46 results found
Yang S, Zhang D, Wong J, et al., 2021, Interactions between Zinc dialkyldithiophosphates (ZDDP) and an oil-soluble ionic liquid additive, Tribology International, ISSN: 0301-679X
Garcia Gonzalez C, Ueda M, Spikes H, et al., 2021, Temperature dependence of Molybdenum dialkyl dithiocarbamate (MoDTC) tribofilms via time-resolved Raman spectroscopy, Scientific Reports, Vol: 11, Pages: 3621-3621, ISSN: 2045-2322
Jiang S, Yuan C, Wong J, 2020, Effectiveness of glycerol-monooleate in high-performance polymer tribo-systems, Tribology International, ISSN: 0301-679X
High performance polymers possessing superior mechanical properties may replace metal components and improve machine efficiency. Successful replacement, however, relies on the compatibility of these polymers with current engineering systems, including lubricants and their additives. This study examines the compatibility of two high performance polymers, polyetheretherketone (PEEK) and polyamide-imide (PAI), with glycerol monooleate (GMO), an organic friction modifier (OFM) commonly used in steel-steel rubbing contacts. Friction tests were conducted with a ball-on-disc geometry in reciprocating motion at 100 °C in polyalphaolefin (PAO) base oil. GMO reduces friction in polymer-steel and polymer-polymer contacts. When steel is involved, the use of GMO and oleic acid (OA) give similar friction coefficients. Since OA is believed to be a hydrolyzed product of GMO in steel-steel contacts, our results show that the interaction of OA with steel controls friction in polymer-steel contacts when GMO is the additive. Results from FTIR and Raman spectroscopies show that steel surfaces contain little to no polymeric materials, nor iron oxides after rubbing against polymers in GMO- and OA-containing PAO. This supports OFM layers are formed on steel surfaces. These OFM layers prevent polymer transfer layer formation and possibly protect steel surfaces from oxidation. Our results show that using OFM that interacts strongly with steel can, contrary to dry friction, eliminate the need of polymeric transfer film on steel for achieving low friction in polymer-steel contacts.
Fry B, Chui MY, Moody G, et al., 2020, Interactions between organic friction modifier additives, Tribology International, Vol: 151, Pages: 1-8, ISSN: 0301-679X
The interactions of different additives in engine oils can create synergistic or antagonistic effects. This paper studies how mixing different organic friction modifier additives affects friction reducing properties of lubricants in the boundary lubrication regime. Amines of different degree of saturation were mixed with either glycerol monooleate (GMO) or oleic acid in hexadecane. The model lubricants thus formed were characterised with Fourier-transform infrared spectroscopy. Friction tests in reciprocating motion using ball-on-disc steel-steel contacts were conducted to examine the tribological performance of these lubricants. Worn surfaces were examined using X-ray photoelectron spectroscopy. Oleic acid and oleylamine, a primary amine. Were found to form a partial ionic liquid, providing synergistic friction reduction. This positive interaction reduces with increasing degree of saturation of the amine. No synergistic effect was observed between GMO and oleylamine,suggesting that GMO does not hydrolyse into a carboxylic acid within a rubbing contact in the presence of amine. Keywords: Boundary Lubrication, Additives, Friction Abbreviations: organic friction modifier (OFM); glycerol monooleate (GMO); ionic liquid (IL); oleylamine (OA); diocylamine (DA); trihexylamine (TA); dimethylhexadecaylamine (DM16); high frequency reciprocating rig (HFRR); X-ray photoelectron spectroscopy (XPS).
Organic friction modifier additives (OFMs) are surfactant molecules added to engine oils to reduce friction in the boundary lubrication regime. They are thought to work by forming an absorbed layer which provides low friction. This paper studied the relationship between the adsorption of OFMs and their friction and wear reducing properties in a rubbing contact formed by a stationary glass ball and a rotating silicon disk under the boundary lubrication regime. The effect of molecular structure was investigated by using OFMs of various tail saturation and head group chemistry. OFM tested were oleic acid, octadecylamine, oleylamine and glycerol monooleate. The thickness of an OFM adsorbed layer in hexadecane, examined in-situ by spectroscopic ellipsometry and quartz crystal microbalance (QCM), depends on the molecular structure and the concentration of the OFM. As expected, saturated, linear chain gives the thickest film. A critical OFM layer thickness of about 0.6 nm is necessary to achieve low initial and maximum friction. The thicker OFM layers are accompanied by narrower wear tracks, which are rougher than the wider, smoother wear tracks formed with thinner OFM layers. The interplay between the thickness of the OFM layer and wear track surface roughness results in all OFM layers having similar steady friction. This shows that the apparent effect of OFM depends on the stage of rubbing test: initially on friction; and then subsequently on surface damage. Despite OFMs and the base oil having similar refractive indices, ellipsometry was found to be a suitable technique for examining the adsorption of OFM additives from an oil based solution, and showed reasonable correlation with QCM results.
