies as the energy landscape evolves. We partially resolve these issues by demonstrating that numerical results can be captured by a model ...


Atomic Friction Modulation On The Reconstructed Au(111) Surface, Qunyang Li, Yalin Dong, Ashlie Martini, Robert Carpick Apr 2015

Atomic Friction Modulation On The Reconstructed Au(111) Surface, Qunyang Li, Yalin Dong, Ashlie Martini, Robert Carpick

Dr. Yalin Dong

Friction between a nanoscale tip and a reconstructed Au(111) surface is investigated both by atomic force microscopy (AFM) and molecular statics calculations. Lateral force AFM images exhibit atomic lattice stick–slip behavior with a superstructure corresponding to the herringbone reconstruction pattern. However, the superstructure contrast is not primarily due to variations in the local frictional dissipation (which corresponds to the local width of the friction loop). Rather, the contrast occurs primarily because the local centerline position of the friction loop is periodically shifted from its usual value of zero. Qualitatively, similar behavior is reproduced in atomistic simulations of an ...


Atomic Roughness Enhanced Friction On Hydrogenated Graphene, Yalin Dong, Xiawa Wu, Ashlie Martini Apr 2015

Atomic Roughness Enhanced Friction On Hydrogenated Graphene, Yalin Dong, Xiawa Wu, Ashlie Martini

Dr. Yalin Dong

Atomic friction on hydrogenated graphene is investigated using molecular dynamics simulations. Hydrogenation is found to increase friction significantly, and the atomic-level information provided by the simulations reveals that atomic roughness induced by hydrogenation is the primary cause of the friction enhancement. Other proposed mechanisms, specifically adhesion and rigidity, are excluded based on the simulation results and analyses performed using the Prandtl–Tomlinson model. In addition, it is found that friction does not monotonically increase with hydrogen coverage on the graphene surface; instead, a maximum friction is observed at a hydrogen coverage between 5 and 10%.


Analytical Models For Atomic Friction, Yalin Dong, Ajay Vadakkepatt, Ashlie Martini Apr 2015

Analytical Models For Atomic Friction, Yalin Dong, Ajay Vadakkepatt, Ashlie Martini

Dr. Yalin Dong

In this methods article, we describe application of Prandtl–Tomlinson models and their extensions to interpret dry atomic-scale friction. The goal is to provide a practical overview of how to use these models to study frictional phenomena. We begin with the fundamental equations and build on them step-by-step—from the simple quasistatic one-spring, one-mass model for predicting transitions between friction regimes to the two-dimensional and multi-atom models for describing the effect of contact area. The intention is to bridge the gap between theoretical analysis, numerical implementation, and predicted physical phenomena. In the process, we provide an introductory manual with example ...


Reinterpretation Of Velocity-Dependent Atomic Friction: Influence Of The Inherent Instrumental Noise In Friction Force Microscopes, Yalin Dong, Hongyu Gao, Ashlie Martini, Philip Egberts Apr 2015

Reinterpretation Of Velocity-Dependent Atomic Friction: Influence Of The Inherent Instrumental Noise In Friction Force Microscopes, Yalin Dong, Hongyu Gao, Ashlie Martini, Philip Egberts

Dr. Yalin Dong

We have applied both the master equation method and harmonic transition state theory to interpret the velocity-dependent friction behavior observed in atomic friction experiments. To understand the discrepancy between attempt frequencies measured in atomic force microscopy experiments and those estimated by theoretical models, both thermal noise and instrumental noise are introduced into the model. It is found that the experimentally observed low attempt frequency and the transition point at low velocity regimes can be interpreted in terms of the instrumental noise inherent in atomic force microscopy. In contrast to previous models, this model also predicts (1) the existence of a ...


