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

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  • 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, ISSN: 0022-2461
  • 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
    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
    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
    Kirkby T, Smith JJ, Berryman J, Fowell M, Reddyhoff Tet al., 2023,

    Soot wear mechanisms in heavy-duty diesel engine contacts

    , Wear, Pages: 204733-204733, ISSN: 0043-1648
  • Journal article
    Yu M, Evangelou S, Dini D,

    Advances in Active Suspension Systems for Road Vehicles

    , Engineering
  • Journal article
    Yuan T, Zhan W, Dini D, 2023,

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

    , Acta Biomater

    Many brain disorders, including Alzheimer's Disease and Parkinson's Disease, and drug delivery procedures are linked to fluid transport in the brain; yet, while neurons are extremely soft and can be easily deformed, how the microscale channel flow interacts with the neuronal structures (especially axons) deformation and how these interactions affect the macroscale tissue function and transport properties is poorly understood. Misrepresenting these relationships may lead to the erroneous prediction of e.g. disease spread, drug delivery, and nerve injury in the brain. However, understanding fluid-neuron interactions is an outstanding challenge because the behaviours of both phases are not only dynamic but also occur at an extremely small length scale (the width of the flow channel is ∼100 nm), which cannot be captured by state-of-the-art experimental techniques. Here, by explicitly simulating the dynamics of the flow and axons at the microstructural level, we, for the first time, establish the link between micromechanical tissue response to the physical laws governing the macroscopic transport property of the brain white matter. We found that interactions between axons and the interstitial flow are very strong, thus playing an essential role in the brain fluid/mass transport. Furthermore, we proposed the first anisotropic pressure-dependent permeability tensor informed by microstructural dynamics for more accurate brain modelling at the macroscale, and analysed the effect of the variation of the microstructural parameters that influence such tensor. These findings will shed light on some unsolved issues linked to brain functions and medical treatments relying on intracerebral transport, and the mathematical model provides a framework to more realistically model the brain and design brain-tissue-like biomaterials. STATEMENT OF SIGNIFICANCE: This study reveals how neurons interact with the fluid flowing around them and how these microscale interactions affect m

  • 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
    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
    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
  • Book
    Liskiewicz T, Dini D, 2022,

    Fretting Wear and Fretting Fatigue Fundamental Principles and Applications

    , Publisher: Elsevier, ISBN: 9780128240977

    Fretting Wear and Fretting Fatigue: Fundamental Principles and Applications takes a combined mechanics and materials approach, providing readers with a fundamental understanding of fretting phenomena, related modeling and experimentation ...

  • 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
    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
    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
    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
    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
    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
    Hu S, Huang W, Li J, Reddyhoff T, Cao X, Shi X, Peng Z, Demello A, Dini Det al., 2022,

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

    , FRICTION, Vol: 11, Pages: 635-646, ISSN: 2223-7690
  • Journal article
    Yap KK, Fukuda K, Vail JR, Wong J, Masen MAet al., 2022,

    Spatiotemporal mapping for in-situ and real-time tribological analysis in polymer-metal contacts

    , Tribology International, Vol: 171, Pages: 1-16, ISSN: 0301-679X

    Spatiotemporal mapping (SMA) is a graphical technique to visualise the evolution of data with time and space during a process. This paper discusses the benefits of SMA in the field of polymer tribology via two highly different polymer/metal sliding systems. The SMA is found useful for the qualitative and quantitative characterisation and analysis of the transfer phenomena at the contact interface during repeated sliding, e.g., the slide-roll mechanism of transfer lumps, the severe-to-mild wear transition due to the tribo-chemical reaction of PTFE, the accumulation of wear debris, and the formation of friction-reducing back-transfer polyimide films. Additionally, the SMA helps spot various abnormal tribological behaviours, such as the local removal of oxides on a misaligned disc that would otherwise be overlooked.