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.
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
Fry B, Moody G, Spikes H, et al., 2019, Effect of surface cleaning on performance of organic friction modifiers, Tribology Transactions, Vol: 63, Pages: 305-313, ISSN: 1040-2004
The performance of surface active additives, such as friction modifiers, depends on their interactions with surfaces. Their effectiveness thus hinges upon the surface conditions. In this work, the effect of cleaning methods of test substrates on the friction reduction capabilities of different organic friction modifier (OFM) additives was investigated. 52100 steel discs and balls were the test specimens. They were cleaned in five different ways. The cleaned surfaces were characterised by using ellipsometry and atomic force microscopy. The tribological performance of stearic acid (STA), octadecylamine (ODA) and octadecanol (ODO) on these surfaces were then tested. As-received steel surfaces were covered with contaminants which may impede the formation of OFM surface layer. Cleaning these surfaces with solvents cannot completely removed these contaminants, with residue layers remain. Cleaning with oxygen or argon plasma results in cleaner surfaces as compared to those cleaned by solvents only. The impact of the choice of cleaning methods on friction depends on the strength of the interaction between the OFM and the steel surface, which determines the ability of an OFM to displace surface contaminations. Cleaner surfaces result in lower initial friction for STA and ODA. Steady state friction is also affected, but to a smaller extent. It may be because most containments remained in the wear track are mechanically removed during rubbing.
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.
Puhan D, Wong J, 2019, Properties of Polyetheretherketone (PEEK) transferred materials in a PEEK-steel contact, Tribology International, Vol: 135, Pages: 189-199, ISSN: 0301-679X
Polyetheretherketone (PEEK) is a high performance polymer that can be an alternative to metal for some moving components in unlubricated conditions. During rubbing, PEEK is transferred to the counterface. The formation and properties of PEEK transfer films on steel and sapphire are studied by in-situ observations of PEEK wear process, contact temperatures and triboemission, as well as FTIR and Raman spectroscopies ex situ. Our results suggest that frictional heating alone may not be sufficient to generate PEEK degradation observed in the transfer materials. Triboplasma observed during rubbing, together with the mechanical shear, may promote generations of radicals and degradation of PEEK, which subsequently influence the properties of PEEK transfer film and performance of polymer-metal tribopair.
Puhan D, Nevshupa R, Wong J, et al., 2019, Transient aspects of plasma luminescence induced by triboelectrification of polymers, Tribology International, Vol: 130, Pages: 366-377, ISSN: 0301-679X
Transient electric gas discharges that occur around sliding interfaces during contact electrification of polymers were studied at millisecond timescales and with micrometre resolution. Deduced vibrational temperatures indicate cold plasma resulting from positive corona discharge. At millisecond timescales, previously unseen rapid discharge events are observed, and modelling suggests that these result from streamer development, triggered by electron emission from the polymer surface. Those which occur over a period of several seconds are shown to be caused by competition between charge generation and the formation of polymer films. The findings explain the interplay between charging and plasma generation and their dependence on wear processes.
Jeffreys S, di Mare L, Liu X, et al., 2019, Elastohydrodynamic lubricant flow with nanoparticle tracking, RSC Advances, Vol: 9, Pages: 1441-1450, ISSN: 2046-2069
Lubricants operating in elastohydrodynamic (EHD) contacts exhibit local variations in rheological properties when the contact pressure rises. Direct evidence of this behaviour has only been obtained by examining through-thickness velocity profiles U(z) of lubricants in a contact using luminescence-based imaging velocimetry. In the present study, nanoparticles (NPs) are added to polybutene (PB) as tracers to investigate the effect of pressure on the flow of PB in an EHD contact. By tracking NPs in the contact, particle velocity distributions f(U) under various pressures are obtained and found to be pressure dependent. Results show quantitatively that f(U) and U(z) are correlated and thus confirm that U(z) of PB changes from Couette flow to partial plug flow above a critical pressure. This confirmation highlights the complexity of lubricant rheology in a high pressure contact.
