1
|
Hörmann JL, Liu C, Meng Y, Pastewka L. Molecular simulations of sliding on SDS surfactant films. J Chem Phys 2023; 158:244703. [PMID: 37377159 DOI: 10.1063/5.0153397] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2023] [Accepted: 06/05/2023] [Indexed: 06/29/2023] Open
Abstract
We use molecular dynamics simulations to study the frictional response of monolayers of the anionic surfactant sodium dodecyl sulfate and hemicylindrical aggregates physisorbed on gold. Our simulations of a sliding spherical asperity reveal the following two friction regimes: at low loads, the films show Amonton's friction with a friction force that rises linearly with normal load, and at high loads, the friction force is independent of the load as long as no direct solid-solid contact occurs. The transition between these two regimes happens when a single molecular layer is confined in the gap between the sliding bodies. The friction force at high loads on a monolayer rises monotonically with film density and drops slightly with the transition to hemicylindrical aggregates. This monotonous increase of friction force is compatible with a traditional plowing model of sliding friction. At low loads, the friction coefficient reaches a minimum at the intermediate surface concentrations. We attribute this behavior to a competition between adhesive forces, repulsion of the compressed film, and the onset of plowing.
Collapse
Affiliation(s)
- Johannes L Hörmann
- Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany
- Cluster of Excellence livMatS, Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
| | - Chenxu Liu
- State Key Laboratory of Tribology in Advanced Equipment, Lee Shau Kee Science and Technology Building, Tsinghua University, Haidian District, Beijing 100084, China
| | - Yonggang Meng
- State Key Laboratory of Tribology in Advanced Equipment, Lee Shau Kee Science and Technology Building, Tsinghua University, Haidian District, Beijing 100084, China
| | - Lars Pastewka
- Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany
- Cluster of Excellence livMatS, Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
| |
Collapse
|
2
|
Sun Q, Liu W, Huang D, Huang X, Xu S, Wang J, Ye Z, Wang X, Wu S, Yue Y. Molecular dynamics study on thermal conductance between a nanotip and a substrate under vertical forces and horizontal sliding. Phys Chem Chem Phys 2023; 25:5510-5519. [PMID: 36723186 DOI: 10.1039/d2cp04655e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
The heat transfer between a nanotip and its substrate is extremely complex but is a key factor in determining the measurement accuracy in tip-assisted nanomanufacturing and thermometry. In this work, the heat transfer from the nanotip to the substrate during sliding is investigated using molecular dynamics simulations. Interfacial interaction and bond formation are analyzed during the sliding process. The results show that the increase of vertical force would greatly improve the interface thermal conductance between the nanotip and the substrate. It is found that more bonds are formed and there are larger contact areas at the interface. In addition, we found that the thermal conductivity of the nanotip is another obstacle for heat transfer between the tip and substrate and it is greatly limited by the nanotip diameter near contact which is close to or even smaller than the phonon mean free path. Meanwhile, the dynamic formation and breakage of the covalent bonds during the sliding could gradually smoothen the tip apex and enhance the thermal transport at the interface. This work provides guidance for the thermal design of a nanotip-substrate system for nanoscale thermal transport measurements.
Collapse
Affiliation(s)
- Qiangsheng Sun
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Wenxiang Liu
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Dezhao Huang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Xiaona Huang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Shen Xu
- School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
| | - Jianmei Wang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Zhijiang Ye
- Department of Mechanical and Manufacturing Engineering, Miami University, Ohio 45056, USA
| | - Xiaosun Wang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Shijing Wu
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Yanan Yue
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China. .,Department of Mechanical and Manufacturing Engineering, Miami University, Ohio 45056, USA
| |
Collapse
|
3
|
Padilla Espinosa IM, Jacobs TDB, Martini A. Atomistic Simulations of the Elastic Compression of Platinum Nanoparticles. NANOSCALE RESEARCH LETTERS 2022; 17:96. [PMID: 36190663 PMCID: PMC9530103 DOI: 10.1186/s11671-022-03734-z] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/15/2022] [Accepted: 09/23/2022] [Indexed: 05/25/2023]
Abstract
The elastic behavior of nanoparticles depends strongly on particle shape, size, and crystallographic orientation. Many prior investigations have characterized the elastic modulus of nanoscale particles using experiments or simulations; however their reported values vary widely depending on the methods for measurement and calculation. To understand these discrepancies, we used classical molecular dynamics simulation to model the compression of platinum nanoparticles with two different polyhedral shapes and a range of sizes from 4 to 20 nm, loaded in two different crystal orientations. Multiple standard methods were used to calculate the elastic modulus from stress-vs-strain data for each nanoparticle. The magnitudes and particle-size dependence of the resulting moduli varied with calculation method and, even for larger nanoparticles where bulk-like behavior may be expected, the effective elastic modulus depended strongly on shape and orientation. Analysis of per-atom stress distributions indicated that the shape- and orientation-dependence arise due to stress triaxiality and inhomogeneity across the particle. When the effective elastic modulus was recalculated using a representative volume element in the center of a large nanoparticle, the elastic modulus had the expected value for each orientation and was shape independent. It is only for single-digit nanoparticles that meaningful differences emerged, where even the very center of the particle had a lower modulus due to the effect of the surface. These findings provide better understanding of the elastic properties of nanoparticles and disentangle geometric contributions (such as stress triaxiality and spatial inhomogeneity) from true changes in elastic properties of the nanoscale material.