  • Journal article
    Hu S, Cao X, Reddyhoff T, Ding X, Shi X, Dini D, deMello AJ, Peng Z, Wang Zet al., 2022,

    Pneumatic programmable superrepellent surfaces

    , Droplet, Vol: 1, Pages: 48-55, ISSN: 2731-4375
  • Journal article
    Ueda M, Kadiric A, Spikes H, 2022,

    Influence of PMA on the anti-scuffing properties of AW/EP additives

    , Tribology International, Vol: 174, Pages: 1-11, ISSN: 0301-679X

    Scuffing is becoming a quite common failure mode in gears and bearings. It has been shown that AW/EP additives are effective in preventing scuffing, but only if they are able to form a thick tribofilm before encountering severe scuffing-type conditions. This study has employed a contra-rotating, step-sliding speed scuffing test to explore the impact of PMAs on the ability of ZDDP and a commercial SP additive-containing package to prevent scuffing when subjected to immediately severe conditions. It is found that some PMAs can greatly enhance the anti-scuffing performance of these AW/EP additives. They do this by forming thick, adsorbed boundary films that can withstand high speed sliding conditions and protect the rubbing surfaces long enough for tribofilms to form.

  • 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
    Yuan T, Zhan W, Jamal A, Dini Det al., 2022,

    On the microstructurally driven heterogeneous response of brain white matter to drug infusion pressure

    , Biomechanics and Modeling in Mechanobiology, Vol: 21, ISSN: 1617-7959

    Delivering therapeutic agents into the brain via convection-enhanced delivery (CED), a mechanically controlled infusion method, provides an efficient approach to bypass the blood–brain barrier and deliver drugs directly to the targeted focus in the brain. Mathematical methods based on Darcy’s law have been widely adopted to predict drug distribution in the brain to improve the accuracy and reduce the side effects of this technique. However, most of the current studies assume that the hydraulic permeability and porosity of brain tissue are homogeneous and constant during the infusion process, which is less accurate due to the deformability of the axonal structures and the extracellular matrix in brain white matter. To solve this problem, a multiscale model was established in this study, which takes into account the pressure-driven deformation of brain microstructure to quantify the change of local permeability and porosity. The simulation results were corroborated using experiments measuring hydraulic permeability in ovine brain samples. Results show that both hydraulic pressure and drug concentration in the brain would be significantly underestimated by classical Darcy’s law, thus highlighting the great importance of the present multiscale model in providing a better understanding of how drugs transport inside the brain and how brain tissue responds to the infusion pressure. This new method can assist the development of both new drugs for brain diseases and preoperative evaluation techniques for CED surgery, thus helping to improve the efficiency and precision of treatments for brain diseases.

  • Journal article
    Collard B, Giuliani F, Ingenbleek G, Verbist G, Dini Det al., 2022,

    A fracture mechanics analysis of the micromechanical events in finite thickness fibre push-out tests

    , Theoretical and Applied Fracture Mechanics, Vol: 121, Pages: 103441-103441, ISSN: 0167-8442

    Understanding the micromechanical events of interfacial failure in fibre reinforced composites is vital to accurately characterising micromechanical properties and, consequently, the macroscopic properties of the composite. A fracture mechanics model of the fibre push-out test is developed, with an emphasis on the effect of sample thickness and residual stresses on the mechanisms of interfacial crack advancement. The model is applied to both a SiC-SiC ceramic matrix composite and a SiC-Ti metal matrix composite. The model demonstrates that previous assumptions about the micromechanical events of interfacial cracking are consistent with the measured values of interfacial fracture energy for ceramic matrix composites. Moreover, the model can identify the range of geometries for which different micromechanical cracking mechanisms occur simultaneously in a given material system. Identifying this range is important in choosing the sample geometry for fibre push-out testing because the interaction of advancing cracks affects the measurement of interfacial fracture energy by classical models.

  • 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

  • Journal article
    Campen S, Fong J, Song W, Wong Jet al., 2022,

    Thermal degradation of n-hexadecane base oil and its impact on boundary friction and surface adsorption

    , Tribology International, Vol: 170, ISSN: 0301-679X

    The chemical and physical properties of lubricants can alter during use. High temperatures may cause thermal autooxidation of the base oil, which could impact the performance of lubricant additives. Here, the effect of high temperature on the properties of n-hexadecane base oil is investigated. n-Hexadecane undergoes an irreversible transition from high to low boundary friction at 122–134 °C when heated in air. FTIR, UV–vis and NMR spectroscopy indicate the presence of carbonyl- and hydroxyl-containing oxidation products (carboxylic acids, alcohols, esters, ketones and aldehydes). ATR-FTIR shows that iron carboxylates form exclusively inside and around the rubbed friction surface. QCM-D is used to investigate the adsorption of degradation products onto an iron(III) oxide surface and reveals that almost half the adsorbed mass is effectively irreversibly adsorbed.