Friction, Slip And Structural Inhomogeneity Of The Buried Interface, Yalin Dong, Q. Li, A. Martini Apr 2015

Friction, Slip And Structural Inhomogeneity Of The Buried Interface, Yalin Dong, Q. Li, A. Martini

Dr. Yalin Dong

An atomistic model of metallic contacts using realistic interatomic potentials is used to study the connection between friction, slip and the structure of the buried interface. Incommensurability induced by misalignment and lattice mismatch is modeled with contact sizes that are large enough to observe superstructures formed by the relative orientations of the surfaces. The periodicity of the superstructures is quantitatively related to inhomogeneous shear stress distributions in the contact area, and a reduced order model is used to clarify the connection between friction and structural inhomogeneity. Finally, the movement of atoms is evaluated before, during and after slip in both ...


Suppression Of Atomic Friction Under Cryogenic Conditions: The Role Of Athermal Instability In Afm Measurements, Yalin Dong, H. Gao, A. Martini Apr 2015

Suppression Of Atomic Friction Under Cryogenic Conditions: The Role Of Athermal Instability In Afm Measurements, Yalin Dong, H. Gao, A. Martini

Dr. Yalin Dong

A theoretical investigation of the behavior of atomic friction at low temperatures is performed using a master equation method with a two-mass, two-spring Prandtl-Tomlinson model of an atomic force microscope experiment. A novel approach is taken in which two distinct instability mechanisms are introduced into the model: thermal activation is described by transition state theory with a prefactor associated with the frequency of the tip apex, and athermal instability is introduced by an Arrhenius-like equation with a prefactor associated with the characteristic frequency of the cantilever. Thermal instability causes the often reported decrease of friction with temperature followed by a ...


Variation Of Friction With Number Of Graphene Layers, Zhijiang Ye, Chun Tang, Yalin Dong, Ashlie Martini Apr 2015

Variation Of Friction With Number Of Graphene Layers, Zhijiang Ye, Chun Tang, Yalin Dong, Ashlie Martini

Dr. Yalin Dong

The article focuses on a research that explains the variation of friction in lubricated system by using graphene layers. It mentions that graphite is widely used in lubricated system as it has weak interlayer interaction strength that cause low friction. It reflects on the use of friction force microscopy (FFM) to study the nanostructure of graphene and mentions that graphene explains the phenomenon of wrinkle formation in lubricants and impact of layers or sheet size on friction.


Rate Theory Description Of Atomic Stick-Slip Friction, Danny Perez, Yalin Dong, Ashlie Martini, Arthur Voter Apr 2015

Rate Theory Description Of Atomic Stick-Slip Friction, Danny Perez, Yalin Dong, Ashlie Martini, Arthur Voter

Dr. Yalin Dong

We assess the validity of assumptions that underpin common low-dimensional rate theory descriptions of nanoscale stick-slip friction by completely specifying harmonic transition state theory kinetic parameters from an atomistic model. The resultant kinetic model is able to reliably reproduce the temperature and velocity dependence of friction as obtained by direct fully atomistic accelerated molecular-dynamics simulations. Analysis of the parameters extracted from the model indicates that, while energetics of the transition pathways can be adequately captured by low-dimensional effective Hamiltonians, rate theory prefactors contain inherently high-dimensional entropic contributions that cannot be accounted for. Despite these limitations, we show that simplified models ...


Numerical Investigation Of Non-Local Electron Transport In Laser-Produced Plasmas, Yalin Dong, Zhao Bin, Zheng Jian Apr 2015

Numerical Investigation Of Non-Local Electron Transport In Laser-Produced Plasmas, Yalin Dong, Zhao Bin, Zheng Jian

Dr. Yalin Dong

Non-local electron transport in laser-produced plasmas under inertial confinement fusion (ICF) conditions is studied based on Fokker–Planck (FP) and hydrodynamic simulations. A comparison between the classical Spitzer–Härm (SH) transport model and non-local transport models has been made. The result shows that among those non-local models the Epperlein and Short (ES) model of heat flux is in reasonable agreement with the FP simulation in overdense region. However, the non-local models are invalid in the hot underdense plasmas. Hydrodynamic simulation is performed with the flux limiting model and the non-local model, separately. The simulation results show that in the underdense ...


