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  • Journal article
    Dawczyk J, Ware E, Ardakani M, Russo J, Spikes Het al., 2018,

    Use of FIB to study ZDDP tribofilms

    , Tribology Letters, Vol: 66, Pages: 155-155, ISSN: 1023-8883

    Focussed ion beam milling (FIB) followed by TEM has been used to study ZDDP tribofilms on rubbed steel surfaces. It has been found that the impact of high energy platinum and gallium ions during FIB causes significant morphological and structural changes to the uppermost 30–50 nm of a ZDDP tribofilm. This can be prevented by the low energy deposition of a quite thick gold layer prior to installation of the sample in the FIB facility. This problem, and its solution, have been quite widely reported in the non-tribology literature but have not previously been highlighted in the application of FIB to study tribological surfaces. It has also been found, using this gold pre-deposition method, that the bulk of the ZDDP tribofilm studied has a polycrystalline structure.

  • Journal article
    Vladescu SC, Marx N, Fernández L, Barceló F, Spikes HAet al., 2018,

    Hydrodynamic friction of viscosity-modified oils in a journal bearing machine

    , Tribology Letters, Vol: 66, ISSN: 1023-8883

    The friction properties of a range of viscosity modifier-containing oils in an engine bearing have been studied in the hydrodynamic regime using a combined experimental and modelling approach. The viscometric properties of these oils were previously measured and single equations derived to describe how their viscosities vary with temperature and shear rate (Marx et al. Tribol Lett 66:92, 2018). A journal bearing machine has been used to measure the friction properties of the test oils at various oil supply temperatures, while simultaneously measuring bearing temperature using an embedded thermocouple. This shows the importance of taking account of thermal response in journal bearings since the operating oil film temperature is often considerably higher than the oil supply temperature. For Newtonian oils, friction coefficient measurements made over a wide range of speeds, loads and oil supply temperatures collapse onto a single Stribeck curve when the viscosity used in determining the Stribeck number is based on an effective oil film temperature. Journal bearing machine measurements on VM-containing oils show that these give lower friction than a Newtonian reference oil. A thermo-hydrodynamic model incorporating shear thinning has been used to explore further the frictional properties of the VM-containing oils. These confirm the findings of the journal bearing experiments and show that two key factors determine the friction of the engine bearing; (i) the low shear rate viscosity of the oil at the effective bearing temperature and (ii) the extent to which the blend shear thins at the high shear rate present in the bearing.

  • Journal article
    Tajabadi-Ebrahimi M, Dini D, Balint DS, Sutton AP, Ozbayraktar Set al., 2018,

    Discrete crack dynamics: a planar model of crack propagation and crack-inclusion interactions in brittle materials

    , International Journal of Solids and Structures, Vol: 152-153, Pages: 12-27, ISSN: 0020-7683

    The Multipole Method (MPM) is used to simulate the many-body self-consistentproblem of interacting elliptical micro-cracks and inclusions in single crystals. Acriterion is employed to determine the crack propagation path based on the stressdistribution; the evolution of individual micro-cracks and their interactions withexisting cracks and inclusions is then predicted using what we coin the DiscreteCrack Dynamics (DCD) method. DCD is fast (semi-analytical) and particularlysuitable for the simulation of evolving low-speed crack networks in brittle orquasi-brittle materials. The method is validated against finite element analysispredictions and previously published experimental data.

  • Journal article
    Verschueren J, Gurrutxaga-Lerma B, Balint D, Sutton A, Dini Det al., 2018,

    Instabilities of high speed dislocations

    , Physical Review Letters, Vol: 121, ISSN: 0031-9007

    Despite numerous theoretical models and simulation results, a clear physical picture of dislocations traveling at velocities comparable to the speed of sound in the medium remains elusive. Using two complementary atomistic methods to model uniformly moving screw dislocations, lattice dynamics and molecular dynamics, the existence of mechanical instabilities in the system is shown. These instabilities are found at material-dependent velocities far below the speed of sound. We show that these are the onset of an atomistic kinematic generation mechanism, which ultimately results in an avalanche of further dislocations. This homogeneous nucleation mechanism, observed but never fully explained before, is relevant in moderate and high strain rate phenomena including adiabatic shear banding, dynamic fracture, and shock loading. In principle, these mechanical instabilities do not prevent supersonic motion of dislocations.

  • Journal article
    Hartinger M, Reddyhoff T, 2018,

    CFD modeling compared to temperature and friction measurements of an EHL line contact

    , Tribology International, Vol: 126, Pages: 144-152, ISSN: 0301-679X

    In this paper, predictions from CFD modeling are compared against measurements of surface temperatures and friction for an EHL line contact lubricated with the fluid Santotrac 50. Two slide-to-roll-ratios (SRR), 50% and 100%, and entrainment velocities ranging from 0.211 to 1.13 m/s are considered. Very good agreement is shown for the 50% SRR cases, with only a 3% deviation in friction coefficient values. At 100% SRR, the deviation in friction increases to 3–7% which is attributed to deficiencies in the modeling approach with regard to shear-thinning. The temperature profiles agree reasonably well at 50% SRR and show larger deviations at 100% SRR. For all cases, the formation of a shear-band in the center of the fluid film is predicted. This is very pronounced for 100% SRR, although likely to be over-estimated by this CFD-approach. The data presented here serve as a basis from which further refinements in the modeling and measurements shown can be made.

