Publications

Journal article

Carman F, Ewen J, Bresme F, Wu B, Dini Det al., 2024,

Molecular Simulations of Thermal Transport across Iron Oxide-Hydrocarbon Interfaces

, ACS Applied Materials and Interfaces, ISSN: 1944-8244

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Journal article

Yang Y, Yuan T, Rodriguez y Baena F, Dini D, Zhan Wet al., 2024,

Effect of infusion direction on convection-enhanced drug delivery to anisotropic tissue

, Journal of the Royal Society Interface, Vol: 21, ISSN: 1742-5662

Convection-enhanced delivery (CED) can effectively overcome the blood–brain barrier by infusing drugs directly into diseased sites in the brain using a catheter, but its clinical performance still needs to be improved. This is strongly related to the highly anisotropic characteristics of brain white matter, which results in difficulties in controlling drug transport and distribution in space. In this study, the potential to improve the delivery of six drugs by adjusting the placement of the infusion catheter is examined using a mathematical model and accurate numerical simulations that account simultaneously for the interstitial fluid (ISF) flow and drug transport processes in CED. The results demonstrate the ability of this direct infusion to enhance ISF flow and therefore facilitate drug transport. However, this enhancement is highly anisotropic, subject to the orientation of local axon bundles and is limited within a small region close to the infusion site. Drugs respond in different ways to infusion direction: the results of our simulations show that while some drugs are almost insensitive to infusion direction, this strongly affects other compounds in terms of isotropy of drug distribution from the catheter. These findings can serve as a reference for planning treatments using CED.

Journal article

Zhu Z, Ewen JP, Kritikos EM, Giusti A, Dini Det al., 2024,

Effect of electric fields on the decomposition of phosphate esters

, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 128, Pages: 15959-15973, ISSN: 1932-7447

Phosphate esters decompose on metal surfaces and form protective polyphosphate films. For many applications, such as in lubricants for electric vehicles and wind turbines, an understanding of the effect of electric fields on molecular decomposition is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others have reported the opposite effect. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri-n-butyl phosphate (TNBP) molecules nanoconfined between ferrous surfaces (iron and iron oxide) under electrostatic fields. The reactive force field (ReaxFF) method is used to model the effects of chemical bonding and molecular dissociation. We show that the charge transfer with the polarization current equalization (QTPIE) method gives more realistic behavior compared to the standard charge equilibration (QEq) method under applied electrostatic fields. The rate of TNBP decomposition via carbon–oxygen bond dissociation is faster in the nanoconfined systems than that in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition. When electric fields are applied to the confined systems, the phosphate anions are pulled toward the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential, leading to asymmetric film growth. Analysis of the temperature- and electric field strength-dependent dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increased reactivity under an applied electric field is driven mostly by an increase in the pre-exponential factor, which is linked to the number of molecule–surface collisions. Conversely, the accelerated decomposition of TNBP on iron oxide surfaces can be attributed to a reduction in the activation

Journal article

Murali M, Cann P, Masen M, 2024,

Development of a biomimetic water-based lubricant: nanoencapsulation of stearic acid using liposomes

, Tribology International, ISSN: 0301-679X

Water-based lubricants are abundant in nature; however, they perform poorly in mechanical systems. Based on research into synovial fluid lubrication a water-based liposomal lubricant was developed using a phospholipid (DSPC) and stearic acid (SA). Compared to pure water the liposomal DSPC lubricant improved wear and friction performance, although it underperforms a low viscosity oil such as hexadecane. The inclusion of SA encapsulated within the liposome further improves the performance to a level comparable to hexadecane. Improved performance is achieved due to the following factors: a) DSPC-SA lubricant forms a growing multi bilayer structure in the contact region thereby decreasing wear b) the electrostatic charge associated with the structure allows for retention of the film within the contact area thereby decreasing friction and wear c) liposomes allow for utilisation of insoluble additives within water-based lubricants. The optimal liposomal lubricant tested demonstrates a 63% reduction of the coefficient of friction and a 99% reduction in resulting wear compared to DI water alone, thereby providing an avenue for effective utilisation of water in mechanical systems.