Collapse
Affiliation(s)
- Ingrid M Padilla Espinosa
- Department of Mechanical Engineering, University of California Merced, 5200 Lake Rd, Merced, CA, 95340, USA
| | - Tevis D B Jacobs
- Department of Mechanical Engineering and Materials Science, University of Pittsburgh, 3700 O'Hara St., Benedum Hall, Room 636, Pittsburgh, PA, 15261, USA
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California Merced, 5200 Lake Rd, Merced, CA, 95340, USA.
| |
Collapse
|
4
|
Vacher R, de Wijn AS. Nanoscale friction and wear of a polymer coated with graphene. BEILSTEIN JOURNAL OF NANOTECHNOLOGY 2022; 13:63-73. [PMID: 35096496 PMCID: PMC8767560 DOI: 10.3762/bjnano.13.4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/28/2021] [Accepted: 12/16/2021] [Indexed: 06/14/2023]
Abstract
Friction and wear of polymers at the nanoscale is a challenging problem due to the complex viscoelastic properties and structure. Using molecular dynamics simulations, we investigate how a graphene sheet on top of the semicrystalline polymer polyvinyl alcohol affects the friction and wear. Our setup is meant to resemble an AFM experiment with a silicon tip. We have used two different graphene sheets, namely an unstrained, flat sheet, and one that has been crumpled before being deposited on the polymer. The graphene protects the top layer of the polymer from wear and reduces the friction. The unstrained flat graphene is stiffer, and we find that it constrains the polymer chains and reduces the indentation depth.
Collapse
Affiliation(s)
- Robin Vacher
- Corrosion and tribology, SINTEF, Richard Birkelands vei 2B, 7034 Trondheim, Norway
| | - Astrid S de Wijn
- Institutt for maskinteknikk og produksjon, NTNU, Richard Birkelands vei 2B, 7034 Trondheim, Norway
| |
Collapse
|
5
|
Stephan S, Dyga M, Alabd Alhafez I, Lenhard J, Urbassek HM, Hasse H. Reproducibility of atomistic friction computer experiments: a molecular dynamics simulation study. MOLECULAR SIMULATION 2021. [DOI: 10.1080/08927022.2021.1987430] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
Affiliation(s)
- Simon Stephan
- Laboratory of Engineering Thermodynamics (LTD), TU Kaiserslautern, Kaiserslautern, Germany
| | - Maximilian Dyga
- Laboratory of Engineering Thermodynamics (LTD), TU Kaiserslautern, Kaiserslautern, Germany
| | - Iyad Alabd Alhafez
- Physics Department and Research Center (OPTIMAS), TU Kaiserslautern, Kaiserslautern, Germany
| | - Johannes Lenhard
- Laboratory of Engineering Thermodynamics (LTD), TU Kaiserslautern, Kaiserslautern, Germany
| | - Herbert M. Urbassek
- Physics Department and Research Center (OPTIMAS), TU Kaiserslautern, Kaiserslautern, Germany
| | - Hans Hasse
- Laboratory of Engineering Thermodynamics (LTD), TU Kaiserslautern, Kaiserslautern, Germany
| |
Collapse
|
6
|
Gao H, Ewen JP, Hartkamp R, Müser MH, Dini D. Scale-Dependent Friction-Coverage Relations and Nonlocal Dissipation in Surfactant Monolayers. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2021; 37:2406-2418. [PMID: 33545003 DOI: 10.1021/acs.langmuir.0c03403] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Surfactant molecules, known as organic friction modifiers (OFMs), are routinely added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases; however, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we investigate various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. We show that the differences between the friction-coverage relations from macroscale and nanoscale experiments are due to molecular plowing in the latter. For our small tip radii, the friction coefficient and indentation depth both have a nonmonotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. We rationalize the nonmonotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, we find that friction predominately originates from plowing of the monolayers by the leading edge of the tip, where gauche defects are created, while thermal dissipation is mostly localized in molecules toward the trailing edge of the tip, where the chains return to a more extended conformation.
Collapse
Affiliation(s)
- Hongyu Gao
- Department of Materials Science and Engineering, Universität des Saarlandes, 66123 Saarbrücken, Germany
| | - James P Ewen
- Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, U.K
| | - Remco Hartkamp
- Process and Energy Department, Delft University of Technology, Leeghwaterstraat 39, 2628 CB Delft, The Netherlands
| | - Martin H Müser
- Department of Materials Science and Engineering, Universität des Saarlandes, 66123 Saarbrücken, Germany
| | - Daniele Dini
- Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, U.K
| |
Collapse
|
7
|
Wang J, Tiwari A, Sivebaek IM, Persson BNJ. Role of lattice trapping for sliding friction. ACTA ACUST UNITED AC 2020. [DOI: 10.1209/0295-5075/131/24006] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
|
8
|
Stephan S, Dyga M, Urbassek HM, Hasse H. The Influence of Lubrication and the Solid-Fluid Interaction on Thermodynamic Properties in a Nanoscopic Scratching Process. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:16948-16960. [PMID: 31815481 DOI: 10.1021/acs.langmuir.9b01033] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Liquid lubricants play an important role in contact processes; for example, they reduce friction and cool the contact zone. To gain better understanding of the influence of lubrication on the nanoscale, both dry and lubricated scratching processes in a model system are compared in the present work using molecular dynamics simulations. The entire range between total dewetting and total wetting is investigated by tuning the solid-fluid interaction energy. The investigated scratching process consists of three sequential movements: A cylindrical indenter penetrates an initially flat substrate, then scratches in the lateral direction, and is finally retracted out of the contact with the substrate. The indenter is fully submersed in the fluid in the lubricated cases. The substrate, the indenter, and the fluid are described by suitably parametrized Lennard-Jones model potentials. The presence of the lubricant is found to have a significant influence on the friction and on the energy balance of the process. The thermodynamic properties of the lubricant are evaluated in detail. A correlation of the simulation results for the profiles of the temperature, density, and pressure of the fluid in the vicinity of the chip is developed. The work done by the indenter is found to mainly dissipate and thereby heat up the substrate and eventually the fluid. Only a minor part of the work causes plastic deformation of the substrate.