  • Journal article
    Yu M, Arana C, Evangelou S, Dini D, Cleaver Get al., 2018,

    Parallel active link suspension: a quarter car experimental study

    , IEEE/ASME Transactions on Mechatronics, Vol: 23, Pages: 2066-2077, ISSN: 1083-4435

    In this paper, a novel electro-mechanical active suspension for cars, the Parallel Active Link Suspension (PALS), is proposed and then experimentally studied. PALS involves the introduction of a rotary-actuator-driven rocker-pushrod mechanism in parallel with the conventional passive suspension assembly, to exert an additional controlled force between the chassis and the wheel. The PALS geometric arrangement is designed and optimized to maximize the rocker torque propagation onto the tire load increment. A quarter car test rig with double wishbone suspension is utilized for the PALS physical implementation. Based on a linear equivalent model of the PALS quarter car, a conservative and an aggressive robust H∞ control schemes are synthesized separately to improve the ride comfort and the road holding, with different levels of control effort allowed in each of the control schemes. Simulations with a theoretical nonlinear model of the PALS quarter car are performed to evaluate the potential in suspension performance enhancement and power demand in the rocker actuator. Experiments with a harmonic road, a smoothed bump and hole, and swept frequency are conducted with the quarter car test rig to validate the practical feasibility of the novel PALS, the ride comfort enhancement, as well as the accuracy of the theoretical model and of a further nonlinear model in which practical features existing in the test rig are identified and included.

  • Journal article
    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
  • Journal article
    Marx N, Fernández L, Barceló F, Spikes HAet al., 2018,

    Shear thinning and hydrodynamic friction of viscosity modifier-containingoils. Part I: shear thinning behaviour

    , Tribology Letters, Vol: 66, Pages: 92-92, ISSN: 1023-8883

    Viscosity versus shear rate curves have been measured up to 107 s−1 for a range of VM solutions and fully formulated oils of known composition at several temperatures. This shows large differences in the shear thinning tendencies of different engine oil VMs. It has been found that viscosity versus shear rate data at different temperatures can be collapsed onto a single master curve using time–temperature superposition based on a shear rate shift factor. This enables shear thinning equations to be derived that are able to predict the viscosity of a given oil at any shear rate and temperature within the range originally tested. One of the tested lubricants does not show this time temperature superposition collapse. This fluid also exhibits extremely high viscosity index and shear thins more easily at high than at low temperature, unlike all the other solutions tested. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. In a companion paper, the master shear thinning curves obtained in this paper are used to explore how VMs impact film thickness and friction in a steadily loaded, isothermal journal bearing [1].

  • Journal article
    Vakis A, Yastrebov V, Scheibert J, Nicola L, Dini D, Minfray C, Almqvist A, Paggi M, Lee S, Limbert G, Molinari JF, Anciaux G, Aghababaei R, Echeverri Restrepo S, Papangelo A, Cammarata A, Nicolini P, Putignano C, Carbone G, Stupkiewicz S, Lengiewicz J, Costagliola G, Bosia F, Guarino R, Pugno NM, Müser MH, Ciavarella Met al., 2018,

    Modeling and simulation in tribology across scales: An overview

    , Tribology International, Vol: 125, Pages: 169-199, ISSN: 0301-679X

    This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.

  • 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
    Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2018,

    Capillary waves with surface viscosity

    , Journal of Fluid Mechanics, Vol: 847, Pages: 644-663, ISSN: 0022-1120

    Experiments over the last 50 years have suggested a tentative correlation between the surface (shear) viscosity and the stability of a foam or emulsion. We examine this link theoretically using small-amplitude capillary waves in the presence of a surfactant solution of dilute concentrations where the associated Marangoni and surface viscosity effects are modelled via the Boussinesq-Scriven formulation. The resulting integro-differential initial value problem is solved analyticallyand surface viscosity is found to contribute an overall damping effect on the amplitude of the capillary wave with varying degrees depending on the lengthscale of the system.Numerically, we find the critical damping wavelength to increase for increasing surface concentration but the rate of increase remains different for both the surface viscosity and the Marangoni effect.

  • Journal article
    Reddyhoff T, Underwood R, Sayles R, Spikes Het al., 2018,

    Temperature measurement of debris particles in EHL contacts

    , Surface Topography: Metrology and Properties, Vol: 6, ISSN: 2051-672X

    Dents caused by entrained debris are now the main cause of fatigue failure in rolling element bearings. It is therefore important to be able to understand and predict the deformation behaviour of particles in elastohydrodynamic contacts. This paper describes a new method to study debris entrainment. This uses a sensitive infrared microscope to map the temperature of a contact between a steel ball and coated sapphire disc as lubricant dispersed with bearing dust is entrained. Full-field thermal maps were acquired at a sufficient rate to monitor the deformation of a single particle on its journey through the contact.Under the low-speed, high-sliding conditions studied, the temperature rise increases from when the particle is trapped by the inlet to reach a peak near the contact centre, where shearing is a maximum. Under these conditions, temperature rises are typically of the order of 10 °C, which is significantly lower than has been predicted theoretically. Even lower temperature rises were observed under pure rolling conditions, since minimal shearing occurs.Experimental results are also compared with existing models used to predict particle behaviour. Measured radiation distributions confirm qualitatively the ductile particle deformation mechanisms originally proposed by Hamer et al.

