Many Tribology Group publications are Open Access thanks to funding from the EPSRC.

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  • Journal article
    Yu X, Xu Y, Morales-Espejel G, Dunne F, Dini Det al., 2024,

    On the importance of Crystal Plasticity Finite Element discretisation for the identification of crack initiation in RCF using energy-based criteria

    , Computational Materials Science, Vol: 232, ISSN: 0927-0256

    Material microstructure plays a key role in crack initiation under rolling contact fatigue. When studying microstructure with crystal plasticity finite element method (CPFE), mesh sensitivity study is of great importance, as the surface-near region is under high uniaxial stresses. In this paper, a new structured mesh strategy is purposed and compared with the classical unstructured mesh strategy. Modelling tests on a bi-grain and a polycrystal model show the calculation of geometrically necessary dislocation (GND) density, recently proposed as a suitable fatigue damage indicator, is highly dependent on mesh morphology, when GND hotspots tend to appear near distorted elements even in homogeneous materials. With uniform mesh size and shape, structured mesh elements can provide physically more acceptable GND calculations, which is particularly important in loading scenarios with complex stresses, such as rolling contact fatigue. Computational efficiency is also improved compared to unstructured models because a smaller number of elements are required in a structured mesh model and pre-processing of the mesh is not required.

  • Journal article
    Knudsen PA, Heyes DM, Niss K, Dini D, Bailey NPet al., 2024,

    Invariant dynamics in a united-atom model of an ionic liquid.

    , J Chem Phys, Vol: 160

    We study a united-atom model of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl)sulfonylamide to determine to what extent there exist curves in the phase diagram along which the microscopic dynamics are invariant when expressed in dimensionless, or reduced, form. The initial identification of these curves, termed isodynes, is made by noting that contours of reduced shear viscosity and reduced self-diffusion coefficient coincide to a good approximation. Choosing specifically the contours of reduced viscosity as nominal isodynes, further simulations were carried out for state points on these, and other aspects of dynamics were investigated to study their degree of invariance. These include the mean-squared displacement, shear-stress autocorrelation function, and various rotational correlation functions. These were invariant to a good approximation, with the main exception being rotations of the anion about its long axis. The dynamical features that are invariant have in common that they are aspects that would be relevant for a coarse-grained description of the system; specifically, removing the most microscopic degrees of freedom in principle leads to a simplification of the potential energy landscape, which allows for the existence of isodynes.

  • Journal article
    Bhamra JS, Everhard EM, Bomidi JAR, Dini D, Ewen JPet al., 2024,

    Comparing the tribological performance of water-based and oil-based drilling fluids in diamond–rock contacts

    , Tribology Letters, Vol: 72, ISSN: 1023-8883

    Oil-based drilling fluids are usually assumed to provide lower friction compared to their water-based alternatives. However, clear evidence for this has only been presented for steel–rock and steel–steel contacts, which are representative of the interface between the drillstring and the borehole or casing. Another crucial interface that needs to be lubricated during drilling is that between the cutter (usually diamond) and the rock. Here, we present pin-on-disc tribometer experiments that show higher boundary friction for n-hexadecane-lubricated diamond–granite contacts than air- and water-lubricated contacts. Using nonequilibrium molecular dynamics simulations of a single-crystal diamond tip sliding on α-quartz, we show the same trend as in the experiments of increasing friction in the order: water < air < n-hexadecane. Analysis of the simulation results suggests that the friction differences between these systems are due to two factors: (i) the indentation depth of the diamond tip into the α-quartz substrate and (ii) the amount of interfacial bonding. The n-hexadecane system had the highest indentation depth, followed by air, and finally water. This suggests that n-hexadecane molecules reduce the hardness of α-quartz surfaces compared to water. The amount of interfacial bonding between the tip and the substrate is greatest for the n-hexadecane system, followed by air and water. This is because water molecules passivate terminate potential reactive sites for interfacial bonds on α-quartz by forming surface hydroxyl groups. The rate of interfacial bond formation increases exponentially with normal stress for all the systems. For each system, the mean friction force increases linearly with the mean number of interfacial bonds formed. Our results suggest that the expected tribological benefits of oil-based drilling fluids are not necessarily realised for cutter–rock interfaces. Further e

  • Journal article
    Ogbomo E, Bhuiyan FH, Latorre CA, Martini A, Ewen JPet al., 2024,

    Effects of surface chemistry on the mechanochemical decomposition of tricresyl phosphate.

    , Physical Chemistry Chemical Physics, Vol: 26, Pages: 278-292, ISSN: 1463-9076

    The growth of protective tribofilms from lubricant antiwear additives on rubbing surfaces is initiated by mechanochemically promoted dissociation reactions. These processes are not well understood at the molecular scale for many important additives, such as tricresyl phosphate (TCP). One aspect that needs further clarification is the extent to which the surface properties affect the mechanochemical decomposition. Here, we use nonequilibrium molecular dynamics (NEMD) simulations with a reactive force field (ReaxFF) to study the decomposition of TCP molecules confined and pressurised between sliding ferrous surfaces at a range of temperatures. We compare the decomposition of TCP on native iron, iron carbide, and iron oxide surfaces. We show that the decomposition rate of TCP molecules on all the surfaces increases exponentially with temperature and shear stress, implying that this is a stress-augmented thermally activated (SATA) process. The presence of base oil molecules in the NEMD simulations decreases the shear stress, which in turn reduces the rate constant for TCP decomposition. The decomposition is much faster on iron surfaces than iron carbide, and particularly iron oxide. The activation energy, activation volume, and pre-exponential factor from the Bell model are similar on iron and iron carbide surfaces, but significantly differ for iron oxide surfaces. These findings provide new insights into the mechanochemical decomposition of TCP and have important implications for the design of novel lubricant additives for use in high-temperature and high-pressure environments.