Journal article

Rahman M, Shen L, Ewen J, Heyes D, Dini D, Smith Eet al., 2024,

Life and death of a thin liquid film

, Communications Physics, Vol: 7, ISSN: 2399-3650

Thin films, bubbles and membranes are central to numerous natural and engineering processes, i.e., in solar cells, coatings, biosensors, foams, and emulsions. Yet, the characterization and understanding of their rupture is limited by the scarcity of atomic detail. We present here the complete life-cycle of freely suspended films using non-equilibrium molecular dynamics simulations of a simple atomic fluid free of surfactants and surface impurities, thus isolating the fundamental rupture mechanisms. We identified a short-term ‘memory’ by rewinding in time from a rupture event, extracting deterministic behaviors from apparent stochasticity. A comprehensive investigation of the key rupture-stages including both unrestrained and frustrated propagation is made—characterization of the latter leads to a first-order correction to the classical film-retraction theory. The highly resolved time window reveals that the different modes of the morphological development, typically characterized as nucleation and spinodal rupture, continuously evolve seamlessly with time from one into the other.

Journal article

Samaras G, Bikos D, Cann P, Masen M, Hardalupas Y, Hartmann C, Vieira J, Charalambides Met al., 2024,

Combining fracture mechanics and rheology to investigate the impact of micro-aeration on chocolate oral processing

, Soft Matter, Vol: 20, Pages: 5134-5152, ISSN: 1744-683X

This study presents a rigorous mechanical characterisation investigation on milk chocolate with varying porosity, at different temperature and strain rate levels. Uniaxial compression tests at temperatures varying from 20 ℃ to 30 ℃ were performed to measure the bulk properties of chocolate as a function of porosity and temperature. Fracture experiments were also conducted to compute the fracture energy at temperature levels between 20 ℃ and 30 ℃ for all tested samples. Additionally, rheological experiments are conducted to compute the viscosity of the different chocolates at 37 ℃. This combined experimental analysis of solid mechanics, fracture mechanics, and rheology aims to define the impact of temperature and chocolate’s phase change from solid to liquid on its mechanical properties. Moreover, the impact of micro-aeration on the relationship between material properties and temperature is discussed. The results demonstrate a significant impact of both temperature and micro-aeration on the chocolate’s material properties; fracture stresses decrease with micro-aeration due to the presence of micro-pores creating weak links in the chocolate matrix, the critical strain energy release rate decreases with micro-aeration at temperatures up to 25 ℃ and increases at temperatures above 30 ℃. Finally, the viscosity at 37 ℃ increases with increasing porosity due to the obstruction of the flow by micro-pores acting as “solid” particles. The results highlight how the impact of micro-aeration on the material properties of chocolate alters as the testing temperature rises and the material changes phase. The relationships between the micro-aeration and material properties and the dependence of temperature on the different mechanical properties are used to explain the difference in textural attributes as obtained from temporal dominance sensation tests. This study seeks to contribute valuable insights into the field of chocolate technology, emphasizing the n

Journal article

Schlichting M-L, Wong J, Masen M, De Pauw S, Van Hoecke H, Kadiric Aet al., 2024,

Experimental study of frictional behaviour of powdered soaps for wire drawing

, Tribology International, Vol: 195, ISSN: 0301-679X

Better understanding of frictional behaviour of powdered soaps used in dry wire drawing can lead to improvements in wire quality and large savings in energy consumption. The latter is particularly significant given the colossal amounts of energy expended in wire drawing processes. However, there is currently a very limited understanding of the subject, including a lack of quantitative data on soap friction. This is at least in part likely to be due to complex tribological behaviour of the powdered soaps, which is very different from that of lubricating oils. This paper studies frictional behaviour of sodium and calcium powdered soaps, using a ball-on-disc tribometer (ETM rig) incorporating a powder ‘scoop’, and under contact conditions pertinent to die-wire contact in wire drawing. Tests employed a WC ball and C15E steel disc to mimic die and wire materials respectively. Results show that under high sliding conditions, friction coefficient with powered soaps is remarkably low, at 0.03 and 0.04 for sodium and calcium soaps respectively. Friction generally decreased with increasing contact pressure and sliding speed. Post-test analysis of ETM specimens showed the presence of thick soap layers on the rubbed surfaces. However, in ETM tests friction was observed to sharply increase leading to the onset of scuffing, soon after the supply of soap to the contact was interrupted, suggesting that any solid deposited layers are easily removed and cannot exclusively explain the low friction. The observed trends suggest that low soap friction may be due to the ability of the soaps to form low shear stress lubricating films at high contact temperatures which are associated with the harsh conditions of high-speed and high-pressure. The results provide new insights into the frictional behaviour of powdered soaps which can be used to optimise the wire drawing processes as well as a direct input to wire deformation models.