Jean-Fulcrand A, Masen MA, Bremner T, et al., 2019, Effect of temperature on tribological performance of polyetheretherketone-polybenzimidazole blend, Tribology International, Vol: 129, Pages: 5-15, ISSN: 0301-679X
© 2018 The Authors Polyetheretherketone (PEEK) is one of the most commonly used High Performance Polymers (HPP) although its high temperature performance is poor. In this study, polybenzimidazole (PBI), a HPP with one of the highest glass transition temperatures currently available, is blended to PEEK to form a 50:50 blend (TU60). Tribological performance of the blend (TU60) was investigated by rubbing it against steel at temperatures up to 280 °C. Results obtained are compared to those from neat PEEK and neat PBI. All three polymers were thermally stable during the duration of tests. However chemical analyses on polymeric transfer layers on steel surfaces and polymer debris suggest polymer degradation. The degradation observed is shear-assisted, possibly promoted by shear heating. Indeed the estimated interfacial temperature based on Jaeger model was above the melting point of PEEK in some cases. TU60 outperforms PEEK in all test conditions and PBI at 280 °C. TU60 formed transfer layers on steel similar to that of PEEK. When contact temperature is closed to the melting point of PEEK, PEEK in the TU60 creates a low strength transfer layer which acts as an interfacial lubricant. This reduces friction which in turn reduces PBI degradation in TU60 at high temperature. This work provides a strategy for creating interfacial layers to improve polymer tribological performance while maintaining the integrity of the polymer.
Dench J, di Mare L, Morgan N, et al., 2018, Comparing the molecular and global rheology of a fluid under high pressures, Physical Chemistry Chemical Physics, Vol: 20, Pages: 30267-30280, ISSN: 1463-9076
The viscosity of liquids is a strong function of pressure. While viscosity is relatively easy to measure at low pressure, high-pressure rheology presents significant experimental challenges. As a result, rheological models are often used to extrapolate viscosity from low pressure measurements to higher pressures. Techniques to obtain data over a wide range of pressures and shear rates, as well as understanding the validity and limitations of methods to fill the gaps in the available data, are therefore of crucial practical and theoretical importance. This work examines the viscosity of polyalphaolefin (PAO) by combining average global area averaged measurements at high pressure and local molecular viscosity measurements at moderate pressures. Viscosities spanning five orders of magnitude are examined at pressures up to 720 MPa. High pressure results were obtained with friction measurements where the fluid is sheared between two surfaces in a loaded point contact. The local molecular microviscosity at medium and low pressures was measured by applying a technique based on fluorescence anisotropy, which probes the rotational motion of dye molecules in a nanoscale film under shear. Both sets of measurements are taken in the same configuration, an elastohydrodynamic (EHD) contact. This is the first set of quantitative local viscosity measurements that have been verified against both friction and high pressure rheometry measurements. Commonly used rheological models were compared to experimental results. Our work shows that fluorescence anisotropy and friction measurements can be used to determine the viscosity of liquids over a wide range of conditions from a single experimental setup. The results obtained match results from low- and high-pressure rheometry for PAO. The importance of correcting friction data for pressure non-uniformity, temperature and shear thinning is also highlighted.
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.
Yang S, Wong J, Zhou F, 2018, Ionic liquid additives for mixed and elastohydrodynamic lubrication, Tribology Transactions, Vol: 61, Pages: 816-826, ISSN: 1040-2004
Ionic liquids (ILs), both as pure lubricants and lubricant additives, have been demonstrated extensively to exhibit excellent tribological performance in terms of friction and wear reduction in the boundary lubrication (BL) regime. Since engineering contacts experience boundary and mixed, as well as full film lubrication depending on operating conditions, it is crucial to examine if lubrication regimes other BL regime can also benefit from the use of ILs. The objective of this work is to investigate the tribological performance of IL additives in the mixed lubrication (ML) and the elastohydrodynamic lubrication (EHL) regimes. Polyethylene glycol (PEG) was used as the base fluid. ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Friction and film thickness measurements were employed to investigate the effectiveness of IL additives at room temperature, 60°C and 80°C; at various loads and slide-roll ratios (SRRs). The effect of IL additives on the rheological behavior of PEG was also investigated. The EHL film thickness increases with increasing IL concentration. EHL friction is however only mildly affected by IL additives. In the ML regime, IL additives can reduce friction and metal wear as compared to pure PEG in mild conditions. It is conjectured that IL forms sacrificial layers and protects the rubbing surfaces.