  • Journal article
    Yuan T, Shen L, Dini D, 2024,

    Porosity-permeability tensor relationship of closely and randomly packed fibrous biomaterials and biological tissues: Application to the brain white matter.

    , Acta Biomater, Vol: 173, Pages: 123-134

    The constitutive model for the porosity-permeability relationship is a powerful tool to estimate and design the transport properties of porous materials, which has attracted significant attention for the advancement of novel materials. However, in comparison with other materials, biomaterials, especially natural and artificial tissues, have more complex microstructures e.g. high anisotropy, high randomness of cell/fibre dimensions/position and very low porosity. Consequently, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive. To fill this gap, we start a mathematical derivation from the fundamental brain white matter (WM) formed by nerve fibres. This is augmented by a numerical characterisation and experimental validations to obtain an anisotropic permeability tensor of the brain WM as a function of the tissue porosity. A versatile microstructure generation software (MicroFiM) for fibrous biomaterial with complex microstructure and low porosity was built accordingly and made freely accessible here. Moreover, we propose an anisotropic poro-hyperelastic model enhanced by the newly defined porosity-permeability tensor relationship which precisely captures the tissues macro-scale permeability changes due to the microstructural deformation in an infusion scenario. The constitutive model, theories and protocols established in this study will both provide improved design strategies to tailor the transport properties of fibrous biomaterials and enable the non-invasive characterisation of the transport properties of biological tissues. This will lead to the provision of better patient-specific medical treatments, such as drug delivery. STATEMENT OF SIGNIFICANCE: Due to the microstructural complexity, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive, which hinders our way of tuning the fluid transport property of the biomaterials by directly programming their microstructure. The same pr

  • Journal article
    Heyes DM, Dini D, Pieprzyk S, Brańka ACet al., 2023,

    Harmonic models and molecular dynamics simulations of isomorph behavior of Lennard-Jones fluids: Excess entropy and high temperature limiting behavior.

    , J Chem Phys, Vol: 159

    Henchman's approximate harmonic model of liquids is extended to predict the thermodynamic behavior along lines of constant excess entropy ("isomorphs") in the liquid and supercritical fluid regimes of the Lennard-Jones (LJ) potential phase diagram. Simple analytic expressions based on harmonic cell models of fluids are derived for the isomorph lines, one accurate version of which only requires as input parameters the average repulsive and attractive parts of the potential energy per particle at a single reference state point on the isomorph. The new harmonic cell routes for generating the isomorph lines are compared with those predicted by the literature molecular dynamics (MD) methods, the small step MD method giving typically the best agreement over a wide density and temperature range. Four routes to calculate the excess entropy in the MD simulations are compared, which includes employing Henchman's formulation, Widom's particle insertion method, thermodynamic integration, and parameterized LJ equations of state. The thermodynamic integration method proves to be the most computationally efficient. The excess entropy is resolved into contributions from the repulsive and attractive parts of the potential. The repulsive and attractive components of the potential energy, excess Helmholtz free energy, and excess entropy along a fluid isomorph are predicted to vary as ∼T-1/2 in the high temperature limit by an extension of classical inverse power potential perturbation theory statistical mechanics, trends that are confirmed by the MD simulations.

  • Journal article
    Masen M, Cann P, 2023,

    Tribology test design for friction measurements with application to oral medicines

    , Biotribology, Vol: 35-36

    In recent years tribology tests have been used to measure friction properties of oral consumables such as semi-solid foods and medicines. The tests aim to simulate thin-film mastication conditions and are intended to correlate with mouth feel or food texture properties. In this paper a new approach is proposed to better simulate shear conditions, fluid supply and friction data capture associated with mastication and swallowing. Two primary changes are suggested: these are the reduction of the inlet influence on lubricant film properties and the ability to measure transient and time-dependent friction. The new test was used to measure friction for a range of oral medicines including a viscous solution (cough syrup) and particulate suspensions (paediatric, calcium carbonate) in combination with an artificial saliva (mucin solution), The tongue-palate was replicated by a PCX glass lens loaded and reciprocating against a textured silicone surface. A short stroke length, comparable to the Hertzian diameter of the contact, was used so the contact operated in a partially replenished lubrication condition. This ensured the film in the contact region has the same composition as the bulk fluid. Friction was measured continuously during reciprocation for up to 5 cycles (comparable to mastication time) and data was sampled at 100 Hz to capture transient friction. Tests were run with and without a mucin layer present. The results showed that tests performed after 20 min adsorption of an artificial saliva solution reduced the friction coefficient from μ = 1 to μ = 0.2–0.3. Tests with the paracetamol suspensions, which contain hard particles, recorded transient friction spikes which were not recorded for the softer calcium carbonate suspensions. Key conclusions for the design of pertinent simulation tests are that the film properties in the oral cavity are not determined by the inlet as for classical lubrication. The (bulk) oral sample is captured in the tongue-palate

  • Journal article
    Afferrante L, Violano G, Dini D, 2023,

    How does roughness kill adhesion?

    , Journal of the Mechanics and Physics of Solids, Vol: 181, ISSN: 0022-5096

    It is well-known that adhesion is strongly influenced by surface roughness. Nevertheless, the literature currently contains an ongoing debate regarding which roughness scales are primarily responsible for adhesion loss. In this study, we aim to contribute to this debate by conducting numerical simulations on self-affine fractal profiles with varying fractal dimensions. Our results reveal that the long-wavelength portion of the roughness spectrum plays a crucial role in killing adhesion when considering profiles with Hurst exponent H>0.5. Conversely, for profiles with H<0.5, results show a different trend, indicating that adhesive stickiness is also influenced by short wavelength roughness. These findings are corroborated by our recent experimental observations. In such case, adhesive hysteresis and pull-off force exhibit a continuous decrease with increasing roughness scales. However, for H>0.5, the pull-off force converges towards a finite value as the magnification increases.