Journal article

Bartolo MK, Newman S, Dandridge O, Halewood C, Accardi MA, Dini D, Amis Aet al., 2024,

An ovine knee simulator: description and proof of concept

, Frontiers in Bioengineering and Biotechnology, Vol: 12, ISSN: 2296-4185

Aims: The ovine stifle is an established model for evaluation of knee treatments, such as meniscus replacement. This study introduces a novel ovine gait simulator for pre-testing of surgical treatments prior to in vivo animal trials. Furthermore, we describe a pilot study that assessed gait kinematics and contact pressures of native ovine stifle joints and those implanted with a novel fiber-matrix reinforced polyvinyl alcohol-polyethylene glycol (PVA-PEG) hydrogel meniscus to illustrate the efficacy of the simulator.Methods: The gait simulator controlled femoral flexion-extension and applied a 980N axial contact force to the distal tibia, whose movement was guided by the natural ligaments. Five right ovine stifle joints were implanted with a PVA-PEG total medial meniscus replacement, fixed to the tibia via transosseous tunnels and interference screws. Six intact and five implanted right ovine stifle joints were tested for 500 k gait cycles at 1.55 Hz. Implanted stifle joint contact pressures and kinematics in the simulator were compared to the intact group. Contact pressures were measured at 55° flexion using pressure sensitive film inserted sub-meniscally. 3D kinematics were measured optically across two 30-s captures.Results: Peak contact pressures in intact stifles were 3.6 ± 1.0 MPa and 6.0 ± 2.1 MPa in the medial and lateral condyles (p < 0.05) and did not differ significantly from previous studies (p > 0.4). Medial peak implanted pressures were 4.3 ± 2.2 MPa (p > 0.4 versus intact), while lateral peak pressures (9.4 ± 0.8 MPa) were raised post medial compartment implantation (p < 0.01). The range of motion for intact joints was flexion/extension 37° ± 1°, varus/valgus 1° ± 1°, external/internal rotation 5° ± 3°, lateral/medial translation 2 ± 1 mm, anterior/posterior translation 3 ± 1 mm and distraction/compression 1 ± 1 mm. Ovine joint kinematics in t

Journal article

Ntioudis S, Ewen JP, Dini D, Turner CHet al., 2024,

PAPRECA: a parallel hybrid off-lattice kinetic MonteCarlo/molecular dynamics simulator

, Journal of Open Source Software, Vol: 9, ISSN: 2475-9066

Kinetic Monte Carlo (kMC) is an atomistic and stochastic simulation technique that capturesthe temporal evolution of various systems in materials science, chemistry, physics, biology, andengineering. Several open-source kMC packages are currently distributed online. Nevertheless,such implementations are typically lattice-based and are mostly designed to study ordered,crystalline materials. In this work, we present PArallel PREdefined CAtalog (PAPRECA), aneasy-to-use and completely lattice-free open-source kMC software suitable for simulations onamorphous materials or systems characterized by a low degree of crystallinity. PAPRECA is aparallel C++ software using the Message Passing Interface (MPI) protocol and coupled withthe Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) (Thompson et al.,2022) to enable pure kMC runs as well as hybrid kMC/Molecular Dynamics (MD) simulations.

