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Zhang J, Ueda M, Campen S, Spikes Het al., 2021, Boundary friction of ZDDP tribofilms, Tribology Letters, Vol: 69, Pages: 1-17, ISSN: 1023-8883

The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundar

Journal article

Zhang J, Ueda M, Campen S, Spikes Het al., 2021, Boundary friction of ZDDP tribofilms (vol 69, 8, 2021), Tribology Letters, Vol: 69, Pages: 1-1, ISSN: 1023-8883

Journal article

Campen S, Moorhouse SJ, Wong JSS, 2020, Mechanism of an asphaltene inhibitor in different depositing environments: Influence of colloid stability, Journal of Petroleum Science and Engineering, Vol: 184, ISSN: 0920-4105

Additives are used to reduce unwanted carbonaceous deposits of asphaltenes on surfaces during petroleum production from natural oil and gas reservoirs. The working mechanism of formulated additive packages can be multifaceted. Additives may be effective in the bulk fluid by preventing asphaltenes aggregation, as well as at the surface by preventing asphaltenes adhesion. In this paper, we investigate the numerous different mechanisms by which an asphaltene inhibitor can interfere with the formation of carbonaceous deposits using a combination of techniques including dynamic light scattering to determine particle size distribution, quartz crystal microbalance with dissipation monitoring to examine deposition behaviour and atomic force microscopy to probe deposit morphology. The tested inhibitor prevents deposition of asphaltenes in toluene, where asphaltenes exist as a stable colloidal dispersion of nanoaggregates, by forming barrier-type films that inhibit asphaltenes adhesion and displacing adsorbed thin films of asphaltenes. However, inhibitor performance in heptane-toluene, where asphaltenes are destabilised, depends on the degree of destabilisation. At low heptane volume fraction, inhibitor slows the rate of deposition and deposition rate decreases with increasing inhibitor concentration. However, at high heptane volume fraction, inhibitor can increase the deposition rate, particularly when used in high concentration. At high heptane volume fraction, inhibitor addition alters the morphology of the deposit from that consisting of large flocculent aggregates to that consisting of smaller, submicrometer aggregates. This is consistent with the finding that inhibitor acts as an anti-agglomerant and prevents the formation of large aggregates in the bulk liquid. This paper shows that the impact of inhibitor addition depends on the environmental conditions encountered and the degree of destabilisation of the asphaltenes. Where inhibitor addition alters the nature of depo

Journal article

Campen SM, Moorhouse SJ, Wong JSS, 2019, Effect of aging on the removal of asphaltene deposits with aromatic solvent, Langmuir, Vol: 35, Pages: 11995-12008, ISSN: 0743-7463

Surface-deposition of destabilised colloidal particles of asphaltenes poses a serious and costly problem during petroleum production. Remediation of asphaltene-fouled well-bore and surface facilities is often undertaken by flowing aromatic solvent to remove deposited films. However, little is known about the properties of deposited asphaltene films during their removal by solvent-rinsing. Here, we carry out quartz crystal microbalance with dissipation monitoring (QCM-D) experiments to investigate surface-deposition of destabilized colloidal particles of asphaltenes and their subsequent removal by solvent-rinsing. It is shown that the properties of deposited films during solvent removal depend on the history of the deposit. Newly formed deposit films are removed immediately without significant change in their mechanical properties during removal. However, deposits that remain on the surface for an extended time in a poor solvent (a low-asphaltene solubility solvent), “aged deposits”, are more difficult to remove and exhibit increased dissipation during the removal period, indicating that they swell and are softer. Liquid-cell atomic force microscopy (AFM) confirms that aged deposits swell when the quality of the solvent is subsequently improved by exchanging for a high-asphaltene solubility solvent. Deposit swelling is accompanied by a change in film morphology, from particulate to continuous. Stubborn deposits of aged asphaltene films, which remain after solvent-rinsing, may be partly removed by flowing dissolved asphaltenes in good solvent. Hence, reinjection of asphaltenes during remediation can aid deposit removal.

Journal article

Campen S, Smith B, Wong J, 2018, Deposition of asphaltene from destabilized dispersions in heptane-toluene, Energy and Fuels, Vol: 32, Pages: 9159-9171, ISSN: 0887-0624

Deposition of carbonaceous materials, such as asphaltene, is a major problem in petroleum production. During production, changing environmental conditions destabilize asphaltene, resulting in dispersions that are out of equilibrium, where asphaltene is aggregating or flocculating. Key to developing the most effective strategies for tackling this problem is a fundamental understanding of asphaltene deposition behavior. A quartz crystal microbalance with dissipation monitoring (QCM-D) is used to study asphaltene deposition from destabilized dispersions generated by in-line mixing of asphaltene in toluene (a solvent) with n-heptane (a precipitant). The effects of heptane:toluene ratio and destabilization time are investigated. At high heptane:toluene ratio, the rate of asphaltene aggregation is faster, and large flocs form by the time the flowing liquid reaches the QCM cell. In this case, the rate of deposition decreases with deposition time. At low heptane:toluene ratio, the rate of asphaltene aggregation is slower; hence large flocs do not form before the flowing liquid reaches the QCM cell, and deposition of smaller aggregates occurs. Here, the deposition rate is constant with time. The deposited mass is greatest before the formation of large flocs and at short destabilization times, where the particle distribution is furthest from equilibrium. Destabilized small particles existing immediately after a destabilization event pose a greater deposition problem than the flocs that subsequently form. This may be a contributing factor in the existence of deposition “hotspots” at certain locations in the production pipeline. Pushing destabilized dispersions to their new equilibrium distributions as quickly as possible may be a preventative strategy to combat deposition. The dissipation–frequency relationship monitored by QCM-D is sensitive to the nature of deposited asphaltene films and may be used as a diagnostic tool.