  • Journal article
    S Bhamra J, P Ewen J, Ayestarán Latorre C, A R Bomidi J, W Bird M, Dini Det al., 2023,

    Atomic-scale insights into the tribochemical wear of diamond on quartz surfaces

    , Applied Surface Science, Vol: 639, Pages: 1-13, ISSN: 0169-4332

    A detailed understanding of diamond wear is crucial due to its use in high-performance cutting tools. Despite being a much harder material, diamond shows appreciable wear when cutting silicon dioxides due to a tribochemical mechanism. Here, we use nonequilibrium molecular dynamics simulations with a reactive force field to investigate the wear of single-crystal diamond tips sliding on α-quartz surfaces. Atom-by-atom attrition of carbon atoms is initiated by the formation of C-O interfacial bonds, followed by C-C cleavage, and either diffusion into the substrate or further oxidation to form CO2 molecules. Water molecules dissociate to form hydroxyl groups, which passivates the surfaces and reduces interfacial bonding and wear. At low loads, the initial wear rate increases exponentially with temperature and normal stress, consistent with stress-augmented thermally activated wear models. At higher loads, the initial wear rate becomes less sensitive to the normal stress, eventually plateauing towards a constant value. This behaviour can be described using the multibond wear model. After long sliding distances, wear also occurs through cluster detachment via tail fracture. Here, wear becomes approximately proportional to the sliding distance and normal load, consistent with the Archard model. The normalised wear rates from the simulations are within the experimentally-measured range.

  • Journal article
    Ebrahimi MT, Balint DS, Dini D, 2023,

    An analytical solution for multiple inclusions subject to a general applied thermal field

    , JOURNAL OF THERMAL STRESSES, Vol: 46, Pages: 1180-1198, ISSN: 0149-5739
  • Journal article
    Yu M, Evangelou S, Dini D, 2023,

    Advances in active suspension systems for road vehicles

    , Engineering, ISSN: 2095-8099

    Active suspension systems (ASSs) have been proposed and developed for a few decades, and nowadays again become a thriving topic in both academia and industry, due to the high demand in driving comfort and safety, and the compatibility with vehicle electrification and autonomy. Existing review papers on ASSs are mainly about dynamics modelling and robust control, however, the gap between academic research outcomes and industrial application requirements is not yet bridged, hindering most ASS research knowledge from transferring to vehicle companies. This paper comprehensively reviews advances in ASSs for road vehicles, focusing on hardware structures and control strategies. Particularly, state-of-the-art ASSs that have been recently adopted in production cars are detailed, including representative solutions of Mercedes Active Body Control and Audi Predictive Active Suspension; novel concepts that could become alternative candidates are also introduced, including the Series Active Variable Geometry Suspension, and the Active Wheel Alignment System. The ASSs with compact structure, small mass increment, low power consumption, high frequency response, acceptable economic costs and high reliability are more likely to be adopted by car manufacturers. In terms of control strategies, future ASSs not only aim to stabilize the chassis attitude and attenuate the chassis vibration, moreover, but also cooperate with other bodies (e.g., steering and braking) and sensors (e.g., camera) within a car, and even with high-level decision (e.g., reference driving speed) in the overall transportation system – these strategies will be compatible with the rapidly developed electric and autonomous vehicles.

  • Journal article
    Ardah S, Profito FJ, Reddyhoff T, Dini Det al., 2023,

    Advanced modelling of lubricated interfaces in general curvilinear grids

    , TRIBOLOGY INTERNATIONAL, Vol: 188, ISSN: 0301-679X
  • Journal article
    Fatti G, Ciniero A, Ko H, Lee HU, Na Y, Jeong CK, Lee S-G, Kwak D, Park K-I, Cho SB, Dini Det al., 2023,

    Rational Design Strategy for Triboelectric Nanogenerators Based on Electron Back Flow and Ionic Defects: The Case of Polytetrafluoroethylene

  • Journal article
    Zhang J, Wheatley A, Pasaribu R, Worthington E, Matthews S, Zinser C, Cann Pet al., 2023,

    Wind turbine lubrication: low temperature fretting wear behaviour of four commercial greases

    , Tribology International, Vol: 187, ISSN: 0301-679X

    Fretting tests on four commercial greases were run on a ball-on-disc machine at 25, 7, − 20, − 40 °C. Post-test wear was measured on the ball and the chemical composition of lubricant films in the wear scar analysed by FTIR (RA-IRS), Raman and SEM-EDS. At 25 and 7 °C the greases had similar friction and wear properties. At − 20 and − 40 °C the highest base-oil viscosity grease gave an inferior friction and wear performance. RA-IRS analysis showed thickener remaining in the wear scar for most test conditions. The highest wear was linked to the absence of thickener in the scar. Better wear and friction performance at low temperatures was associated with thickener present in the lubricated contact and lowest base-oil viscosity.