Collapse
|
9
|
Greenfield ML, Ohtani H. Friction and normal forces of model friction modifier additives in simulations of boundary lubrication. Mol Phys 2019. [DOI: 10.1080/00268976.2019.1670876] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Affiliation(s)
| | - Hiroko Ohtani
- Vehicle Manufacturing and Additive Manufacturing-Metals Department, Ford Motor Company, Dearborn, MI, USA
| |
Collapse
|
10
|
Black JE, Summers AZ, Iacovella CR, Cummings PT, McCabe C. Investigation of the Impact of Cross-Polymerization on the Structural and Frictional Properties of Alkylsilane Monolayers Using Molecular Simulation. NANOMATERIALS 2019; 9:nano9040639. [PMID: 31010181 PMCID: PMC6523262 DOI: 10.3390/nano9040639] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/15/2019] [Revised: 04/05/2019] [Accepted: 04/12/2019] [Indexed: 12/18/2022]
Abstract
Cross-linked chemisorbed n-alkylsilane (CH3(CH2)n−1Si(OH)3) monolayers on amorphous silica surfaces have been studied and their structural properties and frictional performance were compared to those of equivalent monolayers without cross-linkages. The simulations isolated for the first time the effects of both siloxane cross-linkages and the fraction of chains chemisorbed to the surface, providing insight into a longstanding fundamental question in the literature regarding molecular-level structure. The results demonstrate that both cross-linkages and the fraction of chemisorbed chains affect monolayer structure in small but measurable ways, particularly for monolayers constructed from short chains; however, these changes do not appear to have a significant impact on frictional performance.
Collapse
Affiliation(s)
- Jana E Black
- Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235, USA.
| | - Andrew Z Summers
- Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235, USA.
| | - Christopher R Iacovella
- Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235, USA.
| | - Peter T Cummings
- Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235, USA.
| | - Clare McCabe
- Department of Chemical and Biomolecular Engineering and Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA.
| |
Collapse
|
11
|
Ouyang W, de Wijn AS, Urbakh M. Atomic-scale sliding friction on a contaminated surface. NANOSCALE 2018; 10:6375-6381. [PMID: 29560981 DOI: 10.1039/c7nr09530a] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Using non-equilibrium molecular dynamic simulations, we investigate the effect of adsorbates on nanoscopic friction. We find that the interplay between different channels of energy dissipation at the frictional interface may lead to non-monotonic dependence of the friction force on the adsorbate surface coverage and to strongly nonlinear variation of friction with normal load (non-Amontons' behavior). Our simulations suggest that the key parameter controlling the variation of friction force with the normal load, surface coverage and temperature is the time-averaged number of adsorbates confined between the tip and the substrate. Three different regimes of temperature dependence of friction in the presence of adsorbates are predicted. Our findings point on new ways to control friction on contaminated surfaces.
Collapse
Affiliation(s)
- Wengen Ouyang
- School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel. and The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 6997801, Israel
| | - Astrid S de Wijn
- Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway and Department of Physics, Stockholm University, 10691 Stockholm, Sweden
| | - Michael Urbakh
- School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel. and The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 6997801, Israel
| |
Collapse
|
12
|
Summers AZ, Iacovella CR, Cummings PT, McCabe C. Investigating Alkylsilane Monolayer Tribology at a Single-Asperity Contact with Molecular Dynamics Simulation. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:11270-11280. [PMID: 28915731 DOI: 10.1021/acs.langmuir.7b02479] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Chemisorbed monolayer films are known to possess favorable characteristics for nanoscale lubrication of micro- and nanoelectromechanical systems (MEMS/NEMS). Prior studies have shown that the friction observed for monolayer-coated surfaces features a strong dependence on the geometry of contact. Specifically, tip-like geometries have been shown to penetrate into monolayer films, inducing defects in the monolayer chains and leading to plowing mechanisms during shear, which result in higher coefficients of friction (COF) than those observed for planar geometries. In this work, we use molecular dynamics simulations to examine the tribology of model silica single-asperity contacts under shear with monolayer-coated substrates featuring various film densities. It is observed that lower monolayer densities lead to reduced COFs, in contrast to results for planar systems where COF is found to be nearly independent of monolayer density. This is attributed to a liquid-like response to shear, whereby fewer defects are imparted in monolayer chains from the asperity, and chains are easily displaced by the tip as a result of the higher free volume. This transition in the mechanism of molecular plowing suggests that liquid-like films should provide favorable lubrication at single-asperity contacts.
Collapse
Affiliation(s)
- Andrew Z Summers
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Christopher R Iacovella
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Peter T Cummings
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Clare McCabe
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| |
Collapse
|
13
|
Abstract
Every contacting surface inevitably experiences wear. Predicting the exact amount of material loss due to wear relies on empirical data and cannot be obtained from any physical model. Here, we analyze and quantify wear at the most fundamental level, i.e., wear debris particles. Our simulations show that the asperity junction size dictates the debris volume, revealing the origins of the long-standing hypothesized correlation between the wear volume and the real contact area. No correlation, however, is found between the debris volume and the normal applied force at the debris level. Alternatively, we show that the junction size controls the tangential force and sliding distance such that their product, i.e., the tangential work, is always proportional to the debris volume, with a proportionality constant of 1 over the junction shear strength. This study provides an estimation of the debris volume without any empirical factor, resulting in a wear coefficient of unity at the debris level. Discrepant microscopic and macroscopic wear observations and models are then contextualized on the basis of this understanding. This finding offers a way to characterize the wear volume in atomistic simulations and atomic force microscope wear experiments. It also provides a fundamental basis for predicting the wear coefficient for sliding rough contacts, given the statistics of junction clusters sizes.