Journal article

Weiand E, Koenig PH, Rodriguez-Ropero F, Roiter Y, Angioletti-Uberti S, Dini D, Ewen JPet al., 2024,

Boundary lubrication performance of polyelectrolyte–surfactant complexes on biomimetic surfaces

, Langmuir: the ACS journal of surfaces and colloids, Vol: 40, Pages: 7933-7946, ISSN: 0743-7463

Aqueous mixtures of oppositely charged polyelectrolytes and surfactants are useful in many industrial applications, such as shampoos and hair conditioners. In this work, we investigate the friction between biomimetic hair surfaces in the presence of adsorbed complexes formed from cationic polyelectrolytes and anionic surfactants in an aqueous solution. We apply nonequilibrium molecular dynamics (NEMD) simulations using the coarse-grained MARTINI model. We first developed new MARTINI parameters for cationic guar gum (CGG), a functionalized, plant-derived polysaccharide. The complexation of CGG and the anionic surfactant sodium dodecyl sulfate (SDS) on virgin and chemically damaged biomimetic hair surfaces was studied using a sequential adsorption approach. We then carried out squeeze-out and sliding NEMD simulations to assess the boundary lubrication performance of the CGG–SDS complex compressed between two hair surfaces. At low pressure, we observe a synergistic friction behavior for the CGG–SDS complex, which gives lower shear stress than either pure CGG or SDS. Here, friction is dominated by viscous dissipation in an interfacial layer comprising SDS and water. At higher pressures, which are probably beyond those usually experienced during hair manipulation, SDS and water are squeezed out, and friction increases due to interdigitation. The outcomes of this work are expected to be beneficial to fine-tune and screen sustainable hair care formulations to provide low friction and therefore a smooth feel and reduced entanglement.

Journal article

Yu M, Evangelou S, Dini D, 2024,

Advances in active suspension systems for road vehicles

, Engineering, Vol: 33, Pages: 160-177, 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

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

, JOURNAL OF CHEMICAL PHYSICS, Vol: 160, ISSN: 0021-9606

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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 BIOMATERIALIA, Vol: 173, Pages: 123-134, ISSN: 1742-7061

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Journal article

Heyes DM, Dini D, Pieprzyk S, Branka ACet al., 2023,

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

, JOURNAL OF CHEMICAL PHYSICS, Vol: 159, ISSN: 0021-9606

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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

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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

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, theliterature currently contains an ongoing debate regarding which roughness scales are primarilyresponsible for adhesion loss. In this study, we aim to contribute to this debate by conductingnumerical simulations on self-affine fractal profiles with varying fractal dimensions.Our results reveal that the long-wavelength portion of the roughness spectrum plays acrucial 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 adhesivestickiness is also influenced by short wavelength roughness. These findings are corroborated byour recent experimental observations. In such case, adhesive hysteresis and pull-off force exhibita continuous decrease with increasing roughness scales. However, for 𝐻 > 0.5, the pull-off forceconverges towards a finite value as the magnification increases.

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

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

, ADVANCED ELECTRONIC MATERIALS, ISSN: 2199-160X

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

, Nature Communications, Vol: 14, ISSN: 2041-1723

With the resource-intensive meat industry accounting for over 50% of food-linked emissions, plant protein consumption is an inevitable need of the hour. Despite its significance, the key barrier to adoption of plant proteins is their astringent off-sensation, typically associated with high friction and consequently poor lubrication performance. Herein, we demonstrate that by transforming plant proteins into physically cross-linked microgels, it is possible to improve their lubricity remarkably, dependent on their volume fractions, as evidenced by combining tribology using biomimetic tongue-like surface with atomic force microscopy, dynamic light scattering, rheology and adsorption measurements. Experimental findings which are fully supported by numerical modelling reveal that these non-lipidic microgels not only decrease boundary friction by an order of magnitude as compared to native protein but also replicate the lubrication performance of a 20:80 oil/water emulsion. These plant protein microgels offer a much-needed platform to design the next-generation of healthy, palatable and sustainable foods.

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

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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

, COLLOIDS AND SURFACES B-BIOINTERFACES, Vol: 227, ISSN: 0927-7765

Journal article

Ueda M, Wong JSS, Spikes H, 2023,

Influence of dumbbell base oil blends on micropitting

, Tribology International, Vol: 185, ISSN: 0301-679X

Micropitting is a type of surface fatigue that occurs in rolling-sliding contacts. It results from stress fluctuations caused by surface asperity interactions. Both the number and severity of asperity interactions increase at low lambda ratios, so it is becoming a significant challenge to develop low viscosity lubricants for gears and rolling bearings that mitigate micropitting effectively. In this paper, the influence on micropitting of dumbbell blends of PAOs and PIBs with an equivalent kinematic viscosity at test temperature is studied. The result shows that appropriate blends of PAOs and PIBs are able to provide thick oil film thicknesses and reduce micropitting while maintaining low viscosity. The insights here can help with the design of lubricants that are effective in controlling micropitting.