  • Journal article
    Dou P, Wu T, Luo Z, Yang P, Peng Z, Yu M, Reddyhoff Tet al., 2022,

    A finite-element-aided ultrasonic method for measuring central oil film thickness in a roller-raceway tribo-pair

    , Friction, Vol: 10, Pages: 944-962, ISSN: 2223-7690

    Roller bearings support heavy loads by riding on an ultra-thin oil film (between the roller and theraceway), the thickness of which is critical as it reflects the lubrication performance. Ultrasonicinterfacial reflection, which facilitates a non-destructive measurement of oil film thickness, has beenwidely studied. However, insufficient spatial resolution around the rolling line contact zone remainsa barrier, despite the employment of miniature piezoelectric transducers. In this paper, a finiteelement-aided method is utilized to simulate wave propagation through a three-layered structureof roller-oil-raceway under elastohydrodynamic lubrication (EHL), with nonlinear characteristicsof i) the deformed curvature of the cylindrical roller and ii) the non-uniform distribution of fluid bulkmodulus along the circumference of the oil layer taken into account. A load- and speed-dependentlook-up table is then developed to establish an accurate relationship between the overall reflectioncoefficient (directly measured by an embedded ultrasonic transducer) and the objective variable ofcentral oil film thickness. Moreover, the proposed finite-element-aided method is verifiedexperimentally in a roller-raceway test rig, with the ultrasonic measured oil film thicknessessentially corresponding to the calculated values by EHL theory.

  • Journal article
    Dou P, Zheng P, Jia Y, Wu T, Yu M, Reddyhoff T, Liao W-H, Peng Zet al., 2022,

    Ultrasonic measurement of oil film thickness in a four-layer structure for applications including sliding bearings with a thin coating

    , NDT & E International, Pages: 102684-102684, ISSN: 0963-8695
  • Journal article
    Zhang J, Ewen J, Spikes H, 2022,

    Substituent effects on the mechanochemical response of zinc dialkyldithiophosphate

    , Molecular Systems Design & Engineering, Vol: 7, Pages: 1045-1055, ISSN: 2058-9689

    Mechanochemistry is known to play a key role in the function of some lubricant additives, such as the tribofilm growth of zinc dialkyldithiophosphate (ZDDP). This raises the intriguing possibility of tailoring the mechanochemical response of additives by modifying their alkyl substituents. Here, we study the tribofilm formation rate of ZDDPs containing several different alkyl groups on steel surfaces from a high-friction base oil. We use macroscale tribometer experiments under full-film elastohydrodynamic lubrication conditions to enable careful control of the temperature and stress during tribofilm growth. We show how the chain length and the presence of branches or bulky cycloaliphatic groups can lead to large differences in the temperature- and stress-dependencies of the tribofilm formation rate, which can be explained through variations in packing density, steric hindrance, and stress transmission efficiency. Our rate data are successfully fitted using the Bell model; a simple modification of the Arrhenius equation that is commonly employed to model the kinetics of mechanochemical processes. Using this model, we observe large differences in the activation energy, pre-exponential factor, and activation volume for the various ZDDPs. Our findings show how structure–performance relationships can be identified for lubricant additives, which may be useful to optimise their molecular structure.

  • Journal article
    Ewen J, Maffioli L, Smith E, Daivis P, Dini D, Todd Bet al., 2022,

    Slip and stress from low strain-rate nonequilibrium molecular dynamics: The transient-time correlation function technique

    , The Journal of Chemical Physics, Vol: 156, Pages: 1-11, ISSN: 0021-9606

    We derive the transient-time correlation function (TTCF) expression for the computation of phase variables of inhomogenous confined atomistic fluids undergoing boundary-driven planar shear (Couette) flow at constant pressure. Using nonequilibrium molecular dynamics simulations, we then apply the TTCF formalism to the computation of the shear stress and the slip velocity for atomistic fluids at realistic low shear rates, in systems under constant pressure and constant volume. We show that, compared to direct averaging of multiple trajectories, the TTCF method dramatically improves the accuracy of the results at low shear rates and that it is suitable to investigate the tribology and rheology of atomistically detailed confined fluids at realistic flow rates.