Collapse
|
14
|
Yang F, Carpick RW, Srolovitz DJ. Mechanisms of Contact, Adhesion, and Failure of Metallic Nanoasperities in the Presence of Adsorbates: Toward Conductive Contact Design. ACS NANO 2017; 11:490-500. [PMID: 27983792 DOI: 10.1021/acsnano.6b06473] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The properties of contacting interfaces are strongly affected not only by the bulk and surface properties of contacting materials but also by the ubiquitous presence of adsorbed contaminants. Here, we focus on the properties of single asperity contacts in the presence of adsorbates within a molecular dynamics description of metallic asperity normal contact and a parametric description of adsorbate properties. A platinum-platinum asperity contact is modeled with adsorbed oligomers with variable properties. This system is particularly tailored to the context of nanoelectromechanical system (NEMS) contact switches, but the results are generally relevant to metal-metal asperity contacts in nonpristine conditions. Even though mechanical forces can displace adsorbate out of the contact region, increasing the adsorbate layer thickness and/or adsorbate/metal adhesion makes it more difficult for metal asperity/metal surface contact to occur, thereby lowering the electrical contact conductance. Contact separation is a competition between plastic necking in the asperity or decohesion at the asperity/substrate interface. The mechanism which operates at a lower tensile stress dominates. Necking dominates when the adsorbate/metal adhesion is strong and/or the adsorbate layer thickness is small. In broad terms, necking implies larger asperity deformation and mechanical work, as compared with decohesion. Optimal NEMS switch performance requires substantial contact conductance and minimal asperity deformation; these results indicate that these goals can be achieved by balancing the quantity of adsorbates and their adhesion to the metal surface.
Collapse
Affiliation(s)
- Fan Yang
- Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania , Philadelphia, Pennsylvania 19104 United States
| | - Robert W Carpick
- Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania , Philadelphia, Pennsylvania 19104 United States
| | - David J Srolovitz
- Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania , Philadelphia, Pennsylvania 19104 United States
| |
Collapse
|
15
|
Dale SG, Johnson ER. The ionic versus metallic nature of 2D electrides: a density-functional description. Phys Chem Chem Phys 2017; 19:27343-27352. [DOI: 10.1039/c7cp04825d] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The two-dimensional (2D) electrides are a highly unusual class of materials, possessing interstitial electron layers sandwiched between cationic atomic layers of the solid.
Collapse
|
16
|
Manini N, Braun OM, Tosatti E, Guerra R, Vanossi A. Friction and nonlinear dynamics. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2016; 28:293001. [PMID: 27249652 DOI: 10.1088/0953-8984/28/29/293001] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
The nonlinear dynamics associated with sliding friction forms a broad interdisciplinary research field that involves complex dynamical processes and patterns covering a broad range of time and length scales. Progress in experimental techniques and computational resources has stimulated the development of more refined and accurate mathematical and numerical models, capable of capturing many of the essentially nonlinear phenomena involved in friction.
Collapse
Affiliation(s)
- N Manini
- Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
| | | | | | | | | |
Collapse
|
17
|
Summers AZ, Iacovella CR, Billingsley MR, Arnold ST, Cummings PT, McCabe C. Influence of Surface Morphology on the Shear-Induced Wear of Alkylsilane Monolayers: Molecular Dynamics Study. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2016; 32:2348-2359. [PMID: 26885941 DOI: 10.1021/acs.langmuir.5b03862] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Chemisorbed alkylsilane monolayer coatings have been shown to possess favorable lubrication properties; however, film degradation prevents the widespread use of these materials as lubricants in micro- and nanoelectromechanical systems (MEMS/NEMS). In this work, molecular dynamics (MD) simulations are used to provide insight into the conditions that promote the degradation and wear of these materials. This is achieved through removal of interfacial chain-substrate bonds during shear and the examination of the mobility of the resulting free, unbound chains. Specific focus is given to the effects of surface morphology, which has been shown previously to strongly influence frictional forces in monolayer systems. In-plane order of chain attachments is shown to lead to pressure-induced orientational ordering of monolayers, promoting film stability. This behavior is lost as nonideality is introduced into the substrate and chain patterning on the surface becomes disordered. The presence of surface roughness is found to reduce film stability, with localization of wear observed for chain attachment sites nearest the interface of contact. The influence of substrate nonideality on monolayer degradation is shown to diminish as chain length is increased.