In this paper we study the release of cargo from polymeric nano-carriers under shear. Vesicles formed by two star block polymers— A12B6C2 ( ABC ) and A12B6A2 ( ABA )—and one linear block copolymer— A14B6 ( AB ), are investigated using dissipative particle dynamics (DPD) simulations. A - and C -blocks are solvophobic and B -block is solvophilic. The three polymers form vesicles of different structures. The vesicles are subjected to shear both in bulk and between solvophobic walls. In bulk shear, the mechanisms of cargo release are similar for all vesicles, with cargo travelling through vesicle membrane with no preferential release location. When sheared between walls, high cargo release rate is only observed with ABC vesicle after it touches the wall. For ABC vesicle, the critical condition for high cargo release rate is the formation of wall-polymersome interface after which the effect of shear rate in promoting cargo release is secondary. High release rate is achieved by the formation of solvophilic pathway allowing cargo to travel from the vesicle cavity to the vesicle exterior. The results in this paper show that well controlled target cargo release using polymersomes can be achieved with polymers of suitable design and can potentially be very useful for engineering applications. As an example, polymersomes can be used as carriers for surface active friction reducing additives which are only released at rubbing surfaces where the additives are needed most.
Yang S, Wong J, Cai M, et al., 2018, Tribological Properties of In-situ Ionic Liquid Additives for Mixed and Hydrodynamic Lubrication, Mocaxue Xuebao/Tribology, Vol: 38, Pages: 342-348, ISSN: 1004-0595
© 2018, Science Press. All right reserved. This work focused on studying the tribological performance of IL additives in the mixed and hydradynamic lubrication regimes. Polyethylene glycol (PEG-400) was used as the base fluid, ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Mini Traction Machine was used to measure the friction coefficients with entrainment speeds at room temperature, 60 ℃ and 80 ℃ and at various slide-roll ratios for confirming the effect of IL additives on the tribological properties, and the rheological behavior of PEG was also investigated. This work would provide a new research method for further study of the lubrication mechanism of ionic liquids, which is of great significance for guiding the design of new ionic liquid lubricating materials.
Jean-Fulcrand A, Masen M, Bremner T, et al., 2017, High Temperature Tribological Properties of Polybenzimidazole (PBI), Polymer, Vol: 128, Pages: 159-168, ISSN: 0032-3861
Polybenzimidazole (PBI) is a high performance polymer that can potentially replace metal components in some high temperature conditions where lubrication is challenging or impossible. Yet most characterisations so far have been conducted at relatively low temperatures. In this work, the tribological properties of PBI were examined with a steel ball-PBI disc contact at 280 °C under high load and high sliding speed conditions. The dry friction coefficient is relatively low and decreases modestly with increasing applied load. Surface analysis shows that PBI transfer layers are responsible for the low friction observed. In-situ contact temperature measurements were performed to provide for the first time direct links between the morphology and distribution of the transfer layer, and the temperature distribution in the contact. The results show that high pressure and high temperature in heavily loaded contacts promote the removal and the subsequent regeneration of a transfer layer, resulting in a very thin transfer layer on the steel counterface. FeOOH is formed in the contact at high loads, instead of Fe2O3. This may affect the adhesion between PBI and the counterface and thus influence the transfer layer formation process. To control PBI wear, contact temperature management will be crucial.
Campen S, di Mare L, Smith B, et 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.