Role Of Wrinkle Height In Friction Variation With Number Of Graphene Layers, Zhijiang Ye, Chun Tang, Yalin Dong Apr 2015

Role Of Wrinkle Height In Friction Variation With Number Of Graphene Layers, Zhijiang Ye, Chun Tang, Yalin Dong

Dr. Yalin Dong

Molecular dynamics simulations are performed to study the frictional behavior of graphene. It is found that the friction between a diamond tip and graphene decreases with increasing number of graphene layers. This behavior is also affected by the graphene sheet size; specifically, the effect of the number of layers on friction becomes significant only when the modeled graphene sheets exceed a critical length. We further show that the frictional behavior can be directly correlated to the height of near-contact wrinkles that resist sliding. These observations are rationalized in terms of the ability of multiple sheets to act as a single ...


Effect Of Molecular Structure On Liquid Slip, Ajay Vadakkepatt, Yalin Dong, Seth Lichter, Ashlie Martini Apr 2015

Effect Of Molecular Structure On Liquid Slip, Ajay Vadakkepatt, Yalin Dong, Seth Lichter, Ashlie Martini

Dr. Yalin Dong

Slip behavior of three liquids with distinct molecular shapes—linear (hexadecane), branched (pentaerythritol tetra), and a chain of rings (polyphenylether)—is studied using molecular dynamics simulation and reduced-order modeling. Slip at a liquid-solid interface is shown to be affected by the molecular structure of the liquid. A two-dimensional Frenkel-Kontorova model captures the fundamental structural features of the liquid molecules and gives insight into how molecules flex and slip along the surface. We formulate an approximation to the Peierls-Nabarro energy which incorporates both the position of liquid atoms relative to substrate atoms and molecular flexibility. We find that increased molecular flexibility ...


Molecular Dynamics Simulation Of Atomic Friction: A Review And Guide, Yalin Dong, Qunyang Li, Ashlie Martini Apr 2015

Molecular Dynamics Simulation Of Atomic Friction: A Review And Guide, Yalin Dong, Qunyang Li, Ashlie Martini

Dr. Yalin Dong

This paper reviews recent progress in molecular dynamics simulation of atomic-scale friction measured by an atomic force microscopy. Each section of the review focuses on an individual condition or parameter that affects atomic friction including materials, surfaces, compliance, contact area, normal load, temperature, and velocity. The role each parameter plays is described in the context of both experimental measurements and simulation predictions. In addition, the discussion includes an overview of the research community's current understanding of observed effects, guidelines for implementation of those effects in an atomistic simulation, and suggestions for future research to address open questions. Taken together ...


Atomistic Study Of Lateral Contact Stiffness In Friction Force Microscopy, H. Gao, Yalin Dong, A. Martini Apr 2015

Atomistic Study Of Lateral Contact Stiffness In Friction Force Microscopy, H. Gao, Yalin Dong, A. Martini

Dr. Yalin Dong

The effective stiffness of a friction force microscope tip–substrate system is an important parameter that describes the relationship between lateral force and elastic deformation. In this study, we use a multi-spring model to simplify the system, where two contributions, the tip apex stiffness and the lateral contact stiffness, are discussed in detail. Molecular dynamics simulations are used to characterize stiffness by simulating a tip apex subject to shear or sliding over a substrate surface. The results show that, although the height of the tip apex and tip–substrate orientation affect the various stiffness contributions, the contact itself dominates the ...


Environmental Dependence Of Atomic-Scale Friction At Graphite Surface Steps, Philip Egberts, Zhijiang Ye, Xin Liu, Yalin Dong Apr 2015

Environmental Dependence Of Atomic-Scale Friction At Graphite Surface Steps, Philip Egberts, Zhijiang Ye, Xin Liu, Yalin Dong

Dr. Yalin Dong

Atomic force microscopy experiments and molecular dynamics simulations show that friction between a nanoscale tip and atomically stepped surfaces of graphite is influenced by the environment. The presence of a small amount of water increases friction at atomic steps, but does not strongly influence friction on flat terraces.