Collapse
Affiliation(s)
- Andrew Z Summers
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Christopher R Iacovella
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Matthew R Billingsley
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Steven T Arnold
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Peter T Cummings
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Clare McCabe
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| |
Collapse
|
18
|
Black JE, Iacovella CR, Cummings PT, McCabe C. Molecular dynamics study of alkylsilane monolayers on realistic amorphous silica surfaces. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2015; 31:3086-3093. [PMID: 25720502 DOI: 10.1021/la5049858] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Interfacial properties of n-alkylsilane monolayers on silica have been investigated with molecular dynamics simulations using both reactive and classical (i.e., nonreactive) force fields. A synthesis mimetic simulation (SMS) procedure using the reactive force field ReaxFF has been developed to mimic the experimental processing of silicon wafers involved in the preparation of alkylsilane monolayers; in the SMS procedure, amorphous silica surfaces are generated and exposed to hydrogen peroxide (H2O2) to create a hydroxide surface layer. Alkylsilane monolayers are then assembled on these surfaces, and their behavior is studied. To investigate the impact of the SMS procedure on monolayer properties, simulations have also been performed using more idealized monolayers assembled on crystalline surfaces and non-H2O2-processed amorphous surfaces. The simulations reported here demonstrate that processing-induced silica surface roughness plays a key role in the structure and frictional performance of monolayers. Furthermore, ignoring these effects results in a significant underestimation of the coefficient of friction and an overestimation of the orientational ordering of the monolayers.
Collapse
Affiliation(s)
- Jana E Black
- †Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS), and §Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
| | - Christopher R Iacovella
- †Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS), and §Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
| | - Peter T Cummings
- †Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS), and §Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
| | - Clare McCabe
- †Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS), and §Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
| |
Collapse
|
19
|
Ewers BW, Batteas JD. Utilizing atomistic simulations to map pressure distributions and contact areas in molecular adlayers within nanoscale surface-asperity junctions: a demonstration with octadecylsilane-functionalized silica interfaces. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2014; 30:11897-11905. [PMID: 24645696 DOI: 10.1021/la500032f] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity-asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.
Collapse
Affiliation(s)
- Bradley W Ewers
- Department of Chemistry, Texas A&M University , P.O. Box 30012, College Station, Texas 77842-3012, United States
| | | |
Collapse
|
20
|
Meltzer C, Paul J, Dietrich H, Jäger CM, Clark T, Zahn D, Braunschweig B, Peukert W. Indentation and Self-Healing Mechanisms of a Self-Assembled Monolayer—A Combined Experimental and Modeling Study. J Am Chem Soc 2014; 136:10718-27. [DOI: 10.1021/ja5048076] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Christian Meltzer
- Erlangen
Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Paul-Gordan-Strasse
6, 91052 Erlangen, Germany
| | | | | | | | | | - Dirk Zahn
- Cluster
of Excellence − Engineering of Advanced Material (EAM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstrasse
49b, 91052 Erlangen, Germany
| | - Björn Braunschweig
- Erlangen
Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Paul-Gordan-Strasse
6, 91052 Erlangen, Germany
- Cluster
of Excellence − Engineering of Advanced Material (EAM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstrasse
49b, 91052 Erlangen, Germany
| | - Wolfgang Peukert
- Erlangen
Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Paul-Gordan-Strasse
6, 91052 Erlangen, Germany
- Cluster
of Excellence − Engineering of Advanced Material (EAM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstrasse
49b, 91052 Erlangen, Germany
| |
Collapse
|
21
|
Ewers BW, Batteas JD. The role of substrate interactions in the modification of surface forces by self-assembled monolayers. RSC Adv 2014. [DOI: 10.1039/c4ra01427h] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Detailed pressure and strain mapping of atomistic contact simulations elucidate the mechanical and tribochemical mechanisms of surface force modification with SAMs.
Collapse
Affiliation(s)
- B. W. Ewers
- Department of Chemistry
- Texas A&M University
- College Station, USA
| | - J. D. Batteas
- Department of Chemistry
- Texas A&M University
- College Station, USA
| |
Collapse
|
22
|
Yang Y, Singh J, Ruths M. Friction of aromatic thiol monolayers on silver: SFA and AFM studies of adhesive and non-adhesive contacts. RSC Adv 2014. [DOI: 10.1039/c4ra01803f] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
(a) Friction coefficients and (b) critical shear stresses of thiol monolayers on silver, measured with SFA (○) and AFM (red circles).
Collapse
Affiliation(s)
- Y. Yang
- Department of Chemistry
- University of Massachusetts Lowell
- Lowell, USA
| | - J. Singh
- Department of Chemistry
- University of Massachusetts Lowell
- Lowell, USA
| | - M. Ruths
- Department of Chemistry
- University of Massachusetts Lowell
- Lowell, USA
| |
Collapse
|
23
|
Eder SJ, Vernes A, Betz G. On the Derjaguin offset in boundary-lubricated nanotribological systems. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2013; 29:13760-13772. [PMID: 24164715 DOI: 10.1021/la4026443] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
We performed molecular dynamics simulations of boundary-lubricated sliding, varying the boundary lubricant type, its molecular surface coverage, the substrate roughness, and the load. The resulting load versus friction behavior was then analyzed to study how changes in lubricant type, coverage, and roughness affect the extrapolated friction force at zero load, the so-called Derjaguin offset. A smooth-particle-based evaluation method by the authors, applied here for the first time to visualize the sliding interface between the two layers of boundary lubricant, allowed the definition and calculation of a dimensionless normalized sliding resistance area, which was then related to the Derjaguin offset. This relationship excellently reflects the molecular surface coverage, which determines the physical condition of the lubricant, and can differentiate between some lubricant-specific frictional properties.
