1
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Wei P, Gao P, Yang J, Pu W. Investigating the Behavior of Various Lubrication Regimes under Dynamic Conditions Using Nonequilibrium Molecular Dynamics. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2023; 39:12365-12383. [PMID: 37611147 DOI: 10.1021/acs.langmuir.3c01468] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/25/2023]
Abstract
It is crucial to comprehend how the oil film varies under dynamic operating conditions and the accompanying friction properties to better grasp the friction mechanism and control friction behavior. To model the friction characteristics under boundary lubrication (BL) and elastohydrodynamic lubrication (EHL), nonequilibrium molecular dynamics simulations with various numbers of hexadecane molecules as lubricating oil were conducted in this research under the conditions of dynamic speed and dynamic load. All the dynamic operating conditions have the form of sine waves, with various frequencies and amplitudes. According to the findings, the friction force is strongly connected with interfaces where relative sliding takes place, whose number, velocity difference, and the degree of solidification have significant influences. The variation of amplitude under dynamic load can cause a regular change in the density of the lubricating layer, while the variation of frequency can cause a change in molecular layer's range of motion. Both effects are crucial for friction. The structure of the lubricating layer with lower friction varies with various frequencies for dynamic velocity. Both high and small amplitudes of velocity offer advantages to form a stable film structure at low frequencies in the BL and EHL regions, while the amplitude in the BL area has minimal association with friction at high frequencies. At high frequencies in the EHL region, the friction rises as the amplitude of velocity grows and the lubricating layer becomes more unstable.
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Affiliation(s)
- Pengchong Wei
- School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China
| | - Pan Gao
- School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China
| | - Jialong Yang
- School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China
| | - Wei Pu
- School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China
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2
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De Nicola A, Touloupidis V, Kanellopoulos V, Albunia AR, Milano G. A combined experimental and molecular simulation study on stress generation phenomena during the Ziegler-Natta polyethylene catalyst fragmentation process. NANOSCALE ADVANCES 2022; 4:5178-5188. [PMID: 36504732 PMCID: PMC9680958 DOI: 10.1039/d2na00406b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/22/2022] [Accepted: 10/14/2022] [Indexed: 06/17/2023]
Abstract
The morphology of particles obtained under different pre-polymerization conditions has been connected to the stress generation mechanism at the polymer/catalyst interface. A combination of experimental characterization techniques and atomistic molecular dynamics simulations allowed a systematic investigation of experimental conditions leading to a certain particle morphology, and hence to a final polymer with specific features. Atomistic models of nascent polymer phases in contact with magnesium dichloride surfaces have been developed and validated. Using these detailed models, in the framework of McKenna's hypothesis, the pressure increase due to the polymerization reaction has been calculated under different conditions and is in good agreement with experimental scenarios. This molecular scale knowledge and the proposed investigation strategy would allow the pre-polymerization conditions to be better defined and the properties of the nascent polymer to be tuned, ensuring proper operability along the whole polymer production process.
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Affiliation(s)
- Antonio De Nicola
- Scuola Superiore Meridionale Largo San Marcellino 10 80132 Napoli Italy
| | - Vasileios Touloupidis
- Innovation & Technology, Borealis Polyolefine GmbH St. Peter Strasse 25 4021 Linz Austria
| | | | - Alexandra R Albunia
- Innovation & Technology, Borealis Polyolefine GmbH St. Peter Strasse 25 4021 Linz Austria
| | - Giuseppe Milano
- Dipartimento di Ingegneria Chimica dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II Piazzale V. Tecchio 80 80125 Napoli Italy
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3
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Zhao G, Yao Y, Adenutsi CD, Feng X, Wang L, Wu W. Transport Behavior of Oil in Mixed Wettability Shale Nanopores. ACS OMEGA 2020; 5:31831-31844. [PMID: 33344837 PMCID: PMC7745427 DOI: 10.1021/acsomega.0c04678] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/23/2020] [Accepted: 10/30/2020] [Indexed: 06/12/2023]
Abstract
Shale oil reserves play an important role in the oil & gas industry. The investigation of oil transport behavior in shale nanopores is crucial in the successful exploitation of shale oil reservoirs. However, the transport mechanisms of oil in shale nanopores are still not understood. In this paper, a model for oil transport through a single nanopore was established by considering mixed wettability, surface roughness, varying viscosity, and the effects triggered by adsorbed organic matter. The organic surface ratio of a single nanopore was used to quantify mixed wettability, while the effects of adsorbed organic matter were estimated by the surface coverage and the adsorption thickness. The entire mathematical model was simplified into several equations to discuss the contributions of each mechanism. The results showed that to accurately predict the oil transport properties in mixed wettability shale nanopores, it is necessary to consider varying viscosity, wettability alteration, and the oil molecule structure. Adsorbed organic matter led to increase in oil flow capacity by altering the surface wettability. However, the oil flow capacity was greatly reduced when varying viscosity was considered. Additionally, the contributions of each mechanism varied with the pore type. Furthermore, increasing surface roughness significantly reduced the oil flow capacity in both organic and inorganic nanopores. This work provides a better understanding of oil transport behavior in mixed-wettability shale nanopores and a quantitative framework for future research.
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Affiliation(s)
- Guoxiang Zhao
- College
of Petroleum Engineering, China University
of Petroleum, Beijing 102249, China
| | - Yuedong Yao
- College
of Petroleum Engineering, China University
of Petroleum, Beijing 102249, China
| | - Caspar Daniel Adenutsi
- Core
and Rock Properties Laboratory, Department of Petroleum Engineering,
Faculty of Civil and Geo-Engineering, Kwame
Nkrumah University of Science and Technology, Kumasi AK, Ghana
| | - Xiaolong Feng
- No.1
Oil Production Plant, Changqing Oilfield, PetroChina, Yan’an 716000, China
| | - Lian Wang
- College
of Petroleum Engineering, China University
of Petroleum, Beijing 102249, China
| | - Wenwei Wu
- Ningbo
Fengcheng Advanced Energy Materials Research Institute, Ningbo 315000, China
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4
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Lei Z, Lai L, Sun Z, Wu K. Comprehensive Model for Oil Transport Behavior in Nanopores: Interactions between Oil and Pore Surface. Ind Eng Chem Res 2020. [DOI: 10.1021/acs.iecr.0c04025] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Zhengdong Lei
- Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
| | - Lingbin Lai
- Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
| | - Zheng Sun
- State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China
| | - Keliu Wu
- State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, P. R. China
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5
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Chen X, Yang Z, Feng S, Golbek TW, Xu W, Butt HJ, Weidner T, Xu Z, Hao J, Wang Z. How Universal Is the Wetting Aging in 2D Materials. NANO LETTERS 2020; 20:5670-5677. [PMID: 32579374 DOI: 10.1021/acs.nanolett.0c00855] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Previous studies indicate that 2D materials such as graphene, WS2, and MoS2 deposited on oxidized silicon substrate are susceptible to aging due to the adsorption of airborne contamination. As a result, their surfaces become more hydrophobic. However, it is not clear how ubiquitous such a hydrophobization is, and the interplay between the specific adsorbed species and resultant wetting aging remains elusive. Here, we report a pronounced and general hydrophilic-to-hydrophobic wetting aging on 2D InSe films, which is independent of the substrates to synthesize these films (silicon, glass, nickel, copper, aluminum oxide), though the extent of wetting aging is sensitive to the layer of films. Our findings are ascribed to the occurrence and enrichment of airborne contamination that contains alkyl chains. Our results also suggest that the wetting aging effect might be universal to a wide range of 2D materials.