  • Journal article
    Weiand E, Rodriguez-Ropero F, Roiter Y, Koenig P, Angioletti-Uberti S, Dini D, Ewen Jet al., 2023,

    Effects of surfactant adsorption on the wettability and friction of biomimetic surfaces

    , Physical Chemistry Chemical Physics, Vol: 25, Pages: 21916-21934, ISSN: 1463-9076

    The properties of solid–liquid interfaces can be markedly altered by surfactant adsorption. Here, we use molecular dynamics (MD) simulations to study the adsorption of ionic surfactants at the interface between water and heterogeneous solid surfaces with randomly arranged hydrophilic and hydrophobic regions, which mimic the surface properties of human hair. We use the coarse-grained MARTINI model to describe both the hair surfaces and surfactant solutions. We consider negatively-charged virgin and bleached hair surface models with different grafting densities of neutral octadecyl and anionic sulfonate groups. The adsorption of cationic cetrimonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) surfactants from water are studied above the critical micelle concentration. The simulated adsorption isotherms suggest that cationic surfactants adsorb to the surfaces via a two-stage process, initially forming monolayers and then bilayers at high concentrations, which is consistent with previous experiments. Anionic surfactants weakly adsorb via hydrophobic interactions, forming only monolayers on both virgin and medium bleached hair surfaces. We also conduct non-equilibrium molecular dynamics simulations, which show that applying cationic surfactant solutions to bleached hair successfully restores the low friction seen with virgin hair. Friction is controlled by the combined surface coverage of the grafted lipids and the adsorbed CTAB molecules. Treated surfaces containing monolayers and bilayers both show similar friction, since the latter are easily removed by compression and shear. Further wetting MD simulations show that bleached hair treated with CTAB increases the hydrophobicity to similar levels seen for virgin hair. Treated surfaces containing CTAB monolayers with the tailgroups pointing predominantly away from the surface are more hydrophobic than bilayers due to the electrostatic interactions between water molecules and the exposed cationic headgrou

  • Journal article
    Kew B, Holmes M, Liamas E, Ettelaie R, Connell SD, Dini D, Sarkar Aet al., 2023,

    Transforming sustainable plant proteins into high performance lubricating microgels

  • Journal article
    Samaras G, Bikos D, Skamniotis C, Cann P, Masen M, Hardalupas Y, Vieira J, Hartmann C, Charalambides Met al., 2023,

    Experimental and computational models for simulating the oral breakdown of food due to the interaction with molar teeth during the first bite

    , Extreme Mechanics Letters, Vol: 62, Pages: 1-11, ISSN: 2352-4316

    The first bite involves the structural breakdown of foods due to the interaction with teeth and is a crucial process in oral processing. Although in vitro experiments are useful in predicting the oral response of food, they do not facilitate a mechanistic understanding of the relationship between the intrinsic food mechanical properties and the food behaviour in the oral cavity. Computer simulations, on the other hand, allow for such links to be established, offering a promising design alternative that will reduce the need for time consuming and costly in vivo and in vitro trials. Developing virtual models of ductile fracture in soft materials, such as food, with random and non-predefined crack morphology imposes many challenges. One of the most important is to derive results that do not depend on numerical parameters, such as Finite Element (FE) mesh density, but only physical constants obtained through independent standard mechanical tests, such as fracture strain and/or critical energy release rate. We demonstrate here that this challenge can be overcome if a non-local damage approach is used within the FE framework. We develop a first bite FE modelling methodology that provides mesh independent results which are also in agreement with physical first bite experiments performed on chocolate. The model accounts for key features found in chocolate and a wide range of compliant media, such as rate dependent plasticity and pressure dependent fracture initiation strain. As a result, our computational methodology can prove valuable in studying food structure-function relationships that are essential in product development.

  • Journal article
    Kirkby T, Smith JJ, Berryman J, Fowell M, Reddyhoff Tet al., 2023,

    Soot wear mechanisms in heavy-duty diesel engine contacts

    , WEAR, Vol: 524, ISSN: 0043-1648
  • Journal article
    Guo Y, di Mare L, Wong JSS, 2023,

    A statistic study on raspberry vesicles: formation and properties

    , Polymer, Vol: 280, Pages: 1-8, ISSN: 0032-3861

    This paper gives a statistic study on the formation of ABC raspberry vesicles under bulk swelling with DPD simulations. All vesicles formed through a disc wrap-up process, i.e. a disc micelle wraps up to form a vesicle. The lifetimes of the disc micelles before they become vesicles can be characterized as short and long (tfast and tslow). Vesicles formed with tfast have a high loading efficiency and a wide size distribution. Most of them have low membrane permeability. They resist structural deformation under shear due to their high bending rigidity. Vesicles formed with tslow have a narrow size distribution. They are small, and have low loading efficiency. A large portion of them have permeable membranes with low bending rigidity and structural defects. Shear could restructure these vesicles, and hence modify their permeability. Adjusting the repulsion between solvophobic polymers and solvents impacts on lifetimes of disc micelles. A reduction in such repulsion favours tslow. The knowledge obtained can be used to design raspberry vesicles of desired size, loading and cargo release properties.

  • Journal article
    Weston A, Vladescu S-C, Reddyhoff T, Griffiths A, Crouzier T, Fielden M, Garnett JA, Carpenter GHet al., 2023,

    The influence of ions on the lubricative abilities of mucin and the role of sialic acids

  • Journal article
    Ciniero A, Fatti G, Marsili M, Dini D, Righi MCet al., 2023,

    Defects drive the of PTFE: An ab-initio

    , NANO ENERGY, Vol: 112, ISSN: 2211-2855
  • Journal article
    Patino-Ramirez F, O'Sullivan C, Dini D, 2023,

    Percolating contacts network and force chains during interface shear in granular media

    , Granular Matter, Vol: 25, ISSN: 1434-5021

    The concept of force chains transmitting stress through granular materials is well established; however identification of individual force chains and the associated quantitative analysis is non-trivial. This paper proposes two algorithms to (1) find the network of percolating contacts that control the response of loaded granular media, and (2) decompose this network into the individual force chains that comprise it. The new framework is demonstrated considering data from discrete element method simulations of a ribbed interface moving against a granular sample. The subset of contacts in the material that transfers load across the sample, namely the percolating contact network (G perc), is found using the maximum flow algorithm. The resulting network is fully-connected and its maximum flow value corresponds to the force percolating the system in the direction normal to the ribbed wall. G perc re-orientates in response to the ribbed interface movement and transmits 85–95% of the stress, with only 40–65% of the contacts in the sample. Then, is split into individual force chains using a novel implementation of the widest path problem. Results show that denser materials with increased force-chain centrality promote a higher density of force chains, which results in a higher macro-scale strength during interface shearing. The contribution of force chains in the network is revealed to be highly centralized, composed by a small set of strong and long-lived force chains, plus a large set of weak and short-lived force chains.