A Fokker-Planck Code For Laser-Produced Plasmas, Zhao Bin, Yalin Dong, Zheng Jian Apr 2015

A Fokker-Planck Code For Laser-Produced Plasmas, Zhao Bin, Yalin Dong, Zheng Jian

Dr. Yalin Dong

A Fokker-Planck code is developed based upon Epperlein’s scheme [Laser Part. Beams 12, 257 (1994)] for the investigation of laser-produced plasmas in relevance to inertial confinement fusion. The equations are integrated implicitly by the time-splitting method. The test problems are simulated to show the versatility of the code. The comparisons among our computational heat flux and the classical Spitzer-H¨arm (SH) transport model and non-local transport models have been presented. The result shows that the non-local model of heat flux is in reasonable agreement with the FP simulation in overdense region and invalid in the hot underdense plasmas.


Environmental Dependence Of Atomic-Scale Friction At Graphite Surface Steps, Philip Egberts, Zhijiang Ye, Xin Liu, Yalin Dong, Ashlie Martini, Robert Carpick Apr 2015

Environmental Dependence Of Atomic-Scale Friction At Graphite Surface Steps, Philip Egberts, Zhijiang Ye, Xin Liu, Yalin Dong, Ashlie Martini, Robert Carpick

Dr. Yalin Dong

Atomic force microscopy experiments and molecular dynamics simulations show that friction between a nanoscale tip and atomically stepped surfaces of graphite is influenced by the environment. The presence of a small amount of water increases friction at atomic steps, but does not strongly influence friction on flat terraces.


Role Of Wrinkle Height In Friction Variation With Number Of Graphene Layers, Zhijiang Ye, Chun Tang, Yalin Dong, Ashlie Martini Apr 2015

Role Of Wrinkle Height In Friction Variation With Number Of Graphene Layers, Zhijiang Ye, Chun Tang, Yalin Dong, Ashlie Martini

Dr. Yalin Dong

Molecular dynamics simulations are performed to study the frictional behavior of graphene. It is found that the friction between a diamond tip and graphene decreases with increasing number of graphene layers. This behavior is also affected by the graphene sheet size; specifically, the effect of the number of layers on friction becomes significant only when the modeled graphene sheets exceed a critical length. We further show that the frictional behavior can be directly correlated to the height of near-contact wrinkles that resist sliding. These observations are rationalized in terms of the ability of multiple sheets to act as a single ...


Suppression Of Atomic Friction Under Cryogenic Conditions: The Role Of Athermal Instability In Afm Measurements, Yalin Dong, Hongyu Gao, Ashlie Martini Apr 2015

Suppression Of Atomic Friction Under Cryogenic Conditions: The Role Of Athermal Instability In Afm Measurements, Yalin Dong, Hongyu Gao, Ashlie Martini

Dr. Yalin Dong

A theoretical investigation of the behavior of atomic friction at low temperatures is performed using a master equation method with a two-mass, two-spring Prandtl-Tomlinson model of an atomic force microscope experiment. A novel approach is taken in which two distinct instability mechanisms are introduced into the model: thermal activation is described by transition state theory with a prefactor associated with the frequency of the tip apex, and athermal instability is introduced by an Arrhenius-like equation with a prefactor associated with the characteristic frequency of the cantilever. Thermal instability causes the often reported decrease of friction with temperature followed by a ...


Effects Of Substrate Roughness And Electron–Phonon Coupling On Thickness-Dependent Friction Of Graphene, Yalin Dong Apr 2015

Effects Of Substrate Roughness And Electron–Phonon Coupling On Thickness-Dependent Friction Of Graphene, Yalin Dong

Dr. Yalin Dong

Molecular dynamics simulation and the two-temperature method are carried out to model the effects of substrate roughness as well as electron–phonon coupling on thickness-dependent friction on graphene. It is found that substrate roughness can significantly enhance friction of graphene, which is orders of magnitude larger than that on smooth substrate due to puckering effect. Additionally, the adhesive force between graphene and substrate plays opposite roles for smooth and rough substrates. While on a smooth substrate, a larger adhesion hinders the wrinkle formation in graphene, therefore suppressing friction, on a rough substrate, adhesion helps induce atomic roughness in graphene and ...