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Affiliation(s)
- Xuan Chen
- Department of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, China
| | - Zhibin Yang
- Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong 999077, China
| | - Shizhe Feng
- Applied Mechanics Laboratory, Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
| | | | - Wanghuai Xu
- Department of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, China
| | | | - Tobias Weidner
- Department of Chemistry, Aarhus University, Aarhus 8000, Denmark
| | - Zhiping Xu
- Applied Mechanics Laboratory, Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
| | - Jianhua Hao
- Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong 999077, China
| | - Zuankai Wang
- Department of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, China
- Shenzhen Research Institute of City University of Hong Kong, Shenzhen 518057, China
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6
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Zhao Y, Gosai A, Kang K, Shrotriya P. Multiscale Modeling Reveals the Cause of Surface Stress Change on Microcantilevers Due to Alkanethiol SAM Adsorption. J Chem Inf Model 2020; 60:2998-3008. [DOI: 10.1021/acs.jcim.0c00146] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Yue Zhao
- Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States
| | - Agnivo Gosai
- Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States
| | - Kyungho Kang
- Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States
| | - Pranav Shrotriya
- Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States
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7
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Wu M, Vartanian AM, Chong G, Pandiakumar AK, Hamers RJ, Hernandez R, Murphy CJ. Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self-Assembled Monolayers on Gold Nanoparticles: The Effect of Surface Curvature and Ligand Structure. J Am Chem Soc 2019; 141:4316-4327. [PMID: 30763078 DOI: 10.1021/jacs.8b11445] [Citation(s) in RCA: 49] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
We report a solution NMR-based analysis of (16-mercaptohexadecyl)trimethylammonium bromide (MTAB) self-assembled monolayers on colloidal gold nanospheres (AuNSs) with diameters from 1.2 to 25 nm and gold nanorods (AuNRs) with aspect ratios from 1.4 to 3.9. The chemical shift analysis of the proton signals from the solvent-exposed headgroups of bound ligands suggests that the headgroups are saturated on the ligand shell as the sizes of the nanoparticles increase beyond ∼10 nm. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent. Ligand density ranges from ∼3 molecules per nm2 for 25 nm particles to up to 5-6 molecules per nm2 in ∼10 nm and smaller particles for in situ measurements of bound ligands; after I2/I- treatment to etch away the gold cores, ligand density ranges from ∼2 molecules per nm2 for 25 nm particles to up to 4-5 molecules per nm2 in ∼10 nm and smaller particles. T2 relaxation analysis shows greater hydrocarbon chain ordering and less headgroup motion as the diameter of the particles increases from 1.2 nm to ∼13 nm. Molecular dynamics simulations of 4, 6, and 8 nm (11-mercaptoundecyl)trimethylammonium bromide-capped AuNSs confirm greater hydrophobic chain packing order and saturation of charged headgroups within the same spherical ligand shell at larger nanoparticle sizes and higher ligand densities. Combining the NMR studies and MD simulations, we suggest that the headgroup packing limits the ligand density, rather than the sulfur packing on the nanoparticle surface, for ∼10 nm and larger particles. For MTAB-AuNRs, no chemical shift data nor ligand density data suggest that two populations of ligands that might correspond to side-ligands and end-ligands exist; yet T2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.
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Affiliation(s)
- Meng Wu
- Department of Chemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
| | - Ariane M Vartanian
- Department of Chemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
| | - Gene Chong
- Department of Chemistry , Johns Hopkins University , Baltimore , Maryland 21218 , United States
| | - Arun Kumar Pandiakumar
- Department of Chemistry , University of Wisconsin-Madison , Madison , Wisconsin 53706 , United States
| | - Robert J Hamers
- Department of Chemistry , University of Wisconsin-Madison , Madison , Wisconsin 53706 , United States
| | - Rigoberto Hernandez
- Department of Chemistry , Johns Hopkins University , Baltimore , Maryland 21218 , United States
| | - Catherine J Murphy
- Department of Chemistry , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States
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8
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Jaishankar A, Jusufi A, Vreeland JL, Deighton S, Pellettiere J, Schilowitz AM. Adsorption of Stearic Acid at the Iron Oxide/Oil Interface: Theory, Experiments, and Modeling. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:2033-2046. [PMID: 30624939 DOI: 10.1021/acs.langmuir.8b03132] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Improved friction performance is an important objective of equipment manufacturers for meeting improved energy efficiency demands. The addition of friction-reducing additives, or friction modifiers (FMs), to lubricants is a key part of the strategy. The performance of these additives is related to their surface activity and their ability to form adsorbed layers on the metal surface. However, the extent of surface coverage (mass per unit area) required for effective friction reduction is currently unknown. In this article, we show that full coverage is not necessary for significant friction reduction. We first highlight various features of surface adsorption that can influence the surface coverage, packing, and free energy of adsorption of organic FMs on iron oxide surfaces. Using stearic acid in heptane and hexadecane as model lubricant formulations, we employ a combination of experiments and molecular dynamics (MD) simulations to show how the dimerization of acid molecules in the bulk solvent and the crystallographic orientation of the surface modifies surface adsorption. In addition, we show that the solvent can strongly influence the adsorption kinetics, and MD simulations reveal that hexadecane tends to align on the surface, increasing the energy barrier for the adsorption of stearic acid to the surface. Furthermore, we present a combined approach using MD and molecular thermodynamic theory to calculate adsorption isotherms for stearic acid on iron oxide surfaces, which agrees well with experimental data obtained with a quartz crystal microbalance (QCM). Our results suggest that while the friction of systems lubricated with organic FMs decreases with increasing coverage, complete coverage of the surface is neither practically achievable nor necessary for effective friction reduction for the systems and conditions studied here.