  • Journal article
    Ueda M, Wong JSS, Spikes H, 2023,

    Influence of Dumbbell Base Oil Blends on Micropitting

    , Tribology International, Pages: 108578-108578, ISSN: 0301-679X
  • Journal article
    Weiand E, Ewen JP, Roiter Y, Koenig PH, Page SH, Rodriguez-Ropero F, Angioletti-Uberti S, Dini Det al., 2023,

    Nanoscale friction of biomimetic hair surfaces

    , Nanoscale, Vol: 15, Pages: 7086-7104, ISSN: 2040-3364

    We investigate the nanoscale friction between biomimetic hair surfaces using chemical colloidal probe atomic force microscopy experiments and nonequilibrium molecular dynamics simulations. In the experiments, friction is measured between water-lubricated silica surfaces functionalised with monolayers formed from either octadecyl or sulfonate groups, which are representative of the surfaces of virgin and ultimately bleached hair, respectively. In the simulations, friction is monitored between coarse-grained model hair surfaces with different levels of chemical damage, where a specified amount of grafted octadecyl groups are randomly replaced with sulfonate groups. The sliding velocity dependence of friction in the simulations can be described using an extended stress-augmented thermally activation model. As the damage level increases in the simulations, the friction coefficient generally increases, but its sliding velocity-dependence decreases. At low sliding velocities, which are closer to those encountered experimentally and physiologically, we observe a monotonic increase of the friction coefficient with damage ratio, which is consistent with our new experiments using biomimetic surfaces and previous ones using real hair. This observation demonstrates that modified surface chemistry, rather than roughness changes or subsurface damage, control the increase in nanoscale friction of bleached or chemically damaged hair. We expect the methods and biomimetic surfaces proposed here to be useful to screen the tribological performance of hair care formulations both experimentally and computationally.

  • Journal article
    Bartolo MK, Newman S, Dandridge O, Provaggi E, Accardi MA, Dini D, Amis Aet al., 2023,

    Ovine knee kinematics and contact pressures of a novel fibre matrix-reinforced hydrogel total meniscus replacement

    , Orthopaedic Proceedings, Vol: 105-B, Pages: 14-14
  • Journal article
    Heyes DM, Dini D, Pieprzyk S, Branka ACet al., 2023,

    Departures from perfect isomorph behavior in Lennard-Jones fluids and solids

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 158, ISSN: 0021-9606
  • Journal article
    Yuan T, Zhan W, Dini D, 2023,

    Linking fluid-axons interactions to the macroscopic fluid transport properties of the brain

    , ACTA BIOMATERIALIA, Vol: 160, Pages: 152-163, ISSN: 1742-7061
  • Journal article
    Hu S, Huang W, Li J, Reddyhoff T, Cao X, Shi X, Peng Z, Demello A, Dini Det al., 2023,

    Rigid-flexible hybrid surfaces for water-repelling and abrasion-resisting

    , FRICTION, Vol: 11, Pages: 635-646, ISSN: 2223-7690
  • Journal article
    Bikos D, Samaras G, Cann P, Masen M, Hardalupas I, Vieira J, Hartmann C, Huthwaite P, Lan B, Charalambides Met al., 2023,

    Destructive and non-destructive mechanical characterisation of chocolate with different levels of porosity under various modes of deformation

    , Journal of Materials Science, Vol: 58, Pages: 5104-5127, ISSN: 0022-2461

    Chocolate exhibits a complex material response under the varying mechanical loads present during oral processing. Mechanical properties such as Young’s modulus and fracture stress are linked to sensorial attributes such as hardness. Apart from this link with hardness perception, these mechanical properties are important input parameters towards developing a computational model to simulate the first bite. This study aims to determine the mechanical properties of chocolate with different levels of micro-aeration, 0–15%, under varying modes of deformation. Therefore, destructive mechanical experiments under tension, compression, and flexure loading are conducted to calculate the Young’s modulus, yield, and fracture stress of chocolate. The values of Young’s modulus are also confirmed by independent ultrasonic mechanical experiments. The results showed that differences up to 35% were observed amongst the Young’s modulus of chocolate for different mechanical experiments. This maximum difference was found to drop with increasing porosity and a negligible difference in the Young’s modulus measurements amongst the different mechanical experiments is observed for the 15% micro-aerated chocolate. This phenomenon is caused by micro-pores obstructing the microscopic inelastic movement occurring from the early stages of the material’s deformation. This work provides a deeper understanding of the mechanical behaviour of chocolate under different loading scenarios, which are relevant to the multiaxial loading during mastication, and the role of micro-aeration on the mechanical response of chocolate. This will further assist the food industry’s understanding of the design of chocolate products with controlled and/or improved sensory perception.

