Publications
43 results found
Gattinoni C, Ewen JP, Dini D, 2018, Adsorption of Surfactants on α-Fe<sub>2</sub>O<sub>3</sub>(0001): A Density Functional Theory Study, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 20817-20826, ISSN: 1932-7447
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- Citations: 29
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
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.
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- Citations: 29
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
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- Citations: 46
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
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.
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- Citations: 55
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
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- Citations: 75
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
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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- Citations: 99
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