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Affiliation(s)
- Aditya Jaishankar
- ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States
| | - Arben Jusufi
- ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States
| | - Jessica L Vreeland
- ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States
| | - Shane Deighton
- ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States
| | - Joseph Pellettiere
- ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States
| | - Alan M Schilowitz
- ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States
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9
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Accessing the structural and thermodynamic properties of ultra-thin layers of C32 adsorbed on a SiO 2 surface. Chem Phys Lett 2017. [DOI: 10.1016/j.cplett.2017.01.065] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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10
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Li Y, Wu H, Wang F. Effect of a Single Nanoparticle on the Contact Line Motion. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2016; 32:12676-12685. [PMID: 27809545 DOI: 10.1021/acs.langmuir.6b03595] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
Abstract
In this paper, we use a single nanoparticle (NP) to achieve active control of the droplet contact line. When the droplet is out of equilibrium, the resulting excess free energy provides the driving force for the depinning of the contact line and the NP. There are three ways to increase the energy barriers to be surmounted and to realize the pinning of the contact line, namely, the enhancement of the interactions between the NP and the substrate, the increase in substrate hydrophilicity, and the reduction in the NP hydrophilicity. On this basis, we obtained three styles of contact line motion including complete slipping, alternate pinning-depinning, and complete pinning and theoretically interpreted them. The basic theory presented in this paper can be applied to explain and regulate the dynamics of the contact line involved in many physical processes such as evaporation and spreading.
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Affiliation(s)
- YingQi Li
- CAS Key Laboratory of Materials Behavior and Design of Materials, Department of Modern Mechanics, CAS Center for Excellence in Nanoscience, University of Science and Technology of China , Hefei, Anhui 230027, China
| | - HengAn Wu
- CAS Key Laboratory of Materials Behavior and Design of Materials, Department of Modern Mechanics, CAS Center for Excellence in Nanoscience, University of Science and Technology of China , Hefei, Anhui 230027, China
| | - FengChao Wang
- CAS Key Laboratory of Materials Behavior and Design of Materials, Department of Modern Mechanics, CAS Center for Excellence in Nanoscience, University of Science and Technology of China , Hefei, Anhui 230027, China
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11
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Sivebaek IM, Persson BNJ. The effect of surface nano-corrugation on the squeeze-out of molecular thin hydrocarbon films between curved surfaces with long range elasticity. NANOTECHNOLOGY 2016; 27:445401. [PMID: 27668358 DOI: 10.1088/0957-4484/27/44/445401] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
The properties of linear alkane lubricants confined between two approaching solids are investigated by a model that accounts for the roughness, curvature and elastic properties of the solid surfaces. We consider linear alkanes of different chain lengths from [Formula: see text] to [Formula: see text], confined between corrugated solid walls. The pressure necessary to squeeze out the lubricant increases rapidly with the alkane chain length, but is always much lower than in the case of smooth surfaces. The longest alkanes form domains of ordered chains and the squeeze-out appears to nucleate in the more disordered regions between these domains. The short alkanes stay fluid-like during the entire squeeze out process which result in a very small squeeze-out pressure which is almost constant during the squeeze-out of the last monolayer of the fluid. In all cases we observe lubricant trapped in the valley of the surface roughness, which cannot be removed independent of the magnitude of the squeezing pressures.
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Affiliation(s)
- I M Sivebaek
- IFF, FZ-Jülich, D-52425 Jülich, Germany. Novo Nordisk, Device R&D, DK-3400 Hillerød, Denmark. Mechanical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark
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12
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Bekele S, Tsige M. Effect of Polymer/Solid and Polymer/Vapor Instantaneous Interfaces on the Interfacial Structure and Dynamics of Polymer Melt Systems. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2016; 32:7151-7158. [PMID: 27347740 DOI: 10.1021/acs.langmuir.6b01554] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Polymers are used in a wide range of applications that involve chemical and physical processes taking place at surfaces or interfaces which influence the interaction between the polymer material and the substance that comes into contact with it. Polymer surfaces are usually modified either chemically or physically for specific applications such as facilitating wetting, reducing friction, and enhancing adhesion. The variety and complexity of surface and interfacial processes requires a molecular-level understanding of the structural and dynamical properties of the surface/interface layer to help in the design of materials with desired functional properties. Using molecular dynamics (MD) simulations, we investigate the structure and dynamics at the surface of polymer films. We find that the density profiles of the films as a function of distance relative to an instantaneous surface have a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. However, the interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface.
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Affiliation(s)
- Selemon Bekele
- Department of Polymer Science, The University of Akron , Akron, Ohio 44325, United States
| | - Mesfin Tsige
- Department of Polymer Science, The University of Akron , Akron, Ohio 44325, United States
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13
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Iakovlev A, Bedrov D, Müller M. Alkyl-Based Surfactants at a Liquid Mercury Surface: Computer Simulation of Structure, Self-Assembly, and Phase Behavior. J Phys Chem Lett 2016; 7:1546-1553. [PMID: 27045619 DOI: 10.1021/acs.jpclett.6b00494] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Self-assembled organic films on liquid metals feature a very rich phase behavior, which qualitatively differs from the one on crystalline metals. In contrast to conventional crystalline supports, self-assembled alkylthiol monolayers on liquid metals possess a considerably higher degree of molecular order, thus enabling much more robust metal-molecule-semiconductor couplings for organic electronics applications. Yet, compared to crystalline substrates, the self-assembly of organic surfactants on liquid metals has been studied to a much lesser extent. In this Letter we report the first of its kind molecular simulation investigation of alkyl-based surfactants on a liquid mercury surface. The focus of our investigation is the surfactant conformations as a function of surface coverage and surfactant type. First, we consider normal alkanes because these systems set the basis for simulations of all other organic surfactants on liquid mercury. Subsequently, we proceed with the discussion of alkylthiols that are the most frequently used surfactants in the surface science of hybrid organometallic interfaces. Our results indicate a layering transition of normal alkanes as well as alkylthiols from an essentially bare substrate to a completely filled monolayer of laying molecules. As the surface coverage increases further, we observe a partial wetting of the laying monolayer by the bulk phase of alkanes. In the case of alkylthiols, we clearly see the coexistence of molecules in laying-down and standing-up conformations, in which the sulfur headgroups of the thiols are chemically bound to mercury. In the standing-up phase, the headgroups form an oblique lattice. For the first time we were able to explicitly characterize the molecular-scale structure and transitions between phases of alkyl-based surfactants and to demonstrate how the presence of a thiol headgroup qualitatively changes the phase equilibrium and structure in these systems. The observed phenomena are consistent with available direct and indirect experimental evidence.