  • Journal article
    Bikos D, Samaras G, Charalambides M, Cann P, Masen M, Hartmann C, Vieira J, Sergis A, Hardalupas Yet al., 2022,

    Experimental and numerical evaluation of the effect of micro-aeration on the thermal properties of chocolate

    , Food and Function, Vol: 13, Pages: 4993-5010, ISSN: 2042-6496

    Thermal properties, such as thermal conductivity, specific heat capacity and latent heat, influence the melting and solidification of chocolate. The accurate prediction of these properties for micro-aerated chocolate products with varying levels of porosity ranging from 0% to 15% is beneficial for understanding and control of heat transfer mechanisms during chocolate manufacturing and food oral processing. The former process is important for the final quality of chocolate and the latter is associated with sensorial attributes, such as grittiness, melting time and flavour. This study proposes a novel multiscale Finite Element Model to accurately predict the temporal and spatial evolution of temperature across chocolate samples. The model is evaluated via heat transfer experiments at temperatures varying from 16 °C to 45 °C. Both experimental and numerical results suggest that the rate of heat transfer within the micro-aerated chocolate is reduced by 7% when the 15% micro-aerated chocolate is compared to its solid counterpart. More specifically, on average, the thermal conductivity decreased by 20% and specific heat capacity increased by 10% for 15% micro-aeration, suggesting that micro-pores act as thermal barriers to heat flow. The latter trend is unexpected for porous materials and thus the presence of a third phase at the pore’s interface is proposed which might store thermal energy leading to a delayed release to the chocolate system. The developed multiscale numerical model provides a design tool to create pore structures in chocolate with optimum melting or solidifying response.

  • Journal article
    Rahman M, Shen L, Ewen J, Dini D, Smith Eet al., 2022,

    The intrinsic fragility of the liquid-vapor interface: a stress network perspective

    , Langmuir: the ACS journal of surfaces and colloids, Vol: 38, Pages: 4669-4679, ISSN: 0743-7463

    The evolution of the liquid-vapour interface of a Lennard-Jones fluid is examined with molecular dynamics simulations using the intrinsic sampling method. Results suggest, in agreement with capillary wave theory, clear damping of the density profiles as the temperature is increased. We identify a linear variation of the space-filling nature (fractal dimension) of the stress-clusters at the intrinsic surface with increasing surface tension, or equivalently, with decreasing temperature. A percolation analysis of these stress networks indicates that the stress field is more disjointed at higher temperatures. This leads to more fragile interfaces that result in a reduction in surface tension at higher temperature.

  • Journal article
    Beamish S, Reddyhoff T, Hunter A, Dwyer-Joyce RSet al., 2022,

    A method to determine acoustic properties of solids and its application to measuring oil film thickness in bearing shells of unknown composition

    , MEASUREMENT, Vol: 195, ISSN: 0263-2241
  • Journal article
    Kadiric A, Shore J, Christodoulias AI, Kolekar AS, Lockwood FEet al., 2022,

    Prediction of electric vehicle transmission efficiency using a new thermally coupled lubrication model

    , SAE Technical Papers, Pages: 1-20, ISSN: 0148-7191

    We present a new method to predict the power losses in electric vehicle (EV) transmission systems using a thermally coupled gearbox efficiency model. Friction losses in gear teeth contacts are predicted using an iterative procedure to account for the thermal coupling between the tooth temperature, oil viscosity, film thickness, friction, and oil rheology during a gear mesh cycle. Crucially, the prediction of the evolution of the coefficient of friction (COF) along the path of contact incorporates measured lubricant rheological parameters as well as measured boundary friction. This allows the model to differentiate between nominally similar lubricants in terms of their impact on EV transmission efficiency. Bearing and gear churning losses are predicted using existing empirical relationships. The effects of EV motor cooling and heat transfers in the heat exchanger on oil temperature are considered. Finally, heat transfer to the surroundings is accounted for so that the evolution of gearbox temperature over any given drive cycle can be predicted. The general approach presented here is applicable to any automotive gearbox while incorporating features specific to EVs. The model predictions are compared to real road measurements made on a popular current EV, and good agreement is shown over a range of road conditions. It should be noted that at high input speeds, the current model somewhat overpredicts the gearbox losses due to limitations in existing empirical bearing and churning loss models. Analyses of transmission losses breakdown at constant input power show that at low speeds/high torques, it is the losses in the gear meshes and high-load bearings that are most significant whereas at high speeds/low torques the losses in high-speed input shaft bearings, as well as gear churning losses, become more important. It is shown that the gearbox losses can account for 15-25% of the overall power losses in an EV depending on road conditions; a much higher proportion than in