  • Journal article
    Marx N, Fernández L, Barceló F, Spikes HAet al., 2018,

    Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-ContainingOils. Part II: Impact of Shear Thinning on Journal BearingFriction

    , Tribology Letters, Vol: 66, Pages: 91-91, ISSN: 1023-8883
  • Journal article
    Kanca Y, Milner P, Dini D, Amis Aet al., 2018,

    Tribological evaluation of biomedical polycarbonate urethanes against articular cartilage

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 82, Pages: 394-402, ISSN: 1751-6161

    This research investigated the in-vitro wear and friction performance of polycarbonate urethane (PCU) 80A as they interact with articular cartilage, using a customised multidirectional pin-on-plate tester. Condyles were articulated against PCU 80A discs (Bionate® I and Bionate® II) (configuration 1) and the results arising from these tests were compared to those recorded during the sliding of PCU pins against cartilage plates (configuration 2). Configuration 1 produced steadily increasing coefficient of friction (COF) (up to 0.64 ± 0.05) and had the same trend as the cartilage–on–stainless steel articulation (positive control). When synovial fluid rather than bovine calf serum was used as lubricant, average COF significantly decreased from 0.50 ± 0.02–0.38 ± 0.06 for condyle–on–Bionate® I (80AI) and from 0.41 ± 0.02–0.24 ± 0.04 for condyle–on–Bionate® II (80AII) test configurations (p < 0.05). After 15 h testing, the cartilage–on–cartilage articulation (negative control) tests showed no cartilage degeneration. However, different levels of cartilage volume loss were found on the condyles from the positive control (12.5 ± 4.2 mm3) and the PCUs (20.1 ± 3.6 mm3 for 80 AI and 19.0 ± 2.3 mm3 for 80AII) (p > 0.05). A good correlation (R2 =0.84) was found between the levels of average COF and the volume of cartilage lost during testing; increasing wear was found at higher levels of COF. Configuration 2 showed low and constant COF values (0.04 ± 0.01), which were closer to the negative control (0.03 ± 0.01) and significantly lower than configuration 1 (p < 0.05). The investigation showed that PCU is a good candidate for use in hemiarthroplasty components, where only one of the two articulating surfaces is replaced, as long as the synthetic material is implanted in a region where migrating cartilage contact is achieved. Bio

  • Journal article
    Stevenson H, Parkes M, Austin L, Jaggard M, Akhbri P, Vaghela U, Williams H, Gupte C, Cann PMet al., 2018,

    The development of a small-scale wear test for CoCrMo specimens with human synovial fluid

    , Biotribology, Vol: 14, Pages: 1-10, ISSN: 2352-5738

    A new test was developed to measure friction and wear of hip implant materials under reciprocating sliding conditions. The method requires a very small amount of lubricant (<3 ml) which allows testing of human synovial fluid. Friction and wear of Cobalt Chromium Molybdenum (CoCrMo) material pairs were measured for a range of model and human synovial fluid samples. The initial development of the test assessed the effect of fluid volume and bovine calf serum (BCS) concentration on friction and wear. In a second series of tests human synovial fluid (HSF) was used. The wear scar size (depth and volume) on the disc was dependent on protein content and reduced significantly for increasing BCS concentration. The results showed that fluid volumes of <1.5 ml were affected by evaporative loss effectively increasing the protein concentration resulting in anomalously lower wear. At the end of the test thick deposits were observed in and around the wear scars on the disc and ball; these were analysed by Infrared Reflection-Absorption Spectroscopy. The deposits were composed primarily of denatured proteins and similar IR spectra were obtained from the BCS and HSF tests. The analysis confirmed the importance of SF proteins in determining wear of CoCrMo couples.

  • Journal article
    Heyes D, Dini D, Smith E, 2018,

    Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model

    , Journal of Chemical Physics, Vol: 148, ISSN: 0021-9606

    The viscoelastic behavior of sheared fluids is calculated by Non-Equilibrium Molecular Dynamics(NEMD) simulation, and complementary analytic solutions of a time-dependent extension of Eyring’smodel (EM) for shear thinning are derived. It is argued that an “incremental viscosity,”ηi, or IV whichis the derivative of the steady state stress with respect to the shear rate is a better measure of the physicalstate of the system than the conventional definition of the shear rate dependent viscosity (i.e., the shearstress divided by the strain rate). The stress relaxation function,Ci(t), associated withηiis consistentwith Boltzmann’s superposition principle and is computed by NEMD and the EM. The IV of the Eyringmodel is shown to be a special case of the Carreau formula for shear thinning. An analytic solutionfor the transient time correlation function for the EM is derived. An extension of the EM to allow forsignificant local shear stress fluctuations on a molecular level, represented by a gaussian distribution,is shown to have the same analytic form as the original EM but with the EM stress replaced by its timeand spatial average. Even at high shear rates and on small scales, the probability distribution functionis almost gaussian (apart from in the wings) with the peak shifted by the shear. The Eyring formulaapproximately satisfies the Fluctuation Theorem, which may in part explain its success in representingthe shear thinning curves of a wide range of different types of chemical systems.

  • Journal article
    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.

  • Journal article
    Ewen JP, Kannam SK, Todd BD, Dini Det 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.

  • Journal article
    Guo Y, di Mare L, Li R, Wong Jet al., 2018,

    Cargo release from polymeric vesicles under shear

    , Polymers, Vol: 10, Pages: 336-336, ISSN: 2073-4360

    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.

  • Journal article
    Morales-Espejel GE, Rycerz P, Kadiric A, 2018,

    Prediction of micropitting damage in gear teeth contacts considering the concurrent effects of surface fatigue and mild wear

    , Wear, Vol: 398-399, Pages: 99-115, ISSN: 0043-1648

    © 2017 The Authors The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts for mixed lubrication conditions, stress history, and fatigue damage accumulation, is adapted here to deal with transient contact conditions that exist during meshing of gear teeth. The model considers the concurrent effects of surface fatigue and mild wear on the evolution of tooth surface roughness and therefore captures the complexities of damage accumulation on tooth flanks in a more realistic manner than hitherto possible. Applicability of the model to gear contact conditions is first confirmed by comparing its predictions to relevant experiments carried out on a triple-disc contact fatigue rig. Application of the model to a pair of meshing spur gears shows that under low specific oil film thickness conditions, the continuous competition between surface fatigue and mild wear determines the overall level as well as the distribution of micropitting damage along the tooth flanks. The outcome of this competition in terms of the final damage level is dependent on contact sliding speed, pressure and specific film thickness. In general, with no surface wear, micropitting damage increases with decreasing film thickness as may be expected, but when some wear is present micropitting damage may reduce as film thickness is lowered to the point where wear takes over and removes the asperity peaks and hence reduces asperity interactions. Similarly, when wear is negligible, increased sliding can increase the level of micropitting by increasing the number of asperity stress cycles, but when wear is present, an increase in sliding may lead to a reduction in micropitting due to faster removal of asperity peaks. The results suggest that an ideal situation in terms of surface damage prevention is that in which some mild wear at the start of gear pair operation adequately wears-in the tooth surfaces, thus reducing sub