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Affiliation(s)
- Anton Iakovlev
- Institut für Theoretische Physik, Georg-August-Universität Göttingen , Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
| | - Dmitry Bedrov
- Department of Materials Science & Engineering, University of Utah , 122 South Central Campus Drive, Salt Lake City, Utah 84112, United States
| | - Marcus Müller
- Institut für Theoretische Physik, Georg-August-Universität Göttingen , Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
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14
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Glynos E, Frieberg B, Chremos A, Sakellariou G, Gidley DW, Green PF. Vitrification of Thin Polymer Films: From Linear Chain to Soft Colloid-like Behavior. Macromolecules 2015. [DOI: 10.1021/ma502556n] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
| | | | - Alexandros Chremos
- Department
of Chemical Engineering, Centre for System Process System Engineering, Imperial College, South
Kensington Campus, London SW7 2AZ, U.K
| | - Georgios Sakellariou
- Department
of Chemistry, University of Athens, Panepistimiopolis, Zografou, 15771 Athens, Greece
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15
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Wenzel S, Nemec H, Anderson KE, Siepmann JI. Monte Carlo simulations of thin hydrocarbon films: composition heterogeneity and structure at the solid-liquid and liquid-vapor interfaces. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2014; 30:3086-3094. [PMID: 24564283 DOI: 10.1021/la4048598] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The structural properties of 10 nm thick lubricant films consisting of binary and ternary n-alkane mixtures (8 ≤ n ≤ 12) adsorbed on a structureless metal substrate were studied for several temperatures and compositions using Monte Carlo simulations. Configurational-bias Monte Carlo identity switch moves are essential to sample the spatial distribution in these mixtures. Longer alkanes are found to preferentially adsorb onto the substrate while shorter alkanes are enriched at the liquid-vapor interface. This preferential adsorption is evident even when the two chains differ by only one methylene unit and the longer chain is the minor component. Enhanced composition heterogeneity and orientational ordering and fewer gauche defects are characteristic features of the first layer near the substrate.
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Affiliation(s)
- Sara Wenzel
- Department of Chemistry and Chemical Theory Center, University of Minnesota , 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
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17
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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.
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Affiliation(s)
- Stefan J Eder
- Austrian Center of Competence for Tribology , Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria
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18
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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.
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Affiliation(s)
- S Eder
- Austrian Center of Competence for Tribology, Viktor-Kaplan-Strasse 2, 2700 Wiener Neustadt, Austria
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Zammit U, Marinelli M, Mercuri F, Paoloni S, Scudieri F. Effect of Quenched Disorder on the RI−RV, RII−RI, and Liquid−RII Rotator Phase Transitions in Alkanes. J Phys Chem B 2011; 115:2331-7. [DOI: 10.1021/jp111067z] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- U. Zammit
- Dipartimento Ingegneria Meccanica, Universitá di Roma “Tor Vergata”, Rome, Italy
| | - M. Marinelli
- Dipartimento Ingegneria Meccanica, Universitá di Roma “Tor Vergata”, Rome, Italy
| | - F. Mercuri
- Dipartimento Ingegneria Meccanica, Universitá di Roma “Tor Vergata”, Rome, Italy
| | - S. Paoloni
- Dipartimento Ingegneria Meccanica, Universitá di Roma “Tor Vergata”, Rome, Italy
| | - F. Scudieri
- Dipartimento Ingegneria Meccanica, Universitá di Roma “Tor Vergata”, Rome, Italy
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Abstract
In this contribution, we study situations in which nanoparticles in a fluid are strongly heated, generating high heat fluxes. This situation is relevant to experiments in which a fluid is locally heated by using selective absorption of radiation by solid particles. We first study this situation for different types of molecular interactions, using models for gold particles suspended in octane and in water. As already reported in experiments, very high heat fluxes and temperature elevations (leading eventually to particle destruction) can be observed in such situations. We show that a very simple modeling based on Lennard-Jones (LJ) interactions captures the essential features of such experiments and that the results for various liquids can be mapped onto the LJ case, provided a physically justified (corresponding state) choice of parameters is made. Physically, the possibility of sustaining very high heat fluxes is related to the strong curvature of the interface that inhibits the formation of an insulating vapor film.
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Van LP, Kyrylyuk V, Polesel-Maris J, Thoyer F, Lubin C, Cousty J. Experimental three-dimensional description of the liquid hexadecane/graphite interface. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2009; 25:639-642. [PMID: 19072577 DOI: 10.1021/la803665k] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
By using an atomic force microscope based on a quartz tuning fork sensor, a 3-dimensional description of the interface between liquid hexadecane and a highly oriented pyrolytic graphite surface can be achieved at room temperature. The C16H34 monolayer in contact with the substrate surface exhibits a lamellar structure whereas no observation at the liquid/graphite interface by scanning tunnelling microscopy was reported for this alkane. The second layer shows very weak corrugations corresponding to lamella boundaries. Force/distance curves show at least four oscillations separated by 0.4 nm except for the first period with a 0.38 nm distance that corresponds to the layer closer the substrate. Such a description agrees well with molecular dynamics results obtained on alkane/solid interfaces.