  • Journal article
    Vladescu S-C, Tadokoro C, Miyazaki M, Reddyhoff T, Nagamine T, Nakano K, Sasaki S, Tsujii Yet al., 2022,

    Exploiting the Synergy between Concentrated Polymer Brushes and Laser Surface Texturing to Achieve Durable Superlubricity

    , ACS APPLIED MATERIALS & INTERFACES, Vol: 14, Pages: 15818-15829, ISSN: 1944-8244
  • Journal article
    Desai N, Masen M, Cann P, Hanson B, Tuleu C, Orlu Met al., 2022,

    Modernising Orodispersible Film Characterisation to Improve Palatability and Acceptability Using a Toolbox of Techniques

    , PHARMACEUTICS, Vol: 14
  • Journal article
    Heyes DM, Dini D, 2022,

    Intrinsic viscuit probability distribution functions for transport coefficients of liquids and solids

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 156, ISSN: 0021-9606
  • Journal article
    Yuan T, Gao L, Zhan W, Dini Det al., 2022,

    Effect of Particle Size and Surface Charge on Nanoparticles Diffusion in the Brain White Matter

    , PHARMACEUTICAL RESEARCH, Vol: 39, Pages: 767-781, ISSN: 0724-8741
  • Journal article
    Jamal A, Yuan T, Galvan S, Castellano A, Riva M, Secoli R, Falini A, Bello L, Rodriguez y Baena F, Dini Det al., 2022,

    Insights into infusion-based targeted drug delivery in brain: perspectives, challenges and opportunities

    , International Journal of Molecular Sciences, Vol: 23, Pages: 3139-3139, ISSN: 1422-0067

    Targeted drug delivery in the brain is instrumental in the treatment of lethal brain diseases, such as glioblastoma multiforme, the most aggressive primary central nervous system tumour in adults. Infusion-based drug delivery techniques, which directly administer to the tissue for local treatment, as in convection-enhanced delivery (CED), provide an important opportunity; however, poor understanding of the pressure-driven drug transport mechanisms in the brain has hindered its ultimate success in clinical applications. In this review, we focus on the biomechanical and biochemical aspects of infusion-based targeted drug delivery in the brain and look into the underlying molecular level mechanisms. We discuss recent advances and challenges in the complementary field of medical robotics and its use in targeted drug delivery in the brain. A critical overview of current research in these areas and their clinical implications is provided. This review delivers new ideas and perspectives for further studies of targeted drug delivery in the brain.

  • Journal article
    Trovatelli M, Spediacci C, Castellano A, Bernardini A, Dini D, Malfassi L, Pieri V, Falini A, Ravasio G, Riva M, Bello L, Brizzola S, Zani DDet al., 2022,

    Morphometric study of the ventricular indexes in healthy ovine BRAIN using MRI.

    , BMC Veterinary Research, Vol: 18, Pages: 97-97, ISSN: 1746-6148

    BACKGROUND: Sheep (Ovis aries) have been largely used as animal models in a multitude of specialties in biomedical research. The similarity to human brain anatomy in terms of brain size, skull features, and gyrification index, gives to ovine as a large animal model a better translational value than small animal models in neuroscience. Despite this evidence and the availability of advanced imaging techniques, morphometric brain studies are lacking. We herein present the morphometric ovine brain indexes and anatomical measures developed by two observers in a double-blinded study and validated via an intra- and inter-observer analysis. RESULTS: For this retrospective study, T1-weighted Magnetic Resonance Imaging (MRI) scans were performed at 1.5 T on 15 sheep, under general anaesthesia. The animals were female Ovis aries, in the age of 18-24 months. Two observers assessed the scans, twice time each. The statistical analysis of intra-observer and inter-observer agreement was obtained via the Bland-Altman plot and Spearman rank correlation test. The results are as follows (mean ± Standard deviation): Indexes: Bifrontal 0,338 ± 0,032 cm; Bicaudate 0,080 ± 0,012 cm; Evans' 0,218 ± 0,035 cm; Ventricular 0,241 ± 0,039 cm; Huckman 1693 ± 0,174 cm; Cella Media 0,096 ± 0,037 cm; Third ventricle ratio 0,040 ± 0,007 cm. Anatomical measures: Fourth ventricle length 0,295 ± 0,073 cm; Fourth ventricle width 0,344 ± 0,074 cm; Left lateral ventricle 4175 ± 0,275 cm; Right lateral ventricle 4182 ± 0,269 cm; Frontal horn length 1795 ± 0,303 cm; Interventricular foramen left 1794 ± 0,301 cm; Inte