  • Journal article
    Masen M, Cann PME, 2018,

    Friction measurements with molten chocolate

    , Tribology Letters, Vol: 66, ISSN: 1023-8883

    A novel test is reported which allows the measurement of the friction of molten chocolate in a model tongue–palate rubbing contact. Friction was measured over a rubbing period of 150 s for a range of commercial samples with different cocoa content (85–5% w/w). Most of the friction curves had a characteristic pattern: initially a rapid increase occurs as the high-viscosity chocolate melt is sheared in the contact region followed by friction drop as the film breaks down. The exceptions were the very high (85%) and very low (~ 5%) cocoa content samples which gave fairly constant friction traces over the test time. Differences were observed in the initial maximum and final friction coefficients depending on chocolate composition. Generally, the initial maximum friction increased with increasing cocoa content. At the end of the test, the rubbed films on the lower slide were examined by optical microscopy and infrared micro-reflection spectroscopy. In the rubbed track, the chocolate structure was severely degraded and predominately composed of lipid droplets, which was confirmed by the IR spectra. The new test provides a method to distinguish between the friction behaviour of different chocolate formulations in a rubbing low-pressure contact. It also allows us to identify changes in the degraded chocolate film that can be linked to the friction profile. Further development of the test method is required to improve simulation of the tongue–palate contact including the effect of saliva and this will be the next stage of the research.

  • Journal article
    Spikes HA, 2018,

    Stress-augmented thermal activation: Tribology feels the force

    , Friction, Vol: 6, Pages: 1-31, ISSN: 2223-7690

    In stress-augmented thermal activation, the activation energy barrier that controls the rate of atomic and molecular processes is reduced by the application of stress, with the result that the rate of these processes increases exponentially with applied stress. This concept has particular relevance to Tribology, and since its development in the early twentieth century, it has been applied to develop important models of plastic flow, sliding friction, rheology, wear, and tribochemistry. This paper reviews the development of stress-augmented thermal activation and its application to all of these areas of Tribology. The strengths and limitations of the approach are then discussed and future directions considered. From the scientific point of view, the concept of stress-augmented thermal activation is important since it enables the development of models that describe macroscale tribological performance, such as friction coefficient or tribofilm formation, in terms of the structure and behaviour of individual atoms and molecules. This both helps us understand these processes at a fundamental level and also provides tools for the informed design of lubricants and surfaces.

  • Conference paper
    Ferretti A, Giacopini M, Mastrandrea L, Dini Det al., 2018,

    Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

    , WCX World Congress Experience

    © 2018 SAE International. All Rights Reserved. Bearings represent one of the main responsible of friction losses in internal combustion engines and their lubrication performance has a crucial influence on the operating condition of the engine. In particular, the conrod small-end bearing is one of the most critical engine parts from a tribological point of view since limited contact surfaces have to sustain high inertial and combustion forces. In this contribution an analysis is performed of the tribological behaviour of the lubricated contact between the piston pin and the conrod small-end of a high performance motorbike engine. An algorithm is employed based on a complementarity formulation of the cavitation problem. A comparison between two different approaches to simulate the asperity contact problem is performed, the former based on the standard Greenwood-Tripp theory and the latter based on a complementarity formulation of the asperity contact problem. A model validation is performed by comparing the results with those obtained adopting the commercial software AVL Excite Power Unit. Similar results are obtained from both the approaches, if a proper calibration of the model input data is performed. However, a remarkable sensitivity is highlighted of the results obtained using the Greenwood/Tripp model to the adjustment parameters. The realistic (engineering) difficulty in defining and identifying the roughness data and their purely statistical nature returns results that may be afflicted by a dose of uncertainty. Considering that results of such simulations usually offer guidelines for a correct design of the coupling, further investigations are suggested to identify a relationship between simply available roughness data and model input, starting from a direct experimental measurements of real roughness profiles.

  • Journal article
    Parkes M, Sayer K, Goldhofer M, Cann P, Walter WL, Jeffers Jet al., 2017,

    Zirconia phase transformation in retrieved, wear simulated, and artificially aged ceramic femoral heads

    , JOURNAL OF ORTHOPAEDIC RESEARCH, Vol: 35, Pages: 2781-2789, ISSN: 0736-0266
  • Journal article
    Menga N, Carbone G, Dini D, 2017,

    Do uniform tangential interfacial stresses enhance adhesion?

    , Journal of the Mechanics and Physics of Solids, Vol: 112, Pages: 145-156, ISSN: 0022-5096

    We present theoretical arguments, based on linear elasticity and thermodynamics, to show that interfacial tangential stresses in sliding adhesive soft contacts may lead to a significant increase of the effective energy of adhesion. A sizable expansion of the contact area is predicted in conditions corresponding to such scenario. These results are easily explained and are valid under the assumptions that: (i) sliding at the interface does not lead to any loss of adhesive interaction and (ii) spatial fluctuations of frictional stresses can be considered negligible. Our results are seemingly supported by existing experiments, and show that frictional stresses may lead to an increase of the effective energy of adhesion depending on which conditions are established at the interface of contacting bodies in the presence of adhesive forces.