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Affiliation(s)
- L Pham Van
- CEA-Saclay, bat. 462, F-91191 Gif sur Yvette Cedex, France
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Lee WJ, Ju SP, Cheng CH. A molecular dynamics study of nanoindentation on a methyl methacrylate ultrathin film on a Au(111) substrate: interface and thickness effects. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2008; 24:13440-13449. [PMID: 18986180 DOI: 10.1021/la801786u] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
The molecular dynamics simulation model of nanoindentation is proposed in order to study the mechanical and structural deformation properties of an ultrathin MMA (methyl methacrylate) film on a Au(111) surface. First, the significant differences in the structural arrangement of MMA thin films with different thicknesses are observed. Two layers are apparent in the thinnest MMA thin film next to the Au(111) surface, while three layer structures are apparent in the thicker film. Second, this study examines the indentation tip that penetrates the MMA thin film into the Au(111) substrate in order to understand the influence of the interface on the properties and deformation behavior in both the thin film and substrate. The result shows that the indentation force is influenced both by the layer structure and by the thickness of the MMA film. The thinnest case exhibits different deformation behavior from that of the thicker cases. In addition, the deformation of MMA molecules becomes significant at the interface between the MMA film and the Au(111) surface with the increase of film thickness, and detailed deformation behavior of the Au surface for different thicknesses of MMA film is reported in this paper. Finally, both the rigid and the active models for the indentation tip are utilized in the simulation to examine the interaction differences between the tip and the film and the deformation mechanism.
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Affiliation(s)
- Wen-Jay Lee
- Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, Republic of China
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Ivanov VA, Martemyanova JA, Müller M, Paul W, Binder K. Conformational Changes of a Single Semiflexible Macromolecule Near an Adsorbing Surface: A Monte Carlo Simulation. J Phys Chem B 2008; 113:3653-68. [DOI: 10.1021/jp806348y] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- V. A. Ivanov
- Faculty of Physics, Moscow State University, 119992 Moscow, Russia, Institut für Theoretische Physik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany, and Institut für Physik, Johannes-Gutenberg-Universität Mainz, Staudinger Weg 7, 55099 Mainz, Germany
| | - J. A. Martemyanova
- Faculty of Physics, Moscow State University, 119992 Moscow, Russia, Institut für Theoretische Physik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany, and Institut für Physik, Johannes-Gutenberg-Universität Mainz, Staudinger Weg 7, 55099 Mainz, Germany
| | - M. Müller
- Faculty of Physics, Moscow State University, 119992 Moscow, Russia, Institut für Theoretische Physik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany, and Institut für Physik, Johannes-Gutenberg-Universität Mainz, Staudinger Weg 7, 55099 Mainz, Germany
| | - W. Paul
- Faculty of Physics, Moscow State University, 119992 Moscow, Russia, Institut für Theoretische Physik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany, and Institut für Physik, Johannes-Gutenberg-Universität Mainz, Staudinger Weg 7, 55099 Mainz, Germany
| | - K. Binder
- Faculty of Physics, Moscow State University, 119992 Moscow, Russia, Institut für Theoretische Physik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany, and Institut für Physik, Johannes-Gutenberg-Universität Mainz, Staudinger Weg 7, 55099 Mainz, Germany
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Sushko ML, Kalampokidis A, Gal AY, Shluger AL. Modeling of NC-AFM Imaging of Alkanethiols on the Au (111) Surface. Isr J Chem 2008. [DOI: 10.1560/ijc.48.2.99] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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Freedman MA, Becker JS, Rosenbaum AW, Sibener SJ. Polymer surface and thin film vibrational dynamics of poly(methyl methacrylate), polybutadiene, and polystyrene. J Chem Phys 2008; 129:044906. [DOI: 10.1063/1.2939018] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
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Basu S, Satija SK. In-situ X-ray reflectivity study of alkane films grown from the vapor phase. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2007; 23:8331-5. [PMID: 17602677 DOI: 10.1021/la062517f] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
We carried out in-situ X-ray reflectivity study of nine n-alkane chains (CnH2n+2) on Si substrate, n in the range of 17-30. These films formed under vacuum at equilibrium vapor pressure of the respective alkane molecule. For all the alkanes studied we found a bilayer structure on the substrate, a higher density vertical layer at the air-film interface with the layer thickness equal to the all-trans length of the respective molecule, and a lower density layer below it with the molecules lying horizontal on the substrate. This model was earlier proposed for C32 films on Si by Volkmann et al.11 We observe that this model can fit the entire range of data from C17 to C30 in our experiments.
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Affiliation(s)
- S Basu
- National Institute for Standards and Technology, Gaithersburg, Maryland 20899, USA
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29
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Ju SP, Lee WJ, Cheng CH. Molecular dynamics investigation into the effect of temperature on the structure and properties of methyl methacrylate thin films on a Au(111) surface. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2007; 23:8067-73. [PMID: 17585788 DOI: 10.1021/la700643n] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/15/2023]
Abstract
An atomistic modeling approach is performed to investigate the effect of temperature on the structural properties of the MMA (methyl methacrylate) thin film and a Au(111) surface. The density profile and orientation of the MMA molecule in the thin film have been analyzed. We found that there is a significant effect on the density profile and orientation of the MMA molecule in the region near the interface between the thin film and the Au substrate. As the temperature increases, the density clearly decreases in this region; in addition, the orientation of the MMA molecule also changes. Next, we calculated and examined the relationships between the stress, surface tension, and free energy and the density profile. Finally, we analyzed the influence of temperature on the interaction strength between the MMA molecules and between the MMA molecules and the Au(111) surface. We found that the influence of the interaction strength is more significant between MMA molecules than between MMA molecules and the Au(111) surface.
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Affiliation(s)
- Shin-Pon Ju
- Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun-Yat-Sen University, Kaohsiung, Taiwan 804, Republic of China.
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Tavana H, Neumann AW. Recent progress in the determination of solid surface tensions from contact angles. Adv Colloid Interface Sci 2007; 132:1-32. [PMID: 17222380 DOI: 10.1016/j.cis.2006.11.024] [Citation(s) in RCA: 115] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2006] [Accepted: 11/28/2006] [Indexed: 10/23/2022]
Abstract
Advancing contact angles of different liquids measured on the same solid surface fall very close to a smooth curve when plotted as a function of liquid surface tension, i.e., gamma(lv)costheta versus gamma(lv). Changing the solid surface, and hence gamma(sv), shifts the curve in a regular manner. These patterns suggest that gamma(lv)costheta depends only on gamma(lv) and gamma(sv). Thus, an "equation of state for the interfacial tensions" was developed to facilitate the determination of solid surface tensions from contact angles in conjunction with Young's equation. However, a close examination of the smooth curves showed that contact angles typically show a scatter of 1-3 degrees around the curves. The existence of the deviations introduces an element of uncertainty in the determination of solid surface tensions. Establishing that (i) contact angles are exclusively a material property of the coating polymer and do not depend on experimental procedures and that (ii) contact angle measurements with a sophisticated methodology, axisymmetric drop shape analysis (ADSA), are highly reproducible guarantees that the deviations are not experimental errors and must have physical causes. The contact angles of a large number of liquids on the films of four different fluoropolymers were studied to identify the causes of the deviations. Specific molecular interactions at solid-vapor and/or solid-liquid interfaces account for the minor contact angle deviations. Such interactions take place in different ways. Adsorption of vapor of the test liquid onto the solid surface is apparently the only process that influences the solid-vapor interfacial tension (gamma(sv)). The molecular interactions taking place at the solid-liquid interface are more diverse and complicated. Parallel alignment of liquid molecules at the solid surface, reorganization of liquid molecules at the solid-liquid interface, change in the configuration of polymer chains due to contact with certain probe liquids, and intermolecular interactions between solid and liquid molecules cause the solid-liquid interfacial (gamma(sl)) tension to be different from that predicted by the equation of state, i.e., gamma(sl) is not a precise function of gamma(lv) and gamma(sv). In other words, the experimental contact angles deviate from the "ideal" contact angle pattern. Specific criteria are proposed to identify probe liquids which eliminate specific molecular interactions. Octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS) are shown to meet those criteria, and therefore are the most suitable liquids to characterize surface tensions of low energy fluoropolymer films with an accuracy of +/-0.2 mJ/m2.