Collapse
Affiliation(s)
- Stefan J Eder
- Austrian Center of Competence for Tribology , Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria
| | | | | |
Collapse
|
24
|
Solar M, Meyer H, Gauthier C. Analysis of local properties during a scratch test on a polymeric surface using molecular dynamics simulations. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2013; 36:29. [PMID: 23526081 DOI: 10.1140/epje/i2013-13029-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Accepted: 02/21/2013] [Indexed: 06/02/2023]
Abstract
This work demonstrates a possible route to connect a particle (chain) based understanding with continuum mechanical questions about contact mechanics. The bond orientation, chain conformation and stress field of a polymer film were analyzed during scratch tests (tangential contact) using a molecular dynamics (MD) simulation approach. Scratch tests with a conical tip at constant scratching velocity were simulated on linear amorphous polymer surfaces at various temperatures and roughnesses of the tip and for various interactions between the tip and the particles of the polymer chains. The second Legendre polynomial (computed for small domains around the tip) gave the bond orientation inside the polymer film during sliding of the tip. The gyration tensor (layer-resolved in the direction of the polymer film thickness) provided information about the conformation of the polymer chains. These results allowed us to argue in favor of Briscoe's hypothesis (thin film sheared vs. "bulk" compressive behavior) concerning the friction properties of the polymer surfaces. Finally, the first stress measurements of the virial stress tensor (in sub-boxes placed in the MD cell) revealed a complex combination between compressive hydrostatic pressure and shear stress, which may be interpreted as a complex sheared domain at the interface.
Collapse
Affiliation(s)
- M Solar
- Institut Charles Sadron (UPR22-CNRS), University of Strasbourg, 23 rue du Loess, BP 84047, F-67034, Strasbourg, France.
| | | | | |
Collapse
|
25
|
Sun W, Zeng Q, Yu A. Calculation of noncontact forces between silica nanospheres. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2013; 29:2175-2184. [PMID: 23339620 DOI: 10.1021/la305156s] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Quantification of the interactions between nanoparticles is important in understanding their dynamic behaviors and many related phenomena. In this study, molecular dynamics simulation is used to calculate the interaction potentials (i.e., van der Waals attraction, Born repulsion, and electrostatic interaction) between two silica nanospheres of equal radius in the range of 0.975 to 5.137 nm. The results are compared with those obtained from the conventional Hamaker approach, leading to the development of modified formulas to calculate the van der Waals attraction and Born repulsion between nanospheres, respectively. Moreover, Coulomb's law is found to be valid for calculating the electrostatic potential between nanospheres. The developed formulas should be useful in the study of the dynamic behaviors of nanoparticle systems under different conditions.
Collapse
Affiliation(s)
- Weifu Sun
- Laboratory for Simulation and Modelling of Particulate Systems, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
| | | | | |
Collapse
|
26
|
Mendonça ACF, Pádua AAH, Malfreyt P. Nonequilibrium Molecular Simulations of New Ionic Lubricants at Metallic Surfaces: Prediction of the Friction. J Chem Theory Comput 2013; 9:1600-10. [DOI: 10.1021/ct3008827] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
| | | | - Patrice Malfreyt
- Institut de Chimie de Clermont-Ferrand, UMR 6296, Université Blaise Pascal & CNRS, 63171 Aubière, France
| |
Collapse
|
27
|
Nanotribology of self-assembled monolayer with a probe tip investigated using molecular dynamics simulations. Micron 2013; 44:410-8. [DOI: 10.1016/j.micron.2012.09.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2012] [Revised: 09/10/2012] [Accepted: 09/10/2012] [Indexed: 11/22/2022]
|
28
|
Wagner KC, Wang Y, Regen SL, Vezenov DV. Yield strength of glued Langmuir–Blodgett films determined by friction force microscopy. Phys Chem Chem Phys 2013; 15:14037-46. [DOI: 10.1039/c3cp50444a] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
|
29
|
Lewis JB, Vilt SG, Rivera JL, Jennings GK, McCabe C. Frictional properties of mixed fluorocarbon/hydrocarbon silane monolayers: a simulation study. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2012; 28:14218-14226. [PMID: 22937771 DOI: 10.1021/la3024315] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Because of small surface area to volume ratios nanoscale devices can exhibit dominant surface forces that can quickly degrade unlubricated contacting surfaces. While fluorinated materials have been widely used as lubricants, because of their low critical surface tension and high thermal and mechanical stability, fluorinated monolayer coatings, which are suitable for lubricating nanoscale devices, are less effective as lubricants. Although fluorinated monolayers are more stable than their hydrocarbon counterparts against elevated temperature and humidity, they are known to exhibit higher frictional forces. To overcome this issue, here we study mixed monolayers composed of both hydrocarbon and fluorocarbon chains. Hydrocarbon-based monolayers have been widely studied and shown to improve frictional properties and device life. To investigate the frictional behavior of mixed fluorocarbon/hydrocarbon monolayers, molecular dynamics simulations of pure hydrogenated and fluorinated chains and mixed fluorinated/hydrogenated chains on silica surfaces have been performed. The adhesion and friction between the nanoconfined monolayers as a function of normal load, chain length, and chemical composition of the monolayer coating have been investigated, and mixed fluorocarbon/hydrocarbon monolayers found to outperform both pure fluorocarbon and pure hydrocarbon monolayers. Surface coverage was found to have a significant effect on the performance of all systems studied with higher surface coverage resulting in lower frictional forces. The simulations also show that when the hydrocarbon chains in the monolayer are longer than the fluorocarbon chains, a liquidlike layer is formed by the longer hydrocarbon chains that protrudes above the shorter fluorocarbon chains and aids in friction reduction. A frictional load dependence is also seen in these mixed monolayer systems because of repulsive interactions between the fluorocarbon base layer and the hydrocarbon liquidlike layer. A chain length difference of eight carbons between the base layer and the liquidlike layer was found to provide the lowest friction, while both a larger (because of increased entanglement) and a smaller (insufficient atoms between the contacting base layers to form a liquidlike layer) chain length difference increased friction.