Inertia Gap Between Md Simulations And Afm Experiments In Study Of Atomic Friction, Yalin Dong, Qunyang Li, Robert W. Carpick, Ashlie Martini Apr 2015

Inertia Gap Between Md Simulations And Afm Experiments In Study Of Atomic Friction, Yalin Dong, Qunyang Li, Robert W. Carpick, Ashlie Martini

Dr. Yalin Dong

Both atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulation are carried out to investigate atomic stick-slip friction by sliding a Pt tip on an Au substrate. Efforts are taken to match the conditions for AFM experiment and MD simulation as closely as possible. The results show that AFM and MD provide consistent energetic parameters, which suggests that MD simulations can be reliably used to interpret energetic aspects of the interfaces. However, orders of magnitude differences in attempt frequencies are found, which indicates another challenge between the MD simulations and AFM experiments, i.e., the inertia gap.


Dynamics Of Atomic Stick-Slip Friction Examined With Atomic Force Microscopy And Atomistic Simulations At Overlapping Speeds, Xin Liu, Zhijiang Ye, Yalin Dong, Philip Egberts, Robert Carpick, Ashlie Martini Apr 2015

Dynamics Of Atomic Stick-Slip Friction Examined With Atomic Force Microscopy And Atomistic Simulations At Overlapping Speeds, Xin Liu, Zhijiang Ye, Yalin Dong, Philip Egberts, Robert Carpick, Ashlie Martini

Dr. Yalin Dong

Atomic force microscopy (AFM) and atomistic simulations of atomic friction with silicon oxide tips sliding on Au(111) are conducted at overlapping speeds. Experimental data unambiguously reveal a stick-slip friction plateau above a critical scanning speed, in agreement with the thermally activated Prandtl-Tomlinson (PTT) model. However, friction in experiments is larger than in simulations. PTT energetic parameters for the two are comparable, with minor differences attributable to the contact area’s influence on the barrier to slip. Recognizing that the attempt frequency may be determined by thermal vibrations of the larger AFM tip mass or instrument noise fully resolves the ...


Synthesis And Characterization Of Triarylmethane Resin, Xiangquan Ruan, Yalin Dong, Jiaming Zheng Apr 2015

Synthesis And Characterization Of Triarylmethane Resin, Xiangquan Ruan, Yalin Dong, Jiaming Zheng

Dr. Yalin Dong

No abstract provided.


Comment On “A Note On The Two-Spring Tomlinson Model”, Yalin Dong, Ashlie Martini Apr 2015

Comment On “A Note On The Two-Spring Tomlinson Model”, Yalin Dong, Ashlie Martini

Dr. Yalin Dong

A recent note by Lu et al. presented a theoretical study of atomic friction using a two-spring Tomlinson model. In this comment, we argue that the conclusion of the note can be understood in a simple manner, and that the discussion of the criterion for the onset of stick-slip is unnecessarily complicated and potentially misleading.


Reinterpretation Of Velocity-Dependent Atomic Friction: Influence Of The Inherent Instrumental Noise In Friction Force Microscopes, Yalin Dong, Hongyu Gao, Ashlie Martini, Philip Egberts Apr 2015

Reinterpretation Of Velocity-Dependent Atomic Friction: Influence Of The Inherent Instrumental Noise In Friction Force Microscopes, Yalin Dong, Hongyu Gao, Ashlie Martini, Philip Egberts

Dr. Yalin Dong

We have applied both the master equation method and harmonic transition state theory to interpret the velocity-dependent friction behavior observed in atomic friction experiments. To understand the discrepancy between attempt frequencies measured in atomic force microscopy experiments and those estimated by theoretical models, both thermal noise and instrumental noise are introduced into the model. It is found that the experimentally observed low attempt frequency and the transition point at low velocity regimes can be interpreted in terms of the instrumental noise inherent in atomic force microscopy. In contrast to previous models, this model also predicts (1) the existence of a ...