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Affiliation(s)
- H Tavana
- Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, Canada M5S 3G8
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Esenturk O, Walker RA. Surface vibrational structure at alkane liquid/vapor interfaces. J Chem Phys 2006; 125:174701. [PMID: 17100455 DOI: 10.1063/1.2356858] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Broadband vibrational sum frequency spectroscopy (VSFS) has been used to examine the surface structure of alkane liquid/vapor interfaces. The alkanes range in length from n-nonane (C(9)H(20)) to n-heptadecane (C(17)H(36)), and all liquids except heptadecane are studied at temperatures well above their bulk (and surface) freezing temperatures. Intensities of vibrational bands in the CH stretching region acquired under different polarization conditions show systematic, chain length dependent changes. Data provide clear evidence of methyl group segregation at the liquid/vapor interface, but two different models of alkane chain structure can predict chain length dependent changes in band intensities. Each model leads to a different interpretation of the extent to which different chain segments contribute to the anisotropic interfacial region. One model postulates that changes in vibrational band intensities arise solely from a reduced surface coverage of methyl groups as alkane chain length increases. The additional methylene groups at the surface must be randomly distributed and make no net contribution to the observed VSF spectra. The second model considers a simple statistical distribution of methyl and methylene groups populating a three dimensional, interfacial lattice. This statistical picture implies that the VSF signal arises from a region extending several functional groups into the bulk liquid, and that the growing fraction of methylene groups in longer chain alkanes bears responsibility for the observed spectral changes. The data and resulting interpretations provide clear benchmarks for emerging theories of molecular structure and organization at liquid surfaces, especially for liquids lacking strong polar ordering.
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Affiliation(s)
- Okan Esenturk
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
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32
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Lal M, Plummer M, Smith W. Solvent Density Effects on the Solvation Behavior and Configurational Structure of Bare and Passivated 38-Atom Gold Nanoparticle in Supercritical Ethane. J Phys Chem B 2006; 110:20879-88. [PMID: 17048902 DOI: 10.1021/jp0633650] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
In exploring the effects of solvent density on the mode and the degree of solvation of the bare and passivated 38-atom gold particle in supercritical ethane, we have extended the molecular dynamics simulations of the system, reported previously,(34) to cover a range of isotherms in the T > T(c) regime, where T(c) is the critical temperature of the solvent. Consonant with our previous observations, the modes of solvation of the bare and the passivated particle, deduced from the radial distribution of the solvent about the metal core center of mass, are found to be vastly different from each other at all solvent densities: while the molecules solvating the bare particle form a well-defined, two-region layer around it, those solvating the passivated particle are loosely dispersed in the passivating layer. For the bare particle, the degree of solvation (vartheta) as a function of solvent density passes through a maximum occurring in the close vicinity of the critical point, consistent with our previous results and in agreement with Debenedetti's theoretical analysis,(22,23) which predicts a solvation enhancement effect in the critical region for systems where the unlike solvent/solute interaction is much stronger than the solvent/solvent interaction. Taking the degree of solvation (vartheta) as a measure of solvent quality, we have investigated how the solvent quality would vary along the solvent-density isotherms. In the solvent-density regime rho > rho(c), the solvent quality is found to be a decreasing function of the density as a result of progressive dominance of the excluded volume effect over the attractive particle/solvent interactions. The particle/solvent affinity is greatly reduced in the presence of the passivating layer, resulting in considerable shrinkage of the good-solvent-quality domain in the supercritical regime. The solvent environment and the presence of the passivating chains produce significant disorder in the equilibrium structure assumed by the nanoparticle core.
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Affiliation(s)
- Moti Lal
- Centre for Nanoscale Science, The Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, United Kingdom.
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Tartaglino U, Sivebaek IM, Persson BNJ, Tosatti E. Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity. J Chem Phys 2006; 125:014704. [PMID: 16863321 DOI: 10.1063/1.2210008] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The properties of butane (C4H10) lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider the linear n-butane and the branched isobutane. For the linear molecule, well defined molecular layers develop in the lubricant film when the width is of the order of a few atomic diameters. The branched isobutane forms more disordered structures which permit it to stay liquidlike at smaller surface separations. During squeezing the solvation forces show oscillations corresponding to the width of a molecule. At low speeds (<0.1 ms) the last layers of isobutane are squeezed out before those of n-butane. Since the (interfacial) squeezing velocity in most practical applications is very low when the lubricant layer has molecular thickness, one expects n-butane to be a better boundary lubricant than isobutane. With n-butane possessing a slightly lower viscosity at high pressures, our result refutes the view that squeeze-out should be harder for higher viscosities; on the other hand our results are consistent with wear experiments in which n-butane were shown to protect steel surfaces better than isobutane.