  • Journal article
    Song W, Zhang J, Campen S, Yan J, Hongbing J, Wong Jet al., 2023,

    Lubrication mechanism of a strong tribofilm by imidazolium ionic liquid

    , Friction, Vol: 11, Pages: 425-440, ISSN: 2223-7690

    Friction modifiers are surface-active additives added to base fluids to reduce frictionbetween rubbing surfaces. Their effectiveness depends on their interactions with rubbingsurfaces and may be mitigated by the choice of the base fluid. In this work, theperformance of an imidazolium ionic liquid (ImIL) additive in polyethylene-glycol (PEG)and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-likecarbon (DLC-DLC) contacts were investigated. ImIL containing PEG reduces frictionmore effectively in steel-steel than DLC-DLC contacts. In contrast, adding ImIL in1,4-butanediol results in an increase in friction in steel-steel contacts. Results fromRaman spectroscopy, XPS and FIB-TEM reveal that a surface film is formed on steelduring rubbing in ImIL containing PEG. This film consists of two layers. The top layer iscomposed of amorphous carbon and are easily removed during rubbing. The bottom layer,which contains iron oxide and nitride compound, adheres strongly on the steel surface.This film maintains its effectiveness in a steel-steel contact even after ImIL additives are2depleted. Such film is not observed in 1,4-butanediol where the adsorption of ImIL ishindered, as suggested by QCM measurements. No benefit is observed when the basefluid on its own is sufficiently lubricious, as in the case of DLC surfaces.This work provides fundamental insights on how compatibilities among base fluid,friction modifier and rubbing surface affect performance of IL as surface active additives.It reveals the structure of an ionic liquid surface film, which is effective and durable. Theknowledge is useful for guiding future IL additive development.

  • Journal article
    Xu Y, Balint D, Greiner C, Dini Det al., 2023,

    On the origin of plasticity-induced microstructure change under sliding contacts

    , Friction, Vol: 11, Pages: 473-488, ISSN: 2223-7704

    Discrete dislocation plasticity (DDP) calculations are carried out to investigate the response of a single crystal contacted by a rigid sinusoidal asperity under sliding loading conditions to look for causes of microstructure change in the dislocation structure. The mechanistic driver is identified as the development of lattice rotations and stored energy in the subsurface, which can be quantitatively correlated to recent tribological experimental observations. Maps of surface slip initiation and substrate permanent deformation obtained from DDP calculations for varying contact size and normal load suggest ways of optimally tailoring the interface and microstructural material properties for various frictional loads.

  • Journal article
    Kunzelmann B, Rycerz P, Xu Y, Arakere NK, Kadiric Aet al., 2023,

    Prediction of rolling contact fatigue crack propagation in bearing steels using experimental crack growth data and linear elastic fracture mechanics

  • Journal article
    Abdelbar M, Ewen J, Dini D, Angioletti-Uberti Set al., 2023,

    Polymer brushes for friction control: Contributions of molecular simulations

    , Biointerphases, Vol: 18, ISSN: 1934-8630

    When polymer chains are grafted to solid surfaces at sufficiently high density, they form brushes that can modify the surface properties. In particular, polymer brushes are increasingly being used to reduce friction in water-lubricated systems close to the very low levels found in natural systems, such as synovial joints. New types of polymer brush are continually being developed to improve with lower friction and adhesion, as well as higher load-bearing capacities. To complement experimental studies, molecular simulations are increasingly being used to help to understand how polymer brushes reduce friction. In this paper, we review how molecular simulations of polymer brush friction have progressed from very simple coarse-grained models toward more detailed models that can capture the effects of brush topology and chemistry as well as electrostatic interactions for polyelectrolyte brushes. We pay particular attention to studies that have attempted to match experimental friction data of polymer brush bilayers to results obtained using molecular simulations. We also critically look at the remaining challenges and key limitations to overcome and propose future modifications that could potentially improve agreement with experimental studies, thus enabling molecular simulations to be used predictively to modify the brush structure for optimal friction reduction.

  • Journal article
    Bikos D, Samaras G, Charalambides M, Cann P, Masen M, Hartmann C, Vieira J, Sergis A, Hardalupas Iet al., 2023,

    A micromechanical based finite element model approach to accurately predict the effective thermal properties of micro-aerated chocolate

    , Innovative Food Science and Emerging Technologies, Vol: 83, ISSN: 1466-8564

    Micro-aeration is a method to modify the sensorial attributes of chocolate but also affects the material properties of chocolate, which in turn, determine its material response during manufacturing and oral processes. This study aims to define the effect of micro-aeration on the thermal properties of chocolate by considering the changes of chocolate microstructure due to micro-aeration. Micro-aeration was found to alter the chocolate microstructure creating a layer of a third phase at the porous interfaces, which is argued to consist of cocoa butter of higher melting properties. A multiscale Finite Element Model is developed, which was confirmed by macroscale heat transfer measurements, to parametrically simulate the structural changes of micro-porous chocolates at the microscale level and estimate their effective properties, such as thermal conductivity and specific heat capacity. The developed multiscale computational model simulates the porous chocolate as a two-phase (chocolate- pores) or three-phase material (chocolate-cocoa butter layer- pores). The investigation identified a new, complex transient thermal mechanism that controls the behaviour of micro-aerated chocolate during melting and solidification. The results showed a maximum 13% reduction of keff and 15% increase of Cpeff with 15% micro-aeration resulting to a slower transient heat transfer through the micro-aerated chocolate. The reason is that the micro-aerated chocolate can store a larger amount of thermal energy than its solid counterpart. This effect slows down the transient heat transfer rate in the chocolate and modifies melting/solidification rate and impacts sensorial attributes during oral processing and cooling during manufacturing.

