24 results found
Ewen J, Spikes H, Dini D, 2021, Contributions of Molecular Dynamics Simulations to Elastohydrodynamic Lubrication, Tribology Letters, ISSN: 1023-8883
Ayestaran Latorre C, Ewen J, Dini D, et al., 2021, Ab initio insights into the interaction mechanisms between boron, nitrogen and oxygen doped diamond surfaces and water molecules, Carbon, Vol: 171, Pages: 575-584, ISSN: 0008-6223
Diamond and diamond-like carbon coatings are used in many applications ranging from biomedicine to tribology. A wide range of dopants have been tested to modify the hydrophilicity of these surfaces, since this is central to their biocompatibility and tribological performance in aqueous environments. Despite the large number of experimental investigations, an atomistic understanding of the effects of different dopants on carbon film hydrophilicity is still lacking. In this study, we employ ab initio calculations to elucidate the effects of B, N, and O dopants in several mechanisms that could modify interactions with water molecules and thus hydrophilicity. These include the adsorption of intact water molecules on the surfaces, minimum energy pathways for water dissociation, and subsequent interactions of hydrogenated and hydroxylated surfaces with water molecules. We find that all of the dopants considered enhance hydrophilicity, but they do so through different means. Most notably, B dopants can spontaneously chemisorb intact water molecules and increase its interactions in H-bond networks.
Kondratyuk N, Pisarev V, Ewen J, 2020, Probing the high-pressure viscosity of hydrocarbon mixtures using molecular dynamics simulations, Journal of Chemical Physics, Vol: 153, ISSN: 0021-9606
Computational predictions of the high-pressure viscosity of hydrocarbon mixtures could help to accelerate the development of fuels and lubricants with improved performance. In this study, we use molecular dynamics simulations to study the viscosity and density of methylcyclohexane, 1-methylnaphthalene, and their binary mixtures at 323 K and pressures of up to 500 MPa. The simulation results are in excellent agreement with previous experiments available up to 100 MPa for both pure compounds (200 MPa for 1-methylnaphthalene) and the binary mixtures. For 1-methylnaphthalene, the viscosity initially increases slower-than-exponential with pressure before it reaches an inflection point and then increases faster-than-exponential. The inflection point (300 MPa) occurs at a pressure slightly below the one at which 1-methylnaphthalene is expected to enter the supercooled phase (400 MPa). For methylcyclohexane, the increase in viscosity with pressure is slower-than-exponential over the entire pressure range studied. The binary mixtures show intermediate pressure–viscosity responses between the two pure cases. The applicability of equations commonly used to describe the pressure dependence of viscosity, as well as the viscosity of binary mixtures, is evaluated against the computational predictions.
We present a two-part hands-on science outreach demonstration utilizing composite hydrogels to produce realistic models of the human brain. The blends of poly(vinyl alcohol) and Phytagel closely match the mechanical properties of real brain tissue under conditions representative of surgical operations. The composite hydrogel is simple to prepare, biocompatible, and nontoxic, and the required materials are widely available and inexpensive. The first part of the demonstration gives participants the opportunity to feel how soft and deformable our brains are. The second part allows students to perform a mock brain surgery on a simulated tumor. The demonstration tools are suitable for public engagement activities as well as for various student training groups. The activities encompass concepts in polymer chemistry, materials science, and biology.
Ewen JP, Ayestarán Latorre C, Gattinoni C, et al., 2020, Substituent effects on the thermal decomposition of phosphate esters on ferrous surfaces, The Journal of Physical Chemistry C, Vol: 124, Pages: 9852-9865, ISSN: 1932-7447
Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapour phase lubricants and antiwear additives. An atomic-level understanding of phosphate ester tribofilm formation mechanisms is required to improve their tribological performance. A process of particular interest is the thermal decomposition of phosphate esters on steel surfaces, since this initiates polyphosphate film formation. In this study, reactive force field (ReaxFF) molecular dynamics (MD) simulations are used to study the thermal decomposition of phosphate esters with different substituents on several ferrous surfaces. The ReaxFF parameterisation was validated for a representative system using density functional theory (DFT) calculations. During the MD simulations on Fe3O4(001) and α-Fe(110), chemisorption interactions between the phosphate esters and the surfaces occur even at room temperature, and the number of molecule-surface bonds increases as the temperature increases from 300 to 1000 K. Conversely, on hydroxylated, amorphous Fe3O4, most of the molecules are physisorbed and some desorption occurs at high temperature. Thermal decomposition rates were much higher on Fe3O4(001) and particularly α-Fe(110) compared to hydroxylated, amorphous Fe3O4. This suggests that water passivates ferrous surfaces and inhibits phosphate ester chemisorption, decomposition, and ultimately polyphosphate film formation. For the alkyl phosphates, thermal decomposition proceeds mainly through C-O and C-H cleavage on Fe3O4(001). Aryl phosphates show much higher thermal stability, and decomposition on Fe3O4(001) only occurs through P-O and C-H cleavage, which require very high temperature. The onset temperature for C-O cleavage on Fe3O4(001) increases as: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary branched alkyl < aryl. This order is consistent with experimental observations for the thermal stability of antiwear addi
Zhang J, Ewen JP, Ueda M, et al., 2020, Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions, ACS Applied Materials & Interfaces, Vol: 12, Pages: 6662-6676, ISSN: 1944-8244
Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerisation.