Guo YY, di Mare L, Li RKY, et al., 2017, Structure of Amphiphilic Terpolymer Raspberry Vesicles, Polymers, Vol: 9, ISSN: 2073-4360
Terpolymer raspberry vesicles contain domains of different chemical affinities. They are potential candidates as multi-compartment cargo carriers. Their efficacy depends on their stability and load capacity. Using a model star terpolymer system in an aqueous solution, a dissipative particle dynamic (DPD) simulation is employed to investigate how equilibrium aggregate structures are affected by polymer concentration and pairwise interaction energy in a solution. It is shown that a critical mass of polymer is necessary for vesicle formation. The free energy of the equilibrium aggregates are calculated and the results show that the transition from micelles to vesicles is governed by the interactions between the longest solvophobic block and the solvent. In addition, the ability of vesicles to encapsulate solvent is assessed. It is found that reducing the interaction energy favours solvent encapsulation, although solvent molecules can permeate through the vesicle’s shell when repulsive interactions among monomers are low. Thus, one can optimize the loading capacity and the release rate of the vesicles by turning pairwise interaction energies of the polymer and the solvent. The ability to predict and control these aspects of the vesicles is an essential step towards designing vesicles for specific purposes.
Dench J, Morgan N, Wong J, 2016, Quantitative viscosity mapping using fluorescence lifetime measurements, Tribology Letters, Vol: 65, ISSN: 1573-2711
Lubricant viscosity is a key driver in both the tribological performance and energy efficiency of a lubricated contact. Elastohydrodynamic (EHD) lubrication produces very high pressures and shear rates, conditions hard to replicate using conventional rheometry. In situ rheological measurements within a typical contact are therefore important to investigate how a fluid behaves under such conditions. Molecular rotors provide such an opportunity to extract the local viscosity of a fluid under EHD lubrication. The validity of such an application is shown by comparing local viscosity measurements obtained using molecular rotors and fluorescence lifetime measurements, in a model EHD lubricant, with reference measurements using conventional rheometry techniques. The appropriateness of standard methods used in tribology for high-pressure rheometry (combining friction and film thickness measurements) has been verified when the flow of EHD lubricant is homogeneous and linear. A simple procedure for calibrating the fluorescence lifetime of molecular rotors at elevated pressure for viscosity measurements is proposed.
Laux K, Jean-Fulcrand A, SUE HJ, et al., 2016, The influence of surface properties on sliding contact temperatureand friction for Polyetheretherketone (PEEK), Polymer, Vol: 103, Pages: 397-404, ISSN: 0032-3861
Polyetheretherketone (PEEK) polymers are increasingly used intribological applications. An important aspect of PEEK tribology is thesurface temperature reached during sliding. However, most knowledge offrictional heating in PEEK is based on post-hoc analysis of debris andwear surfaces. In this study, infrared thermography was used to observethe full field temperature map of PEEK during ball-on-disc sliding.Although the measured temperatures were below any thermal transition, theresults matched closely to those predicted by flash temperature models.Additionally, friction studies were performed with steel and sapphirecounterfaces. It was observed that PEEK debris was readily deposited tosteel but not on sapphire. The friction studies also indicated a greateradhesive friction response for PEEK against steel compared to sapphire.The transfer of PEEK material to the steel surface may elevate thetemperature at the sliding interface. Analysis of films formed on steelsuggests that the transferred PEEK was oriented in the direction ofsliding. The deposition of debris and formation of oriented filmsresembled a high temperature drawing process, which was likely to be dueto localized frictional heating. The results of this study illustrate theimportant role transfer films play in determining both the friction andtemperature response of the PEEK wear interface.
Tysoe W, Spencer N, 2016, Looking at lube in a new light, Publisher: Society of Tribologists and Lubrication Engineers
Galmiche B, Ponjavic A, Wong J, 2016, Flow measurements of a polyphenyl ether oil in an elastohydrodynamic contact, Journal of Physics: Condensed Matter, Vol: 28, ISSN: 0953-8984
A novel methodology, based on the use of phosphorescence imaging, is applied to determine the local through-thickness velocity profile of lubricant in an elastohydrodynamic (EHD) contact. The technique has spatial and temporal resolutions of 40 µm and 340 µs respectively and thus allows lubricant rheology to be investigated at conditions close to service conditions. The capability of the newly-developed method is verified by examining the flow of 5P4E polyphenyl ether, a lubricant base fluid used in very high temperature applications and well-known for its high viscosity-pressure coefficient. Experimental results highlight the effect of the contact pressure on the velocity profile of this fluid in lubricated contacts. At low pressures, the velocity profile of 5P4E is close to linear, characteristic of Couette flow. As the local pressure increases, its velocity profile progressively deviates from a Couette profile and shear banding is evident at high pressure.