Collapse
Affiliation(s)
- J Ben Lewis
- Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States
| | | | | | | | | |
Collapse
|
30
|
Junge T, Molinari JF. Molecular dynamics nano-scratching of aluminium: a novel quantitative energy-based analysis method. ACTA ACUST UNITED AC 2012. [DOI: 10.1016/j.piutam.2012.03.013] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
|
31
|
Huang CH, Hsiao PY, Tseng FG, Fan SK, Fu CC, Pan RL. Pore-spanning lipid membrane under indentation by a probe tip: a molecular dynamics simulation study. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2011; 27:11930-42. [PMID: 21859109 DOI: 10.1021/la201977d] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
We study the indentation of a free-standing lipid membrane suspended over a nanopore on a hydrophobic substrate by means of molecular dynamics simulations. We find that in the course of indentation the membrane bends at the point of contact and the fringes of the membrane glide downward intermittently along the pore edges and stop gliding when the fringes reach the edge bottoms. The bending continues afterward, and the large strain eventually induces a phase transition in the membrane, transformed from a bilayered structure to an interdigitated structure. The membrane is finally ruptured when the indentation goes deep enough. Several local physical quantities in the pore regions are calculated, which include the tilt angle of lipid molecules, the nematic order, the included angle, and the distance between neighboring lipids. The variations of these quantities reveal many detailed, not-yet-specified local structural transitions of lipid molecules under indentation. The force-indentation curve is also studied and discussed. The results make a connection between the microscopic structure and the macroscopic properties and provide deep insight into the understanding of the stability of a lipid membrane spanning over nanopore.
Collapse
Affiliation(s)
- Chen-Hsi Huang
- Department of Engineering and System Science, National Tsing Hua University, Taiwan, Republic of China
| | | | | | | | | | | |
Collapse
|
32
|
Eder S, Vernes A, Vorlaufer G, Betz G. Molecular dynamics simulations of mixed lubrication with smooth particle post-processing. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2011; 23:175004. [PMID: 21493976 DOI: 10.1088/0953-8984/23/17/175004] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
A post-processing method for molecular dynamics (MD) simulations of friction based on the smooth particle approach is proposed, allowing--among other features--the introduction and evaluation of a solid-solid contact area arising due to direct asperity interaction. In order to illustrate the feasibility of this scheme, a large number of MD calculations of lubricated nanotribological systems with various asperity geometries and carefully selected numbers of lubricant molecules were carried out and analysed. In this manner, it is shown that the friction force as a function of load agrees very well with a three-parameter friction law which, in addition to the adhesion- and the load-controlled terms, contains a load-independent offset.
Collapse
Affiliation(s)
- S Eder
- Austrian Center of Competence for Tribology, Viktor-Kaplan-Strasse 2, 2700 Wiener Neustadt, Austria
| | | | | | | |
Collapse
|
33
|
Gosvami NN, Egberts P, Bennewitz R. Molecular Order and Disorder in the Frictional Response of Alkanethiol Self-Assembled Monolayers. J Phys Chem A 2011; 115:6942-7. [DOI: 10.1021/jp1121728] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Nitya Nand Gosvami
- INM—Leibniz Institute for New Materials, Campus D2 2, 66123, Saarbrücken, Germany
- Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
| | - Philip Egberts
- INM—Leibniz Institute for New Materials, Campus D2 2, 66123, Saarbrücken, Germany
| | - Roland Bennewitz
- INM—Leibniz Institute for New Materials, Campus D2 2, 66123, Saarbrücken, Germany
| |
Collapse
|
34
|
Toton D, Lorenz CD, Rompotis N, Martsinovich N, Kantorovich L. Temperature control in molecular dynamic simulations of non-equilibrium processes. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:074205. [PMID: 21386383 DOI: 10.1088/0953-8984/22/7/074205] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Thermostats are often used in various condensed matter problems, e.g. when a biological molecule undergoes a transformation in a solution, a crystal surface is irradiated with energetic particles, a crack propagates in a solid upon applied stress, two surfaces slide with respect to each other, an excited local phonon dissipates its energy into a crystal bulk, and so on. In all of these problems, as well as in many others, there is an energy transfer between different local parts of the entire system kept at a constant temperature. Very often, when modelling such processes using molecular dynamics simulations, thermostatting is done using strictly equilibrium approaches serving to describe the NVT ensemble. In this paper we critically discuss the applicability of such approaches to non-equilibrium problems, including those mentioned above, and stress that the correct temperature control can only be achieved if the method is based on the generalized Langevin equation (GLE). Specifically, we emphasize that a meaningful compromise between computational efficiency and a physically appropriate implementation of the NVT thermostat can be achieved, at least for solid state and surface problems, if the so-called stochastic boundary conditions (SBC), recently derived from the GLE (Kantorovich and Rompotis 2008 Phys. Rev. B 78 094305), are used. For SBC, the Langevin thermostat is only applied to the outer part of the simulated fragment of the entire system which borders the surrounding environment (not considered explicitly) serving as a heat bath. This point is illustrated by comparing the performance of the SBC and some of the equilibrium thermostats in two problems: (i) irradiation of the Si(001) surface with an energetic CaF(2) molecule using an ab initio density functional theory based method, and (ii) the tribology of two amorphous SiO(2) surfaces coated with self-assembled monolayers of methyl-terminated hydrocarbon alkoxylsilane molecules using a classical atomistic force field. We discuss the differences in behaviour of these systems due to applied thermostatting, and show that in some cases a qualitatively different physical behaviour of the simulated system can be obtained if an equilibrium thermostat is used.