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

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Citations: 1

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

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

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Citations: 1

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

, International Journal of Fatigue, Vol: 168, ISSN: 0142-1123

Rolling contact fatigue (RCF) is a major life limiting factor for machine elements that employ non-conformal, rolling sliding, lubricated contacts such as rolling bearings and gears. This paper explores the application of linear elastic fracture mechanics (LEFM) principles, as commonly used in structural fatigue, for prediction of RCF crack propagation. A triple-disc contact fatigue machine is used to generate RCF cracks of varying lengths in AISI 52100 bearing steel roller specimens. Crack propagation rates across the surface are measured using optical inspection of test specimens and the final crack geometry is established through specimen sectioning. A numerical finite element model of surface breaking RCF cracks based on LEFM methods is devised to predict the evolution of stress intensity factors (SIFs) during over-rolling of the contact over the experimentally observed crack geometries. The model employs a suitable fracture mechanics mesh to resolve stresses at the crack tip and accounts for Hertzian contact stresses, contact friction and crack face friction. Potential effects of lubricant pressurisation within the crack are not modelled. The predicted SIFs are then related to the experimentally measured crack propagation rates to establish the applicability of the LEFM principles to RCF crack propagation. Results show that LEFM can be used to predict the growth of surface braking RCF cracks. For cracks longer than about 100 μm, a Paris law relationship with the stress intensity exponent of about 4 is derived. Mode II was seen to be the dominant mode of propagation for surface braking RCF cracks. Mode I SIFs are much smaller but can exhibit significant values when the contact is located just ahead of the crack mouth. Decreasing the crack face friction significantly increases mode II stress intensity suggesting that this is one important mechanism by which lubricant entry into the surface braking crack can accelerate its propagation. The findings can help in

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

Ardah S, Profito FJ, Dini D, 2023,

An integrated finite volume framework for thermal elasto-hydrodynamic lubrication

, TRIBOLOGY INTERNATIONAL, Vol: 177, ISSN: 0301-679X

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Citations: 2

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

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Citations: 2

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

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Citations: 2

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: 508-509, ISSN: 0043-1648

Micropitting is a major failure mode in gears and rolling bearings. Despite its practical importance, there currently exist no universally accepted design guidelines for its prevention primarily due to the great number of influencing parameters. In the absence of an established criterion, the Λ-ratio (the ratio of lubricant film thickness to surface roughness) is often used as a simple way to assess the risk of micropitting. In this paper we present new data to establish and decouple the individual effects of two of the most important influencing parameters in micropitting: the surface roughness amplitude and Λ-ratio. The experiments are conducted on a triple disc contact fatigue rig and carefully designed to decouple the effect of roughness from that of the Λ-ratio. To isolate the influence of the Λ ratio, we use specimens with the same and very tightly controlled surface roughness and then vary the film thickness by blending different viscosities of the same PAO base oil while keeping all other influential parameters constant. To isolate the influence of surface roughness, we keep the Λ ratio the same using the same PAO oils with appropriate viscosities while changing the RMS roughness of the specimen and keeping other roughness parameters as constant as possible. All tests use case carburised 16MnCr5 gear steel specimens. Results show that higher Λ ratio at fixed roughness produces less micropitting as may intuitively be expected. More importantly, at a fixed Λ ratio the amount of micropitting damage was extremely sensitive to the actual surface roughness, with lower roughness both lengthening the micropitting incubation phase and decreasing the rate of material loss in the micropitting wear phase. This shows that Λ ratio on its own cannot be used to assess the risk of micropitting because it is not able to account for the effects of different roughness levels on micropitting, despite including roughness in

Journal article

Zhang J, Yu M, Joedicke A, Reddyhoff Tet al., 2023,