  • Journal article
    Lu J, Reddyhoff T, Dini D, 2017,

    3D Measurements of Lubricant and Surface Temperatures Within an Elastohydrodynamic Contact

    , Tribology Letters, Vol: 66, ISSN: 1023-8883

    We present an infrared microscopy technique, capable of measuring the temperature of both the bounding surfaces and the oil film in an elastohydrodynamic contact. This technique can, for the first time, spatially resolve the oil film temperature in three dimensions. The contact is produced by loading a steel ball against a sapphire disc, and the film is viewed using an infrared microscope focussing through the disc. Two band pass filters are used to isolate the radiation from the oil film, and Planck’s law is applied to data obtained at a known temperature as part of the calibration procedure. The proposed technique requires the emissivity of the oil film to be measured, which is acquired in situ and is shown to vary strongly as a function of thickness and temperature. The technique is validated under pure rolling conditions, when the temperature of the oil film is equal to the controlled lubricant reservoir temperature, and also compared to an equation commonly used to predict average film temperatures, confirming the value of the unknown constant. The technique is then used to gain insights into the thermal/rheological behaviour within a contact. This is important since the temperature of elastohydrodynamic contacts is critical in determining friction and hence the efficiency of machine components and this technique enables much needed validation and provides input data for CFD and numerical simulations.

  • Journal article
    Tan Z, Parisi C, Di Silvio L, Dini D, Forte AEet al., 2017,

    Cryogenic 3D printing of super soft hydrogels

    , Scientific Reports, Vol: 7, ISSN: 2045-2322

    Conventional 3D bioprinting allows fabrication of 3D scaffolds for biomedical applications. In this contribution we present a cryogenic 3D printing method able to produce stable 3D structures by utilising the liquid to solid phase change of a composite hydrogel (CH) ink. This is achieved by rapidly cooling the ink solution below its freezing point using solid carbon dioxide (CO2) in an isopropanol bath. The setup was able to successfully create 3D complex geometrical structures, with an average compressive stiffness of O(1) kPa (0.49 ± 0.04 kPa stress at 30% compressive strain) and therefore mimics the mechanical properties of the softest tissues found in the human body (e.g. brain and lung). The method was further validated by showing that the 3D printed material was well matched to the cast-moulded equivalent in terms of mechanical properties and microstructure. A preliminary biological evaluation on the 3D printed material, coated with collagen type I, poly-L-lysine and gelatine, was performed by seeding human dermal fibroblasts. Cells showed good attachment and viability on the collagen-coated 3D printed CH. This greatly widens the range of applications for the cryogenically 3D printed CH structures, from soft tissue phantoms for surgical training and simulations to mechanobiology and tissue engineering.

  • Journal article
    Shen L, Denner F, Morgan N, van Wachem B, Dini Det al., 2017,

    Marangoni effect on small-amplitude capillary waves in viscous fluids

    , Physical Review E, Vol: 96, Pages: 053110-053110, ISSN: 1539-3755

    We derive a general integro-differential equation for the transient behavior of small-amplitude capillary waves on the planar surface of a viscous fluid in the presence of the Marangoni effect. The equation is solved for an insoluble surfactant solution in concentration below the critical micelle concentration undergoing convective-diffusive surface transport. The special case of a diffusion-driven surfactant is considered near the the critical damping wavelength. The Marangoni effect is shown to contribute to the overall damping mechanism, and a first-order term correction to the critical wavelength with respect to the surfactant concentration difference and the Schmidt number is proposed.

  • Journal article
    Kontou A, Southby M, Spikes HA, 2017,

    Effect of steel hardness on soot wear

    , Wear, Vol: 390-391, Pages: 236-245, ISSN: 0043-1648

    Due to incomplete combustion, high levels of soot can accumulate in engine lubricants between drain intervals. This soot can promote wear of engine parts such as timing chains and cam followers. One standard approach to reducing wear is to increase the hardness of the rubbing components used. According to the Archard wear equation, wear rate should be broadly inversely proportional to hardness. To explore this approach for controlling soot wear, wear tests have been conducted in a High Frequency Reciprocating Rig (HFRR) with HFRR steel discs of various hardness against a hard steel ball. Carbon black (soot surrogate) dispersions in model lubricants based on solutions of ZDDP and dispersant in GTL base oils have been studied. Wear volumes have been measured and wear scars and tribofilms analysed using scanning white light interferometry and SEM-EDS. It is found that, while most oils show wear that reduces with increasing hardness, for blends that contain both ZDDP and carbon black, wear rate markedly increases with disc hardness as the latter approaches the hardness of the ball. The results support the prevalence of a corrosive-abrasive wear mechanism when carbon black and ZDDP are both present in a lubricant and suggests that selection of very hard surfaces may not be a useful way to control soot.

  • Journal article
    Putignano, Dini D, 2017,

    Soft matter lubrication: does solid viscoelasticity matter?

    , ACS Applied Materials and Interfaces, Vol: 9, Pages: 42287-42295, ISSN: 1944-8244

    Classical lubrication theory is unable to explain a variety of phenomena and experimental observations involving soft viscoelastic materials, which are ubiquitous and increasingly used in e.g. engineering and biomedical applications. These include unexpected ruptures of the lubricating film and a friction–speed dependence, which cannot be elucidated by means of conventional models, based on time-independent stress–strain constitutive laws for the lubricated solids. A new modeling framework, corroborated through experimental measurements enabled via an interferometric technique, is proposed to address these issues: Solid/fluid interactions are captured thanks to a coupling strategy that makes it possible to study the effect that solid viscoelasticity has on fluid film lubrication. It is shown that a newly defined visco-elasto-hydrodynamic lubrication (VEHL) regime can be experienced depending on the degree of coupling between the fluid flow and the solid hysteretic response. Pressure distributions show a marked asymmetry with a peak at the flow inlet, and correspondingly, the film thickness reveals a pronounced shrinkage at the flow outlet; friction is heavily influenced by the viscoelastic hysteresis which is experienced in addition to the viscous losses. These features show significant differences with respect to the classical elasto-hydrodynamic lubrication (EHL) regime response that would be predicted when solid viscoelasticity is neglected. A simple yet powerful criterion to assess the importance of viscoelastic solid contributions to soft matter lubrication is finally proposed.