Ewen J, Ramos Fernandez E, Smith E, et al., 2020, Nonequilibrium Molecular Dynamics Simulations of Tribological Systems, Modeling and Simulation of Tribological Problems in Technology, Editors: Paggi, Hills, Publisher: Springer Nature, Pages: 95-130, ISBN: 978-3-030-20376-4
Ayestarán Latorre C, Ewen JP, Gattinoni C, et al., 2019, Simulating surfactant-iron oxide interfaces: from density functional theory to molecular dynamics, The Journal of Physical Chemistry B, Vol: 123, Pages: 6870-6881, ISSN: 1520-6106
Understanding the behaviour of surfactant molecules on iron oxide surfaces is important for many industrial applications. Molecular dynamics (MD) simulations of such systems have been limited by the absence of a force-field (FF) which accurately describes the molecule-surface interactions. In this study, interaction energies from density functional theory (DFT) + U calculations with a van der Waals functional are used to parameterize a classical FF for MD simulations of amide surfactants on iron oxide surfaces. The Original FF, which was derived using mixing rules and surface Lennard-Jones (LJ) parameters developed for nonpolar molecules, were shown to significantly underestimate the adsorption energy and overestimate the equilibrium adsorption distance compared to DFT. Conversely, the Optimized FF showed excellent agreement with the interaction energies obtained from DFT calculations for a wide range of surface coverages and molecular conformations near to and adsorbed on α-Fe2O3(0001). This was facilitated through the use of a Morse potential for strong chemisorption interactions, modified LJ parameters for weaker physisorption interactions, and adjusted partial charges for the electrostatic interactions. The Original FF and Optimized FF were compared in classical nonequilibrium molecular dynamics (NEMD) simulations of amide molecules confined between iron oxide surfaces. When the Optimized FF was employed, the amide molecules were pulled closer to the surface and the orientation of the headgroups was more similar to that observed in the DFT calculations compared to the Original FF. The Optimized FF proposed here facilitates classical MD simulations of anhydrous amide-iron oxide interfaces in which the interactions are representative of accurate DFT calculations.
Restrepo SE, van Eijk MCP, Ewen JP, 2019, Behaviour of n-alkanes confined between iron oxide surfaces at high pressure and shear rate: A nonequilibrium molecular dynamics study, Tribology International, Vol: 137, Pages: 420-432, ISSN: 0301-679X
The behaviour of n-alkanes confined and sheared between iron oxide surfaces has been studied using nonequilibrium molecular dynamics simulations. The molecular extension, orientation, film structure, flow, and friction have been investigated for a range of n-alkane chain lengths under conditions representative of the elastohydrodynamic lubrication regime. At high pressure, the molecules show strong layering and long-range order, suggesting solid-like films. Conversely, high shear rates result in less elongated, layered, and ordered molecules; indicating more liquid-like films. Although Couette flow is usually observed for short n-alkanes, the flow is often non-linear for long n-alkanes. The friction coefficient increases logarithmically with shear rate, but the slope decreases with increasing pressure such that it becomes insensitive to shear rate for long n-alkanes.
Ewen J, Gao H, Mueser M, et al., 2019, Shear heating, flow, and friction of confined molecular fluids at high pressure, Physical Chemistry Chemical Physics, Vol: 21, Pages: 5813-5823, ISSN: 1463-9076
Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.