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Yung KL, He L, Xu Y, Shen YW. Study of surface conditions and shear flow of LCP melts in nanochannels through molecular dynamics simulation. POLYMER 2006. [DOI: 10.1016/j.polymer.2006.04.028] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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Tay K, Bresme F. Hydrogen bond structure and vibrational spectrum of water at a passivated metal nanoparticle. ACTA ACUST UNITED AC 2006. [DOI: 10.1039/b600252h] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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36
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Determination of accurate surface tensions of maleimide copolymers containing fluorinated side chain from contact angle measurements. Colloid Polym Sci 2005. [DOI: 10.1007/s00396-005-1410-6] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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Tavana H, Simon F, Grundke K, Kwok DY, Hair ML, Neumann AW. Interpretation of contact angle measurements on two different fluoropolymers for the determination of solid surface tension. J Colloid Interface Sci 2005; 291:497-506. [PMID: 15925379 DOI: 10.1016/j.jcis.2005.05.001] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2004] [Revised: 04/29/2005] [Accepted: 05/02/2005] [Indexed: 11/25/2022]
Abstract
Contact angle measurements with a large number of liquids on the semi-fluorinated acryl polymer EGC-1700 films are reported. The surface tension was determined to be gammasv=13.84 mJ/m2 from contact angles of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS). Inertness of these two liquids makes them ideal for determination of surface tension of low-energy fluoropolymers. On the other hand, contact angles of many other liquids deviated somewhat from a smooth contact angle pattern that represents the EGC-1700 surface tension. It is argued that noninertness of the molecules of these liquids gives rise to specific interactions with the polymer film, causing the deviations. Furthermore, contact angles of a series of n-alkanes (n-hexane to n-hexadecane) showed systematic deviations from this curve, similar to the trend observed for n-alkanes/Teflon AF 1600 systems studied earlier. Adsorption of vapor of short-chain liquids onto the polymer film caused their contact angles to fall above the gammasv=13.84 mJ/m2 curve, and a parallel alignment of molecules of the long-chain n-alkanes in the vicinity of the solid was the explanation for the deviation of their contact angles below it. It is found that vapor adsorption effect is more significant in the case of Teflon AF 1600, while the alignment of liquid molecules close to the surface is more pronounced for EGC-1700.
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Affiliation(s)
- H Tavana
- Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, ON M5S 3G8, Canada
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Kruchten F, Knorr K, Volkmann UG, Taub H, Hansen FY, Matthies B, Herwig KW. Ellipsometric and neutron diffraction study of pentane physisorbed on graphite. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2005; 21:7507-12. [PMID: 16042486 DOI: 10.1021/la050338c] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
High-resolution ellipsometry and neutron diffraction measurements have been used to investigate the structure, growth, and wetting behavior of fluid pentane (n-C(5)H(12)) films adsorbed on graphite substrates. We present isotherms of the thickness of pentane films adsorbed on the basal-plane surfaces of a pyrolytic graphite substrate as a function of the vapor pressure. These isotherms are measured ellipsometrically for temperatures between 130 and 190 K. We also describe neutron diffraction measurements in the temperature range 11-140 K on a deuterated pentane (n-C(5)D(12)) monolayer adsorbed on an exfoliated graphite substrate. Below a temperature of 99 K, the diffraction patterns are consistent with a rectangular centered structure. Above the pentane triple point at 143.5 K, the ellipsometric measurements indicate layer-by-layer adsorption of at least seven fluid pentane layers, each having the same optical thickness. Analysis of the neutron diffraction pattern of a pentane monolayer at a temperature of 130 K is consistent with small clusters having a rectangular-centered structure and an area per molecule of approximately 37 A(2) in coexistence with a fluid monolayer phase. Assuming values of the polarizability tensor from the literature and that the monolayer fluid has the same areal density as that inferred for the coexisting clusters, we calculate an optical thickness of the fluid pentane layers in reasonable agreement with that measured by ellipsometry. We discuss how these results support the previously proposed "footprint reduction" mechanism of alkane monolayer melting. In the hypercritical regime, we show that the layering behavior is consistent with the two-dimensional Ising model and determine the critical temperatures for layers n = 2-5.
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Affiliation(s)
- Frank Kruchten
- Technische Physik, Universität des Saarlandes, D 66041 Saarbrücken, Germany, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago 22, Chile
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Daoulas KC, Harmandaris VA, Mavrantzas VG. Detailed Atomistic Simulation of a Polymer Melt/Solid Interface: Structure, Density, and Conformation of a Thin Film of Polyethylene Melt Adsorbed on Graphite. Macromolecules 2005. [DOI: 10.1021/ma050176r] [Citation(s) in RCA: 171] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Kostas Ch. Daoulas
- Department of Chemical Engineering, University of Patras, GR 26504, Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH-ICE/HT), GR 26504, Patras, Greece
| | - Vagelis A. Harmandaris
- Department of Chemical Engineering, University of Patras, GR 26504, Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH-ICE/HT), GR 26504, Patras, Greece
| | - Vlasis G. Mavrantzas
- Department of Chemical Engineering, University of Patras, GR 26504, Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH-ICE/HT), GR 26504, Patras, Greece
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Tay K, Bresme F. Computer simulations of two dimensional gold nanoparticle arrays: the influence of core geometry. MOLECULAR SIMULATION 2005. [DOI: 10.1080/08927020500035879] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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Samoilov VN, Sivebaek IM, Persson BNJ. The effect of surface roughness on the adhesion of solid surfaces for systems with and without liquid lubricant. J Chem Phys 2004; 121:9639-47. [PMID: 15538886 DOI: 10.1063/1.1806814] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We present molecular dynamics results for the interaction between two solid elastic walls during pull-off for systems with and without octane (C(8)H(18)) lubricant. We used two types of substrate--flat and corrugated--and varied the lubricant coverage from approximately 1/8 to approximately 4 ML (monolayers) of octane. For the flat substrate without lubricant the maximum adhesion was found to be approximately three times larger than for the system with the corrugated substrate. As a function of the octane coverage (for the corrugated substrate) the pull-off force first increases as the coverage increases from 0 to approximately 1 ML, and then decreases as the coverage is increased beyond monolayer coverage. It is shown that at low octane coverage, the octane molecules located in the substrate corrugation wells during squeezing are pulled out of the wells during pull-off, forming a network of nanocapillary bridges around the substrate nanoasperities, thus increasing the adhesion between two surfaces. For greater lubricant coverages a single capillary bridge is formed. The adhesion force saturates for lubricant coverages greater than 3 ML. For the flat substrate, during pull-off we observe discontinuous, thermally activated changes in the number n of lubricant layers (n-1-->n layering transitions), whereas for the corrugated substrate these transitions are "averaged" by the substrate surface roughness.