  • Journal article
    Milner P, Parkes M, Puetzer J, Chapman R, Cann P, Stevens M, Jeffers Jet al., 2017,

    A Low Friction, Biphasic and Boundary Lubricating Hydrogel for Cartilage Replacement

    , Acta Biomaterialia, Vol: 65, Pages: 102-111, ISSN: 1742-7061

    Partial joint repair is a surgical procedure where an artificial material is used to replace localised chondral damage. These artificial bearing surfaces must articulate against cartilage, but current materials do not replicate both the biphasic and boundary lubrication mechanisms of cartilage. A research challenge therefore exists to provide a material that mimics both boundary and biphasic lubrication mechanisms of cartilage.In this work a polymeric network of a biomimetic boundary lubricant, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), was incorporated into an ultra-tough double network (DN) biphasic (water phase + polymer phase) gel, to form a PMPC triple network (PMPC TN) hydrogel with boundary and biphasic lubrication capability. The presence of this third network of MPC was confirmed using ATR-FTIR. The PMPC TN hydrogel had a yield stress of 26 MPa, which is an order of magnitude higher than the peak stresses found in the native human knee. A preliminary pin on plate tribology study was performed where both the DN and PMPC TN hydrogels experienced a reduction in friction with increasing sliding speed which is consistent with biphasic lubrication. In the physiological sliding speed range, the PMPC TN hydrogel halved the friction compared to the DN hydrogel indicating the boundary lubricating PMPC network was working.A biocompatible, tough, strong and chondral lubrication imitating PMPC TN hydrogel was synthesised in this work. By complementing the biphasic and boundary lubrication mechanisms of cartilage, PMPC TN hydrogel could reduce the reported incidence of chondral damage opposite partial joint repair implants, and therefore increase the clinical efficacy of partial joint repair.Statement of SignificanceThis paper presents the synthesis, characterisation and preliminary tribological testing of a new biomaterial that aims to recreate the primary chondral lubrication mechanisms: boundary and biphasic lubrication. This work has demonstrated that the

  • Journal article
    Kanca Y, Milner P, Dini D, Amis AAet al., 2017,

    Tribological properties of PVA/PVP blend hydrogels against articular cartilage.

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 78, Pages: 36-45, ISSN: 1751-6161

    This research investigated in-vitro tribological performance of the articulation of cartilage-on- polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) blend hydrogels using a custom-designed multi-directional wear rig. The hydrogels were prepared by repeated freezing-thawing cycles at different concentrations and PVA to PVP fractions at a given concentration. PVA/PVP blend hydrogels showed low coefficient of friction (COF) values (between 0.12 ± 0.01 and 0.14 ± 0.02) which were closer to the cartilage-on-cartilage articulation (0.03 ± 0.01) compared to the cartilage-on-stainless steel articulation (0.46 ± 0.06). The COF increased with increasing hydrogel concentration (p = 0.03) and decreasing PVP content at a given concentration (p < 0.05). The cartilage-on-hydrogel tests showed only the surface layers of the cartilage being removed (average volume loss of the condyles was 12.5 ± 4.2mm3). However, the hydrogels were found to be worn/deformed. The hydrogels prepared at a higher concentration showed lower apparent volume loss. A strong correlation (R2 = 0.94) was found between the COF and compressive moduli of the hydrogel groups, resulting from decreasing contact congruency. It was concluded that the hydrogels were promising as hemiarthroplasty materials, but that improved mechanical behaviour was required for clinical use.

  • Journal article
    Heyes D, Dini D, Smith E, Branka Aet al., 2017,

    Nanowire stretching by Non-equilibrium Molecular Dynamics

    , Physica Status Solidi B: Basic Solid State Physics, Vol: 254, ISSN: 0370-1972

    Non-equilibrium Molecular Dynamics (NEMD) simulations of a stretched Lennard-Jones (LJ) model single crystal nanowire with square cross-section are carried out. The microstructural and mechanical properties are examined as a function of strain and strain rate. The instantaneous Poisson's ratio and Young's modulus are shown to be strongly time (strain) dependent from the start of the pulling process. The structural transformation as a result of straining initially involves the (100) layers moving further apart and then slipping at ca. math formula when the shear slip stress along that direction is about 1% of the shear modulus, which is typical of plastic deformation of noble gas solid crystals, and in accordance with Schmid's law.

  • Journal article
    Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2017,

    Before the bubble ruptures

    , Physical Review Fluids, Vol: 2, Pages: 090505-090505, ISSN: 2469-990X

    This paper is associated with a video winner of a 2016 APS/DFD Gallery of Fluid Motion Award. The original video is available from the Gallery of Fluid Motion,

  • Journal article
    Mackowiak S, Heyes D, Pieprzyk S, Dini D, Branka ACet al., 2017,

    Non-equilibrium phase behavior of confined molecular films at low shear rates

    , Physica Status Solidi B - Basic Solid State Physics, Vol: 254, ISSN: 0370-1972

    In a recent publication [Maćkowiak et al., J. Chem. Phys. 145, 164704 (2016)] the results of Non-Equilibrium Molecular Dynamics (NEMD) simulations of confined sheared Lennard-Jones molecular films have been presented. The present work builds on that study by focusing on the low wall speed (shear rate) regime. Maps are given of the steady-state structures and corresponding friction coefficients in the region where a transition from static to kinetic friction is observed. The boundary between static and kinetic friction regions is determined as a function of wall speed and applied pressure, which is located for wall speeds up to about 0.8 m s−1. It was found that stick-slip behavior extends to pressures as high as 1000 MPa. The NEMD equations of motion are shown to be consistent with the Prandtl–Tomlinson model in the ‘soft spring’ limit, which leads to a new expression for the friction coefficient. This study provides new details and insights into the nature of anomalous friction behavior in the so-called Plug-Slip part of the nonquilibrium phase diagram regime.