  • Journal article
    Vladescu S-C, Agurto MG, Myant C, Boehm MW, Baier SK, Yakubov GE, Carpenter G, Reddyhoff Tet al., 2023,

    Protein-induced delubrication: How plant-based and dairy proteins affect mouthfeel

    , FOOD HYDROCOLLOIDS, Vol: 134, ISSN: 0268-005X
  • Journal article
    Ardah S, Profito FJ, Dini D, 2023,

    An integrated finite volume framework for thermal elasto-hydrodynamic lubrication

    , TRIBOLOGY INTERNATIONAL, Vol: 177, ISSN: 0301-679X
  • Journal article
    Shi Y, Liu J, Li J, Xiong D, Dini Det al., 2022,

    Improved mechanical and tribological properties of PAAm/PVA hydrogel-Ti6Al4V alloy configuration for cartilage repair

    , JOURNAL OF POLYMER RESEARCH, Vol: 29, ISSN: 1022-9760
  • Journal article
    Wainwright B, Takeuchi H, Makino T, Kadiric Aet al., 2022,

    The influence of A ratio and surface roughness on the initiation and progression of micropitting damage

    , WEAR, Vol: 508, ISSN: 0043-1648
  • Journal article
    Zhang J, Yu M, Joedicke A, Reddyhoff Tet al., 2023,

    Characterising the effects of simultaneous water and gasolinedilution on lubricant performance

    , Tribology International, ISSN: 0301-679X
  • Journal article
    Jia Y, Dou P, Zheng P, Wu T, Yang P, Yu M, Reddyhoff Tet al., 2022,

    High-accuracy ultrasonic method for in-situ monitoring of oil film thickness in a thrust bearing

    , Mechanical Systems and Signal Processing, Vol: 180, Pages: 109453-109453, ISSN: 0888-3270
  • Journal article
    Bernardini A, Trovatelli M, Klosowski M, Pederzani M, Zani D, Brizzola S, Porter A, Rodriguez y Baena F, Dini Det al., 2022,

    Reconstruction of ovine axonal cytoarchitecture enables more accurate models of brain biomechanics

    , Communications Biology, Vol: 5, ISSN: 2399-3642

    There is an increased need and focus to understand how local brain microstructure affects the transport of drug molecules directly administered to the brain tissue, for example in convection-enhanced delivery procedures. This study reports a systematic attempt to characterize the cytoarchitecture of commissural, long association and projection fibres, namely the corpus callosum, the fornix and the corona radiata, with the specific aim to map different regions of the tissue and provide essential information for the development of accurate models of brain biomechanics. Ovine samples are imaged using scanning electron microscopy combined with focused ion beam milling to generate 3D volume reconstructions of the tissue at subcellular spatial resolution. Focus is placed on the characteristic cytological feature of the white matter: the axons and their alignment in the tissue. For each tract, a 3D reconstruction of relatively large volumes, including a significant number of axons, is performed and outer axonal ellipticity, outer axonal cross-sectional area and their relative perimeter are measured. The study of well-resolved microstructural features provides useful insight into the fibrous organization of the tissue, whose micromechanical behaviour is that of a composite material presenting elliptical tortuous tubular axonal structures embedded in the extra-cellular matrix. Drug flow can be captured through microstructurally-based models using 3D volumes, either reconstructed directly from images or generated in silico using parameters extracted from the database of images, leading to a workflow to enable physically-accurate simulations of drug delivery to the targeted tissue.

  • Journal article
    Bonari J, Paggi M, Dini D, 2022,

    A new finite element paradigm to solve contact problems with roughness

    , International Journal of Solids and Structures, Vol: 253, ISSN: 0020-7683

    This article's main scope is the presentation of a computational method for the simulation of contact problems within the finite element method involving complex and rough surfaces. The approach relies on the MPJR (eMbedded Profile for Joint Roughness) interface finite element proposed in [Paggi, M., Reinoso, J., 2020. Mech. Adv. Mater. Struct. 27:1731–1747], which is nominally flat but can embed at the nodal level any arbitrary height to reconstruct the displacement field due to contact in the presence of roughness. Here, the formulation is generalized to handle 3D surface height fields and any arbitrary nonlinear interface constitutive relation, including friction and adhesion. The methodology is herein validated with BEM solutions for linear elastic contact problems. Then, a selection of nonlinear contact problems prohibitive to be simulated by BEM and by standard contact algorithms in FEM are detailed, to highlight the promising aspects of the proposed method for tribology.

  • Journal article
    Dou P, Zou L, Wu T, Yu M, Reddyhoff T, Peng Zet al., 2022,

    Simultaneous measurement of thickness and sound velocity of porous coatings based on the ultrasonic complex reflection coefficient

    , NDT & E International, Vol: 131, Pages: 102683-102683, ISSN: 0963-8695
  • Journal article
    Yang X, Liu H, Dhawan S, Politis D, Zhang J, Dini D, Hu L, Gharbi M, Wang Let al., 2022,

    Digitally-enhanced lubricant evaluation scheme for hot stamping applications

    , Nature Communications, Vol: 13, ISSN: 2041-1723

    Digitally-enhanced technologies are set to transform every aspect of manufacturing. Networks of sensors that compute at the edge (streamlining information flow from devices and providing real-time local data analysis), and emerging Cloud Finite Element Analysis technologies yield data at unprecedented scales, both in terms of volume and precision, providing information on complex processes and systems that had previously been impractical. Cloud Finite Element Analysis technologies enable proactive data collection in a supply chain of, for example the metal forming industry, throughout the life cycle of a product or process, which presents revolutionary opportunities for the development and evaluation of digitally-enhanced lubricants, which requires a coherent research agenda involving the merging of tribological knowledge, manufacturing and data science. In the present study, data obtained from a vast number of experimentally verified finite element simulation results is used for a metal forming process to develop a digitally-enhanced lubricant evaluation approach, by precisely representing the tribological boundary conditions at the workpiece/tooling interface, i.e., complex loading conditions of contact pressures, sliding speeds and temperatures. The presented approach combines the implementation of digital characteristics of the target forming process, data-guided lubricant testing and mechanism-based accurate theoretical modelling, enabling the development of data-centric lubricant limit diagrams and intuitive and quantitative evaluation of the lubricant performance.