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Affiliation(s)
- V N Samoilov
- Institut für Festkörperforschung, FZ-Jülich, 52425 Jülich, Germany
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Tavana H, Lam CNC, Grundke K, Friedel P, Kwok DY, Hair ML, Neumann AW. Contact angle measurements with liquids consisting of bulky molecules. J Colloid Interface Sci 2004; 279:493-502. [PMID: 15464816 DOI: 10.1016/j.jcis.2004.06.090] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2003] [Accepted: 06/30/2004] [Indexed: 10/26/2022]
Abstract
Well-measured contact angles with different solid-liquid systems fall approximately on smooth patterns when plotted versus liquid surface tension. However, there are deviations of 1 degrees -3 degrees , which are outside the error limits. It is the purpose of this paper to elucidate the reasons for such deviations. Two types of liquids were selected for advancing contact angle measurements on Teflon AF 1600 coated surfaces: a series of n-alkanes ranging from n-hexane to n-hexadecane and five liquids consisting of bulky molecules, octamethylcyclotetrasiloxane (OMCTS), methyl salicylate, tetralin, cis-decalin, and octamethyltrisiloxane (OMTS). It was found that contact angles of the liquids with bulky molecules fall on a perfectly smooth curve corresponding to a solid surface tension of 13.64 +/- 0.1 mJ/m2. However, contact angles of n-alkanes deviated from this curve by up to 3 degrees in a complicated fashion. The observed trend suggests that more than one mechanism is responsible for the deviations. Substrate-induced rearrangement of liquid molecules in the close vicinity of the surface in the case of long-chain n-alkanes and adsorption of vapor onto the solid surface in the case of short-chain n-alkanes are the most likely explanations. The results suggest that liquids with bulky molecules appear to be suitable for contact angle measurements to characterize energetics of polymeric surfaces.
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Affiliation(s)
- H Tavana
- Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, ON M5S 3G8, Canada
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Lal M, Plummer M, Richmond NJ, Smith W. Solvation of Metal Nanoparticles in a Subcritical — Supercritical Fluid: A Computer Simulation Study. J Phys Chem B 2004. [DOI: 10.1021/jp036776e] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Moti Lal
- Centre for Nanoscale Science, The Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, U.K., and Computational Science & Engineering Department, CCLRC Daresbury Laboratory, Daresbury, Cheshire WA4 4AD, U.K
| | - Martin Plummer
- Centre for Nanoscale Science, The Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, U.K., and Computational Science & Engineering Department, CCLRC Daresbury Laboratory, Daresbury, Cheshire WA4 4AD, U.K
| | - Nicola J. Richmond
- Centre for Nanoscale Science, The Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, U.K., and Computational Science & Engineering Department, CCLRC Daresbury Laboratory, Daresbury, Cheshire WA4 4AD, U.K
| | - William Smith
- Centre for Nanoscale Science, The Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, U.K., and Computational Science & Engineering Department, CCLRC Daresbury Laboratory, Daresbury, Cheshire WA4 4AD, U.K
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Rosenbaum AW, Freedman MA, Darling SB, Popova I, Sibener SJ. Surface vibrations in alkanethiol self-assembled monolayers of varying chain length. J Chem Phys 2004; 120:3880-6. [PMID: 15268554 DOI: 10.1063/1.1643353] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The effect of chain length on the low-energy vibrations of alkanethiol striped phase self-assembled monolayers on Au(111) was studied. We have examined the low-energy vibrational structure of well-ordered, low-density 1-decanethiol (C10), 1-octanethiol (C8), and 1-hexanethiol (C6) to further understand the interaction between adsorbate and substrate. Dispersionless Einstein mode phonons, polarized perpendicularly to the surface, were observed for the striped phases of C10, C8, and C6 at 8.0, 7.3, and 7.3 meV, respectively. An overtone at 12.3 meV was also observed for C6/Au(111). These results, in concert with molecular dynamics simulations, indicate that the forces between the adsorbate and substrate can be described using simple van der Waals forces between the hydrocarbon chains and the Au substrate with the sulfur chemisorbed in the threefold hollow site.
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Affiliation(s)
- A W Rosenbaum
- The James Franck Institute, The University of Chicago, Illinois 60637, USA
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Hansen FY, Criswell L, Fuhrmann D, Herwig KW, Diama A, Dimeo RM, Neumann DA, Volkmann UG, Taub H. Intramolecular diffusive motion in alkane monolayers studied by high-resolution quasielastic neutron scattering and molecular dynamics simulations. PHYSICAL REVIEW LETTERS 2004; 92:046103. [PMID: 14995389 DOI: 10.1103/physrevlett.92.046103] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/06/2003] [Indexed: 05/24/2023]
Abstract
Molecular dynamics simulations of a tetracosane (n-C24H50) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of approximately 0.1-4 ns. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.
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Affiliation(s)
- F Y Hansen
- Department of Chemistry, Technical University of Denmark, IK 207 DTU, DK-2800 Lyngby, Denmark
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Abstract
We present molecular-dynamics results for the squeezing of octane (C8H18) between two approaching solid elastic walls with different wetting properties. The interaction energy between the octane bead units and the solid walls is varied from a very small value (1 meV), corresponding to a nonwetting surface with a very large contact angle (nearly 180 degrees), to a high value (18.6 meV) corresponding to complete wetting. When at least one of the solid walls is wetted by octane we observe well defined molecular layers develop in the lubricant film when the thickness of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers (n-->n-1 layering transitions). With increasing interaction energy between the octane bead units and the solid walls, the transitions from n to n-1 layers occur at higher average pressure. This results from the increasing activation barrier to nucleate the squeeze-out with increasing lubricant-wall binding energy (per unit surface area) in the contact zone. Thus, strongly wetting lubricant fluids are better boundary lubricants than the less wetting ones, and this should result in less wear. We analyze in detail the effect of capillary bridge formation (in the wetting case) and droplets formation (in the nonwetting case) on the forces exerted by the lubricant on the walls. For the latter case small liquid droplets may be trapped at the interface, resulting in a repulsive force between the walls during squeezing, until the solid walls come into direct contact, where the wall-wall interaction may be initially attractive. This effect is made use of in some practical applications, and we give one illustration involving conditioners for hair care application.
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Affiliation(s)
- V N Samoilov
- IFF, FZ-Julich, 52425 Julich, GermanyPhysics Faculty, Moscow State University, 117234 Moscow, Russia.
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Abstract
The energy barrier for sliding of octane on Cu(111) is estimated from an experimental datum for the Brillouin-zone-center gap for translation of a monolayer solid of the octane.
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Affiliation(s)
- L W Bruch
- Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA
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