  • Journal article
    Jean-Fulcrand A, Masen M, Bremner T, Wong Jet 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.

  • Journal article
    Shimizu Y, Spikes HA, 2017,

    The Influence of Aluminium–Silicon Alloy on ZDDP Tribofilm Formation on the Counter-Surface

    , Tribology Letters, Vol: 65, ISSN: 1023-8883

    In order to reduce vehicle weight and thus improve fuel economy, aluminium (Al) alloys have been increasingly adopted as contact surfaces such as piston skirts and cylinder liners in current engines. In general, hypereutectic Al–Si alloys are used, in which hard silicon grains are embedded in a softer Al–Si single phase matrix. It is reported that after rubbing, the matrix is removed to leave silicon grains protruding from the surface. However, the response of the counter-surface by these silicon grains is rarely investigated. In this study, mini traction machine–space layer imaging (MTM–SLIM) has been used to monitor tribofilm formation in situ and investigate the evolution of both surfaces in the contact of a steel ball on an Al–Si disc lubricated by ZDDP solution. In low-load conditions, the top layer of aluminium on the Al–Si disc is removed physically to leave silicon grains protruding from the surface, while ZDDP tribofilm pads are formed mainly on the grains. On the counter-surface (steel ball), ZDDP tribofilms are formed and build up with no wear scars. In high-load conditions, deep gaps are observed to form initially round the silicon grains on the disc. During rubbing, these become shallower, while the silicon grains start to protrude. On the steel ball, ZDDP tribofilm is generated initially over the whole rubbing track, but then the tribofilm in the middle of the track is almost completely removed by rubbing against the protruding silicon grains. Wear of the underlying steel surface then ensues.

  • Journal article
    Forte AE, galvan S, Dini D, 2017,

    Models and tissue mimics for brain shift simulations

    , Biomechanics and Modeling in Mechanobiology, Vol: 17, Pages: 249-261, ISSN: 1617-7940

    Capturing the deformation of human brain during neurosurgical operations is an extremely important task to improve the accuracy or surgical procedure and minimize permanent damage in patients. This study focuses on the development of an accurate numerical model for the prediction of brain shift during surgical procedures and employs a tissue mimic recently developed to capture the complexity of the human tissue. The phantom, made of a composite hydrogel, was designed to reproduce the dynamic mechanical behaviour of the brain tissue in a range of strain rates suitable for surgical procedures. The use of a well-controlled, accessible and MRI compatible alternative to real brain tissue allows us to rule out spurious effects due to patient geometry and tissue properties variability, CSF amount uncertainties, and head orientation. The performance of different constitutive descriptions is evaluated using a brain–skull mimic, which enables 3D deformation measurements by means of MRI scans. Our combined experimental and numerical investigation demonstrates the importance of using accurate constitutive laws when approaching the modelling of this complex organic tissue and supports the proposal of a hybrid poro-hyper-viscoelastic material formulation for the simulation of brain shift.

  • Journal article
    Hu H, Wen J, Jia L, Song D, Song B, Pan G, Scaraggi M, Dini D, Xue Q, Zhou Fet al., 2017,

    Significant and stable drag reduction with air rings confined by alternated superhydrophobic and hydrophilic strips

    , Science Advances, Vol: 3, ISSN: 2375-2548

    Superhydrophobic surfaces have the potential to reduce the viscous drag of liquids by significantly decreasing friction at a solid-liquid interface due to the formation of air layers between solid walls and interacting liquids. However, the trapped air usually becomes unstable due to the finite nature of the domain over which it forms. We demonstrate for the first time that a large surface energy barrier can be formed to strongly pin the three-phase contact line of air/water/solid by covering the inner rotor of a Taylor-Couette flow apparatus with alternating superhydrophobic and hydrophilic circumferential strips. This prevents the disruption of the air layer, which forms stable and continuous air rings. The drag reduction measured at the inner rotor could be as much as 77.2%. Moreover, the air layers not only significantly reduce the strength of Taylor vortexes but also influence the number and position of the Taylor vortex pairs. This has strong implications in terms of energy efficiency maximization for marine applications and reduction of drag losses in, for example, fluid transport in pipelines and carriers.

  • Journal article
    Cann P, Masen M, 2017,

    The 3 rd International Conference on Biotribology (ICoBT) Imperial College London, 11-14 th September 2016

    , Biotribology, Vol: 11, Pages: 1-2, ISSN: 2352-5738
  • Journal article
    De Laurentis N, Cann P, Lugt P, Kadiric Aet al., 2017,

    The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

    , Tribology Letters, Vol: 65, ISSN: 1023-8883

    This study investigates the influence of base oil type and viscosity on the frictional behaviour of lithium-thickened bearing greases. A series of model lithium greases were manufactured by systematically varying viscosity and type of base oil, so that the influence of a single base oil property could be studied in isolation. In addition, selected greases were blended with oleic acid, with the purpose of evaluating its effectiveness in further reducing grease friction. Friction coefficient and film thickness were measured in laboratory ball-on-disc tribometers over a range of speeds and temperatures. For a specific oil type, the influence of base oil viscosity on friction was found to be closely related to its effect on film thickness: greases formulated with PAO oils covering a wide range of viscosities gave very similar friction at the same nominal film thickness. For a given base oil viscosity, base oil type was found to have a strong influence on grease friction under all test conditions. PAO-based greases generally produced lower friction than mineral- and ester-based greases. Addition of oleic acid to the test greases did not significantly affect friction within the range of test conditions employed in this study. The results provide new insight into the frictional behaviour of greases, which may be used to help inform new low-friction grease formulations for rolling bearing applications.