  • Journal article
    Bastola A, Stewart D, Dini D, 2022,

    Three-dimensional finite element simulation and experimental validation of sliding wear

    , WEAR, Vol: 504-505, ISSN: 0043-1648
  • Journal article
    Malik S, O'Sullivan C, Reddyhoff T, Dini D, Holmes Aet al., 2022,

    An acoustic 3D positioning system for robots operating underground

    , IEEE Sensors Letters, Vol: 6, Pages: 1-4, ISSN: 2475-1472

    Underground robots are potentially helpful in many application domains, including geotechnical engineering, agriculture, and archaeology. One of the critical challenges in developing underground robotics is the accurate estimation of the positions of the robots. Acoustic-based positioning systems have been explored for developing an underground 3D positioning system. However, the positioning range is limited due to attenuation in the medium. This letter proposes an underground positioning system that utilizes a novel and easy-to-implement electronic approach for measuringthe acoustic propagation times between multiple transmitters and a receiver. We demonstrate a prototype using four transmitters at the surface and a single buried acoustic sensor as a proof-of-concept. The times of arrival for signals emitted by the different sources are measured by correlating the transmitted and received signals. The distances between the multiple transmitters and a receiver are estimated, and a tri-linearization algorithm is used to estimate the position of the buried sensor in 3D with respect to reference coordinates. The system is tested in a soil tank. The experimental results show that the proposed system is able to estimate the 3D position of buried sensors with an error of less than ±2.5 cm within a measurement field of size 50 cm × 50 cm × 35 cm in X, Y, and Z (width × length × depth). The proposed electronic synchronization approach allows increasing the positioning range of the system by increasing the number of transmittersat the surface. This paves the way for the development of a positioning system for robots operating underground.

  • Journal article
    Ueda M, Spikes H, Kadiric A, 2022,

    In-situ observation of the effect of the tribofilm growth on scuffing in rolling-sliding contact

    , Tribology Letters, Vol: 70, Pages: 1-21, ISSN: 1023-8883

    General reductions in lubricant viscosities in many machine components mean that the role of lubricant additives in forming tribofilms has become increasingly important to provide adequate surface protection against scuffing. However, the relationship between scuffing and the formation and removal of tribofilms has not been systematically demonstrated. In this study, a step-sliding speed scuffing test based on contra-rotation using MTM-SLIM and ETM-SLIM has been employed to observe concurrently tribofilm thickness and the onset of scuffing. The initial sliding speed used was found to significantly affect scuffing performance since it determines the extent to which a tribofilm can form before critical sliding speed conditions are reached. In general, additives that formed thicker tribofilms, especially ZDDPs and triphenyl phosphate, gave effective protection against scuffing, though their protective tribofilms were progressively removed at higher sliding speeds, eventually resulting in scuffing.

  • Journal article
    Yu M, Evangelou S, Dini D, 2022,

    Parallel active link suspension: full car application with frequency-dependent multi-objective control strategies

    , IEEE Transactions on Control Systems Technology, Vol: 30, Pages: 2046-2061, ISSN: 1063-6536

    In this article, a recently proposed at basic level novel suspension for road vehicles, the parallel active link suspension (PALS), is investigated in the realistic scenario of a sport utility vehicle (SUV) full car. The involved rocker-pushrod assembly is generally optimized to maximize the PALS capability in improving the suspension performance. To fully release the PALS functions of dealing with both low- and high-frequency road cases, a PID control scheme is first employed for the chassis attitude stabilization, focusing on the minimization of both the roll and pitch angles; based on a derived linear equivalent model of the PALS-retrofitted full car, an H∞ control scheme is designed to enhance the ride comfort and road holding; moreover, a frequency-dependent multiobjective control strategy that combines the developed PID and H∞ control is proposed to enable: 1) chassis attitude stabilization at 0-1 Hz; 2) vehicle vibration attenuation at 1-8 Hz; and 3) control effort penalization (for energy saving) above 10 Hz. With a group of ISO-defined road events tested, numerical simulation results demonstrate that, compared to the conventional passive suspension, the PALS has a promising potential in full-car application, with up to 70% reduction of the chassis vertical acceleration in speed bumps and chassis leveling capability of dealing with up to 4.3-m/s² lateral acceleration.

  • Journal article
    Chennaoui M, Fowell M, Liang H, Kadiric Aet al., 2022,

    A novel set-up for in situ measurement and mapping of lubricant film thickness in a model rolling bearing using interferometry and ratiometric fluorescence imaging

    , Tribology Letters, Vol: 70, Pages: 1-17, ISSN: 1023-8883

    This paper describes a unique experimental set-up constructed for studies of lubricant behaviour in an operating rolling element bearing including in situ quantitative measurements of film thickness in and around the element-raceway contact. The set-up is based on a deep groove ball bearing in which the outer race is made of sapphire to allow full optical access to the zone in which the rolling elements are loaded against it. This allows direct imaging of lubricant films under both steady-state and transient conditions and at contact pressures and rotational speeds representative of those present in real rolling element bearings. Optical interferometry is used to measure thin EHL films inside the ball–raceway contacts while a specific laser induced fluorescence approach, referred to as ratiometric fluorescence, is implemented to observe the lubricant distribution and quantify its thickness ahead of the ball–raceway contact. Results are presented to validate the accuracy of the method and to investigate the influence of bulk lubricant viscosity and bearing speed on contact film thickness, inlet starvation and lubricant distribution around the ball–raceway contact. To the best of our knowledge, the work described here is the first to directly measure lubricant distribution and EHL film thickness in a ball–raceway contact in an operating radial rolling bearing.

  • Journal article
    Zhang X, Scaraggi M, Zheng Y, Li X, Wu Y, Wang D, Dini D, Zhou Fet al., 2022,

    Quantifying Wetting Dynamics with Triboelectrification


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