Ewen J, Heyes D, Dini D, 2018, Advances in nonequilibrium molecular dynamics simulations of lubricants and additives, Friction, Vol: 6, Pages: 349-386, ISSN: 2223-7704
Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.
Gattinoni C, Ewen JP, Dini D, 2018, Adsorption of Surfactants on alpha-Fe2O3(0001): A Density Functional Theory Study, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 20817-20826, ISSN: 1932-7447
Clark RH, Ewen JP, Heins RJ, et al., 2018, Fuel composition, EP3337877A1
Clark RH, Ewen JP, Wardle RWM, et al., 2018, High power fuel compositions, EP3022278B1
Ewen JP, Kannam SK, Todd BD, et al., 2018, Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces, Langmuir, Vol: 34, Pages: 3864-3873, ISSN: 0743-7463
© 2018 American Chemical Society. The slip and friction behavior of n-hexadecane, confined between organic friction modifier surfactant films adsorbed on hematite surfaces, has been studied using nonequilibrium molecular dynamics simulations. The influence of the surfactant type and coverage, as well as the applied shear rate and pressure, has been investigated. A measurable slip length is only observed for surfactant films with a high surface coverage, which provide smooth interfaces between well-defined surfactant and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes toward a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behavior is well-described using previously proposed slip models. This study provides a more detailed understanding of the slip of alkanes on surfactant monolayers. It also suggests that high coverage surfactant films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant.
Ewen JP, Kannam S, Todd B, et al., 2018, Slip of hexadecane on organic friction modifier monolayers, APS March Meeting 2018, Publisher: American Physical Society, ISSN: 0003-0503
Ewen J, Echeverri Restrepo S, 2017, LAMMPS_Builder
This software is suitable as a starting point for performing confined nonequilibrium molecular dynamics (NEMD) simulations of organic friction modifier (OFM) films adsorbed to iron surfaces, separated by a layer of n-alkane molecules:This software generates a LAMMPS datafile and basic input file containing:* Two a-Fe or a-Fe2O3 slabs with/without random nanoscale roughness* Two OFM monolayers above/below bottom/top slabs* A central region of n-alkaneshttps://doi.org/10.5281/zenodo.1043868
Ewen J, 2017, Molecular dynamics simulations of lubricants and additives
Ewen JP, Gattinoni C, Zhang J, et al., 2017, On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 19, Pages: 17883-17894, ISSN: 1463-9076
Ewen J, Gattinoni C, Spikes H, et al., 2017, Nonequilibrium molecular dynamics simulations of organic friction modifiers, 253rd National Meeting of the American-Chemical-Society (ACS) on Advanced Materials, Technologies, Systems, and Processes, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Ewen JP, Echeverri Restrepo S, Morgan N, et al., 2017, Nonequilibrium molecular dynamics simulations of stearic acid adsorbed on iron surfaces with nanoscale roughness, Tribology International, Vol: 107, Pages: 264-273, ISSN: 0301-679X
© 2016 The Authors Nonequilibrium molecular dynamics (NEMD) simulations have been used to examine the structure and friction of stearic acid films adsorbed on iron surfaces with nanoscale roughness. The effect of pressure, stearic acid coverage, and level of surface roughness were investigated. The direct contact of asperities was prevented under all of the conditions simulated due to strong adsorption, which prevented squeeze-out. An increased coverage generally resulted in lower lateral (friction) forces due to reductions in both the friction coefficient and Derjaguin offset. Rougher surfaces led to more liquidlike, disordered films; however, the friction coefficient and Derjaguin offset were only slightly increased. This suggests that stearic acid films are almost as effective on contact surfaces with nanoscale roughness as those which are atomically-smooth.
Ewen JP, Gattinoni C, Thakkar FM, et al., 2016, Nonequilibrium Molecular Dynamics Investigation of the Reduction in Friction and Wear by Carbon Nanoparticles Between Iron Surfaces, Tribology Letters, Vol: 63, ISSN: 1023-8883
Ewen JP, Gattinoni C, Thakkar FM, et al., 2016, A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants, MATERIALS, Vol: 9, ISSN: 1996-1944
Ewen JP, Gattinoni C, Morgan N, et al., 2016, Nonequilibrium molecular dynamics simulations of organic friction modifiers adsorbed on iron oxide surfaces, Langmuir, Vol: 32, Pages: 4450-4463, ISSN: 0743-7463
© 2016 American Chemical Society. For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.
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