  • 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
    Vlădescu S-C, Ciniero A, Tufail K, Gangopadhyay A, Reddyhoff Tet al., 2017,

    Optimisation of Pocket Geometry for Friction Reduction in Piston-Liner Contacts

    , Tribology Transactions, Pages: 00-00, ISSN: 1040-2004
  • Journal article
    ciniero A, Le-Rouzic J, Reddyhoff T, 2017,

    The Use of Triboemission Imaging and Charge Measurements to Study DLC Coating Failure

    , Coatings, Vol: 7, ISSN: 2079-6412

    We present a study on the simultaneous evolution of the electron emission and surface charge accumulation that occurs during scratching tests in order to monitor coating failure. Steel discs coated with a diamond-like-carbon (DLC) film were scratched in both vacuum (~10−5 Torr) and atmospheric conditions, with electron emission and surface charge being measured by a system of microchannel plates and an electrometer, respectively. The results highlight a positive correlation between emission intensity values, surface charge measurements and surface damage topography, suggesting the effective use of these techniques to monitor coating wear in real time.

  • Journal article
    Mueser MH, Dapp WB, Bugnicourt R, Sainsot P, Lesaffre N, Lubrecht TA, Persson BNJ, Harris K, Bennett A, Schulze K, Rohde S, Ifju P, Sawyer WG, Angelini T, Esfahani HA, Kadkhodaei M, Akbarzadeh S, Wu J-J, Vorlaufer G, Vernes A, Solhjoo S, Vakis AI, Jackson RL, Xu Y, Streator J, Rostami A, Dini D, Medina S, Carbone G, Bottiglione F, Afferrante L, Monti J, Pastewka L, Robbins MO, Greenwood JAet al., 2017,

    Meeting the Contact-Mechanics Challenge

    , TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883

    This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

  • Journal article
    Parkes M, Cann P, Jeffers J, 2017,

    Real-time observation of fluid flows in tissue during stress relaxation using Raman spectroscopy

    , JOURNAL OF BIOMECHANICS, Vol: 60, Pages: 261-265, ISSN: 0021-9290
  • Journal article
    Ewen JP, Gattinoni C, Zhang J, Heyes DM, Spikes HA, Dini Det 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
  • Journal article
    wang A, liu JUN, gao H, Wang L, masen Met al., 2017,

    Hot stamping of AA6082 tailor welded blanks: experiments and knowledge based cloud FE (KBC-FE) simulation

    , Journal of Materials Processing Technology, Vol: 250, Pages: 228-238, ISSN: 0924-0136

    A novel hot stamping technique known as ‘Solution Heat treatment, Forming and in-die Quenching (HFQ®)’ was employed to manufacture lightweight structural components from AA6082 tailor-welded blanks (TWBs) of different thickness combinations: 1.5–1.5 and 2.0–1.0 mm. A finite element (FE) model was built to study the deformation characteristics during the hot stamping process. The FE model was successfully validated by comparing simulation results with experimental ones. Subsequently, the verified simulation results were analysed through a novel multi-objective FE platform known as ‘Knowledge-Based Cloud – Finite Element (KBC-FE)’. KBC-FE operates in a cloud environment and offers various advanced unique functions via functional modules. The ‘formability’ module was implemented in the current study to predict the limiting dome height and failure mode during the hot stamping process. Good agreements were achieved between the predicted and experimental results, from which studies were extended to predict the forming features of 2.0–1.5 mm TWBs. The ‘formability’ module has successfully captured the complex nature of a hot stamping process, featuring a non-isothermal and non-linear loading path. The formability of TWBs was found to be dependent on forming speed and blank thickness, out of which the latter has a dominant effect.

  • Journal article
    Guo YY, di Mare L, Li RKY, Wong JSSet 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.

  • Journal article
    Marx N, Ponjavic A, Taylor RI, Spikes HAet al., 2017,

    Study of permanent shear thinning of VM polymer solutions

    , Tribology Letters, Vol: 65, ISSN: 1023-8883

    The ultrashear viscometer (USV) has been adapted and employed to investigate the permanent shear thinning of polystyrene solutions in a series of phthalate ester base fluids. The permanent shear stability index based on viscosities measured at 106 s−1, PSSI(106), has been found to be a convenient way to express the magnitude of permanent shear thinning. When comparing permanent shear thinning at various shear rates in the USV, it is very important to take account of the different times of shear that are present at different shear rates. The PSSI(106) value divided by the total time of shear is then a useful way of quantifying and comparing permanent shear thinning rates. Tests using polystyrene in different viscosity base fluids have shown that this rate of permanent shear thinning depends on shear stress and not shear rate and varies linearly with polymer concentration. The rate of permanent shear thinning also varies exponentially with shear stress, suggestive of a stress-promoted polymer breakdown process. By using a small volume of test fluid in the USV and solvent extraction after a test, it has proved possible to obtain molecular weight distributions of polymer after shear using gel permeation chromatography (GPC). This indicates that the polymer breakdown process is different at low and high polymer concentrations, with molecule fragmentation at low polymer concentration but mid-chain scission at high concentration. A key feature of the USV is that, unlike other methods currently used to measure permanent shear thinning behaviour of engine oils, it subjects the test fluid to well-defined, controllable high shear conditions. Coupled with the use of GPC, this makes it possible for the first time to relate quantitatively the permanent shear thinning of engine oils to shear conditions and to polymer degradation response.

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