Journal article

Campen S, di Mare L, Smith B, Wong Jet al., 2017, Determining the kinetics of asphaltene adsorption from toluene; a new reaction-diffusion model, Energy and Fuels, Vol: 31, Pages: 9101-9116, ISSN: 0887-0624

Fouling by asphaltene, which constitutes the densest, most polar fraction of crude oil, poses a serious problem for the oil production industry. In order to obtain a fundamental understanding of asphaltene deposition, it is necessary to determine both the thermodynamics and kinetics that govern this process. In recent years, there have been numerous studies of the kinetics of asphaltene adsorption; however, a consensus on the model that best describes asphaltene adsorption remains elusive. In this work the adsorption of asphaltene from solution in toluene onto a gold surface is investigated using a quartz crystal microbalance inside a flow cell. The kinetics of adsorption depends on the state of the asphaltene in solution, and the adsorption behavior is altered with long-time aging of asphaltene solutions. A model is developed that links the kinetics of asphaltene adsorption to the bulk solution properties in terms of coexisting monomer and multimer states. A large portion of deposited asphaltene is effectively irreversibly bound and not easily removed by rinsing with toluene. The model suggests that asphaltene–asphaltene interactions play an important role in the formation of irreversibly bound deposits, which could lead to fouling problems.

Journal article

Campen S, Green JH, Lamb GD, Spikes HAet al., 2015, In situ study of model organic friction modifiers using liquid cell AFM: self-assembly of octadecylamine, Tribology Letters, Vol: 58, ISSN: 1573-2711

Liquid cell AFM has been applied to study in situ the formation and properties of self-assembled films formed on mica surfaces by octadecylamine from alkane solution. Mica surfaces immersed in hexadecane or dodecane at room temperature show no identifiable surface films. However, when octadecylamine solution is injected into the cell, a boundary film forms almost immediately. This film takes the form of irregular islands of mean diameter approximately 1–3 µm and thickness typically 1.5 nm when measured in contact mode. These islands are believed to correspond to patches of vertically oriented but tilted octadecylamine or ammonium salt held together primarily by van der Waals forces between adjacent alkyl chains. These films are quite labile in that during scanning of the tip in both tapping mode and contact mode changes to the shape of the islands take place, including consolidation of the island density in the scanned region and depletion from around this area. In situ experiments in which the temperature of the cell is varied over time show that the initially formed islands disappear at a temperature of ca. 35 °C but are reformed when the cell is re-cooled. Similar tests on samples that remain immersed in solution for extended periods show more stable films, with islands being lost only above about 50–60 °C. The work shows that liquid cell AFM has great promise for studying the formation and properties of the boundary films formed by organic friction modifiers.

Journal article

Campen S, Green JH, Lamb GD, Spikes HAet al., 2015, In situ study of model organic friction modifiers using liquid cell AFM; saturated and mono-unsaturated carboxylic acids, Tribology Letters, Vol: 57, Pages: 1-20, ISSN: 1023-8883

Fatty acids and their derivatives have been used as model organic friction modifiers for almost a century, but there is still debate as to the nature of the boundary films that they form on rubbed surfaces. In this study, in situ liquid cell atomic force microscopy (AFM) is used to monitor the self-assembly of boundary films from solutions of fatty acids in alkanes on to mica surfaces. Because the mica surfaces are wholly immersed in solution, it is possible to study directly changes in the morphology and friction of these films over time and during heating and cooling. It has been found that stearic acid and elaidic acid, which are able to adopt linear molecular configurations, form irregular islands on mica that are tens to hundreds of microns in diameter and typically 1.6 nm thick, corresponding to domains of tilted single monolayers. At a relatively high concentration of 0.01 M, stearic acid in hexadecane forms an almost complete monolayer, but at lower concentrations, in dodecane solution and for elaidic acid solutions, these films remain incomplete after prolonged immersion of more than a day. The films formed by fatty acids on mica are displaced by repeated scanning in contact mode AFM but can be imaged without damage using tapping mode AFM. Rubbed quartz surfaces from a sliding ball-on-disc test were also scanned ex situ using AFM, and these showed that stearic acid forms similar monolayer island films on quartz in macro-scale friction experiments as are found on mica. Oleic acid solutions behave quite differently from stearic acid and elaidic acid, forming irregular globular films on both mica and rubbed quartz surfaces. This is believed to be because its cis-double bond geometry means that, unlike its trans-isomer elaidic acid or saturated stearic acid, it is unable to adopt a linear molecular configuration and so is less able to form close-packed monolayers.

Journal article

Campen S, Green J, Lamb G, Atkinson D, Spikes Het al., 2012, On the Increase in Boundary Friction with Sliding Speed, TRIBOLOGY LETTERS, Vol: 48, Pages: 237-248, ISSN: 1023-8883

Journal article

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