The viscosity of liquids governs crucial physical and engineering phenomena, ranging from diffusion and transport processesof nutrients and chemicals, to the generation of friction and the physics of damping. Engineering fluids frequently experiencelocal conditions that change their bulk rheological properties. While viscosity data can easily be acquired using conventionalrheometers, the results are not always applicable to fluids under engineering conditions. This is particularly the case forfluids being sheared at high pressure under severe confinement, which experience very high shear stresses and often showextensive shear thinning. There is a lack of suitable methods for measuring fluid viscosity under such conditions. This workdescribes a novel in-situ viscosity measurement technique to fill this gap. It involves the quantification of the fluorescencelifetime of a fluorescent dye that is sensitive to viscosity. The capability of the developed technique is verified by takingmeasurements in submicron thick films of two model fluids confined in a ball on flat contact. Viscosity measurements weresuccessfully performed at pressures up to 1.2 GPa and shear rates up to 105s-1. Spatial heterogeneity in viscosity caused byvariations in pressure within the thin fluid film could be observed using the technique. It was also possible to detectdifferences in the rheological responses of a Newtonian and a non-Newtonian fluid. These first in-situ high pressure, highshear viscosity measurements demonstrate the versatility of the proposed technique in providing information on theviscosity in conditions where contemporary techniques are insufficient. More importantly it highlights the complexity of therheology of engineering fluids and provides a means of verifying existing theories by performing in-situ measurements.Information on local viscosity is crucial for understanding the physics of confined fluids and to facilitate improvements inengineering technology.
Parkes M, Myant C, Cann PM, et al., 2015, Synovial fluid lubrication: The effect of protein interactions on adsorbed and lubricating films, Biotribology, Vol: 1-2, Pages: 51-60, ISSN: 2352-5738
© 2015 Elsevier Ltd. All rights reserved. Synovial fluid lubrication is dependent on protective protein films that form between joint surfaces. Under static conditions surface film formation occurs through adsorption, while under dynamic conditions protein aggregation under shear and load becomes the dominant mechanism. This work examines how the protein content of six model synovial fluids affects film formation under static and rolling conditions and if the changes in properties can be correlated. With an increase in the statically adsorbed mass and the rate of adsorption the film thickness under rolling increased. These increases did not correlate with the total protein content of the fluid, but were dependent on the type of protein. An increase in pH reduced the adsorbed mass, rate of adsorption and film thickness, but was of secondary importance to the type of protein. The rolling film thickness was also correlated with the viscoelastic properties of the films formed under static conditions. In this case thinner rolling films corresponded to the more hydrated, viscoelastic adsorbed films. The strong correlations found between the properties of the adsorbed films and those formed under rolling indicate that the same protein-protein and protein-surface interactions may govern both mechanisms of film formation despite the differences in the film structures.
Wong JSP, Hu M, Shi D, et al., 2014, In-situ Monitoring on Dynamics of Solute Transport in Polymer Films, Polymer, Vol: 58, Pages: 67-75, ISSN: 0032-3861
A new and non-invasive technique based on confocal laser scanning microscopy (CLSM) that allows the visualization of penetrant diffusion in-situ has been developed and was applied to quantify local solute dynamics in polymeric films. The effectiveness of the proposed technique was demonstrated using a model penetrant, rhodamine-6G (Rh-6G), and a system of polyvinyl alcohol (PVA) films with different degree of cross-linking, and different content of montmorillonite (MMT) clay. The penetrant's transport across PVA films were monitored by measuring the time evolutions of through thickness fluorescence intensity profiles. These profiles were then converted to concentration profiles, which allow local diffusion coefficients of the model solute (i.e. Rh-6G) to be determined. The developed methodology was applied to both single layer and bilayers films and local diffusion heterogeneity was detected. Hence the technique developed can be applied to multi-layer films, and can be beneficial to film developments for packaging and filtration technology.
Liu X, Spikes H, Wong JSS, 2014, In-situ pH responsive fluorescent probing of localized iron corrosion, Corrosion Science, ISSN: 0010-938X
Parkes M, Myant C, Cann PM, et al., 2014, The effect of buffer solution choice on protein adsorption and lubrication, Tribology International, Vol: 72, Pages: 108-117, ISSN: 0301-679X
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