Low-Speed Atomistic Simulation Of Stick–Slip Friction Using Parallel Replica Dynamics, Ashlie Martini, Yalin Dong, Danny Perez, Arthur Voter Apr 2015

Low-Speed Atomistic Simulation Of Stick–Slip Friction Using Parallel Replica Dynamics, Ashlie Martini, Yalin Dong, Danny Perez, Arthur Voter

Dr. Yalin Dong

Atomic stick–slip friction has been predicted by molecular dynamics simulation and observed in experiments. However, direct quantitative comparison of the two has thus far not been possible because of the large difference between scanning velocities accessible to simulations and experiments. In general, the slowest sliding speeds in MD simulations are at least five orders of magnitude larger than the upper limit available to experimentalists. To take a step toward bridging this gap, we have applied parallel replica dynamics, an accelerated molecular dynamics method, to the simulation of atomic stick–slip. The method allows molecular simulations to run parallel in ...


Atomic Roughness Enhanced Friction On Hydrogenated Graphene, Yalin Dong, Xiawa Wu, Ashlie Martini Apr 2015

Atomic Roughness Enhanced Friction On Hydrogenated Graphene, Yalin Dong, Xiawa Wu, Ashlie Martini

Dr. Yalin Dong

Atomic friction on hydrogenated graphene is investigated using molecular dynamics simulations. Hydrogenation is found to increase friction significantly, and the atomic-level information provided by the simulations reveals that atomic roughness induced by hydrogenation is the primary cause of the friction enhancement. Other proposed mechanisms, specifically adhesion and rigidity, are excluded based on the simulation results and analyses performed using the Prandtl–Tomlinson model. In addition, it is found that friction does not monotonically increase with hydrogen coverage on the graphene surface; instead, a maximum friction is observed at a hydrogen coverage between 5 and 10%.


Rate Theory Description Of Atomic Stick-Slip Friction, Danny Perez, Yalin Dong, Ashlie Martini, Arthur Voter Apr 2015

Rate Theory Description Of Atomic Stick-Slip Friction, Danny Perez, Yalin Dong, Ashlie Martini, Arthur Voter

Dr. Yalin Dong

We assess the validity of assumptions that underpin common low-dimensional rate theory descriptions of nanoscale stick-slip friction by completely specifying harmonic transition state theory kinetic parameters from an atomistic model. The resultant kinetic model is able to reliably reproduce the temperature and velocity dependence of friction as obtained by direct fully atomistic accelerated molecular-dynamics simulations. Analysis of the parameters extracted from the model indicates that, while energetics of the transition pathways can be adequately captured by low-dimensional effective Hamiltonians, rate theory prefactors contain inherently high-dimensional entropic contributions that cannot be accounted for. Despite these limitations, we show that simplified models ...


Atomic Friction Studied By Modeling The Buried Inteface, Yalin Dong, Ashlie Martini, Qunyang Li, Robert W. Carpick Apr 2015

Atomic Friction Studied By Modeling The Buried Inteface, Yalin Dong, Ashlie Martini, Qunyang Li, Robert W. Carpick

Dr. Yalin Dong

Molecular dynamics simulation is carried out to model the single-asperity friction in atomic force microscope experiments. Superlubricity is achieved through misalignment between the AFM tip and substrate. Direct observation of the buried interface reveals that incommensurability-induced inhomogeneous shear stress can cause ultra-low atomic scale friction.


Numerical Investigation Of Non-Local Electron Transport In Laser-Produced Plasmas, Yalin Dong, Zhao Bin, Zheng Jian Apr 2015

Numerical Investigation Of Non-Local Electron Transport In Laser-Produced Plasmas, Yalin Dong, Zhao Bin, Zheng Jian

Dr. Yalin Dong

Non-local electron transport in laser-produced plasmas under inertial confinement fusion (ICF) conditions is studied based on Fokker–Planck (FP) and hydrodynamic simulations. A comparison between the classical Spitzer–Härm (SH) transport model and non-local transport models has been made. The result shows that among those non-local models the Epperlein and Short (ES) model of heat flux is in reasonable agreement with the FP simulation in overdense region. However, the non-local models are invalid in the hot underdense plasmas. Hydrodynamic simulation is performed with the flux limiting model and the non-local model, separately. The simulation results show that in the underdense ...