Collapse
Affiliation(s)
- Dawid Toton
- Physics, King's College London, The Strand, London WC2R 2LS, UK
| | | | | | | | | |
Collapse
|
35
|
Cheng S, Luan B, Robbins MO. Contact and friction of nanoasperities: effects of adsorbed monolayers. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:016102. [PMID: 20365427 DOI: 10.1103/physreve.81.016102] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/22/2009] [Indexed: 05/29/2023]
Abstract
Molecular dynamics simulations are used to study contact between a rigid, nonadhesive, and spherical tip with radius of order 30 nm and a flat elastic substrate covered with a fluid monolayer of adsorbed chain molecules. Previous studies of bare surfaces showed that the atomic scale deviations from a sphere that are present on any tip constructed from discrete atoms lead to significant deviations from continuum theory and dramatic variability in friction forces. Introducing an adsorbed monolayer leads to larger deviations from continuum theory but decreases the variations between tips with different atomic structure. Although the film is fluid, it remains in the contact and behaves qualitatively like a thin elastic coating except for certain tips at high loads. Measures of the contact area based on the moments or outer limits of the pressure distribution and on counting contacting atoms are compared. The number of tip atoms making contact during a time interval Deltat grows as a power of Deltat when the film is present and as the logarithm of Deltat for bare surfaces. Friction is measured by displacing the tip at a constant velocity or pulling the tip with a spring. Both static and kinetic friction rise linearly with load at small loads. Transitions in the state of the film lead to nonlinear behavior at large loads. The friction is less clearly correlated with contact area than load.
Collapse
Affiliation(s)
- Shengfeng Cheng
- Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
| | | | | |
Collapse
|
36
|
Yang Y, Ruths M. Friction of polyaromatic thiol monolayers in adhesive and nonadhesive contacts. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2009; 25:12151-12159. [PMID: 19785396 DOI: 10.1021/la901678c] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
We have used friction force microscopy to study the effects of adhesion on the boundary friction of self-assembled monolayers of the aromatic compounds thiophenol, p-phenylthiophenol, p-terphenyl thiol, 2-naphthalenethiol, and benzyl mercaptan on gold. To control the adhesion between the monolayer-covered tip and substrate, the friction measurements were made in dry N(2) gas or in ethanol. At low loads, low adhesion (in ethanol) resulted in a linear dependence of the friction force on load (i.e., F = muL) whereas higher adhesion between the same monolayers (in N(2)) gave an apparent area-dependent friction. The friction in the adhesive systems was well described by F = S(c)A with the contact area, A, calculated for a thin, linearly elastic film confined between rigid substrates using the thin-coating contact mechanics (TCCM) model in a transition regime between its DMT- and JKR-like limits. With increasing packing density of the monolayers, a systematic decrease was found in the friction coefficient (mu) obtained in ethanol and the critical shear stress (S(c)) obtained in N(2). To describe these aromatic monolayers with the extended TCCM model, a higher Young's modulus was neeeded than for fatty acid monolayers of similar packing density.
Collapse
Affiliation(s)
- Y Yang
- Department of Chemistry, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854, USA
| | | |
Collapse
|
37
|
|
38
|
Lane JMD, Chandross M, Lorenz CD, Stevens MJ, Grest GS. Water penetration of damaged self-assembled monolayers. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2008; 24:5734-5739. [PMID: 18341361 DOI: 10.1021/la703492j] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
The interaction of water with self-assembled monolayers (SAMs) on amorphous silica is investigated using classical molecular dynamics simulation. Damage is induced through shear simulations with model atomic force microscopy (AFM) tips and separately with controlled extraction. We find that SAM coatings that have been slightly damaged (by normal loads close to 10 nN from a 10-nm-diameter AFM tip) are susceptible to water penetration and migration to the underlying hydrophilic substrate. The controlled damage studies indicate that the presence of water tends to heal damage below a threshold radius and exploits and magnifies damage above this threshold. For the systems studied here, Si(OH)3(CH2)10CH3 alkylsilane chains on amorphous silica, this threshold radius is between 0.5 and 1.0 nm.
Collapse
|
39
|
Xu C, Jones RL, Batteas JD. Dynamic variations in adhesion of self-assembled monolayers on nanoasperities probed by atomic force microscopy. SCANNING 2008; 30:106-117. [PMID: 18320599 DOI: 10.1002/sca.20095] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Octadecyltriethoxysilane (OTE) self- assembled monolayers (SAMs) and their effects on friction and adhesion have been investigated on various combinations of functionalized and unfunctionalized silicon oxide surfaces including the oxidized surface of crystalline Si(100), silica nanoparticle films, and oxidized Si atomic force microscopy (AFM) tips. Force-distance spectroscopy was utilized to probe and compare the properties of the OTE SAMs on silica asperities with nanoscale curvature against these same monolayers on surfaces with sub-1 nm roughness (flat surfaces). It was found that adhesion between SAMs and silicon oxide surfaces can vary significantly when assembly takes place on surfaces with nanoscopic curvature as compared to flat surfaces. Observations indicate that the SAM structure present during force measurements is dynamic in nature, which yields different adhesion values when measured with variations of both tip-sample contact time and tip-approach/retract rates. These results point the need in reporting a number of measurement parameters when probing adhesion by SAM functionalized tips.
Collapse
Affiliation(s)
- Chang Xu
- Department of Polymer Science and Engineering, University of Massachusetts-Amherst, Amherst, Massachusetts, USA
| | | | | |
Collapse
|