  • Journal article
    Yu M, Reddyhoff T, Dini D, Holmes A, O'Sullivan Cet al., 2022,

    Acoustic emission enabled particle size estimation via low stress-varied axial interface shearing

    , IEEE Transactions on Instrumentation and Measurement, Vol: 71, ISSN: 0018-9456

    Acoustic emission (AE) refers to a rapid release of localized stress energy that propagates as a transient elastic wave and is typically used in geotechnical applications to study stick-slip during shearing, and breakage and fracture of particles. This article develops a novel method of estimating the particle size, an important characteristic of granular materials, using axial interface shearing-induced AE signals. Specifically, a test setup that enables axial interface shearing between a one-dimensional compression granular deposit and a smooth shaft surface is developed. The interface sliding speed (up to 3mm/s), the compression stress (0-135kPa), and the particle size (150μm-5mm) are varied to test the acoustic response. The start and end moments of a shearing motion, between which a burst of AE data is produced, are identified through the variation of the AE count rates, before key parameters can be extracted from the bursts of interests. Linear regression models are then built to correlate the AE parameters with particle size, where a comprehensive evaluation and comparison in terms of estimation errors is performed. For granular samples with a single size, it is found that both the AE energy related parameters and AE counts, obtained using an appropriate threshold voltage, are effective in differentiating the particle size, exhibiting low fitting errors. The value of this technique lies in its potential application to field testing, for example as an add-on to cone penetration test systems and to enable in-situ characterization of geological deposits.

  • Journal article
    Bahshwan M, Gee M, Nunn J, Myant CW, Reddyhoff Tet al., 2022,

    In situ observation of anisotropic tribological contact evolution in 316L steel formed by selective laser melting

    , Wear, Vol: 490-491, Pages: 1-12, ISSN: 0043-1648

    A consensus on the tribological performance of components by additive-versus conventional manufacturing has not been achieved; mainly because the tribological test set-ups thus far were not suited for investigating the underlying microstructure's influence on the tribological properties. As a result, utilization of additive manufacturing techniques, such as selective laser melting (SLM), for tribological applications remains questionable. Here, we investigate the anisotropic tribological response of SLM 316L stainless steel via in situ SEM reciprocating micro-scratch testing to highlight the microstructure's role. As-built 316L SLM specimens were compared against annealed wire-drawn 316L. We found that: (i) microgeometric conformity was the main driver for achieving steady-state friction, (ii) the anisotropic friction of the additively manufactured components is limited to the break-in and is caused by the lack of conformity, (iii) the cohesive bonds, whose strength is proportional to frictional forces, are stronger in the additively manufactured specimens likely due to the dislocation-dense, cellular structures, (iv) low Taylor-factor grains with large dimension stimulate microcutting in the form of long, thin sheets with serrated edges. These findings uncover some microstructurally driven tribological complexities when comparing additive to conventional manufacturing.

  • Journal article
    Weiand E, Ewen J, Koenig P, Roiter Y, Page S, Angioletti-Uberti S, Dini Det al., 2022,

    Coarse-grained molecular models of the surface of hair

    , Soft Matter, Vol: 2022, ISSN: 1744-683X

    We present a coarse-grained molecular model of the surface of human hair, which consists of a supported lipid monolayer, in the MARTINI framework. Using coarse-grained molecular dynamics (MD) simulations, we identify a lipid grafting distance that yields a monolayer thickness consistent with both atomistic MD simulations and experimental measurements of the hair surface. Coarse-grained models for fully-functionalised, partially damaged, and fully damaged hair surfaces are created by randomly replacing neutral thioesters with anionic sulfonate groups. This mimics the progressive removal of fatty acids from the hair surface by bleaching and leads to chemically heterogeneous surfaces. Using molecular dynamics (MD) simulations, we study the island structures formed by the lipid monolayers at different degrees of damage in vacuum and in the presence of polar (water) and non-polar (n-hexadecane) solvents. We also use MD simulations to compare the wetting behaviour of water and n-hexadecane droplets on the model surfaces through contact angle measurements, which are compared to experiments using virgin and bleached hair. The model surfaces capture the experimentally-observed transition of the hair surface from hydrophobic (and oleophilic) to hydrophilic (and oleophobic) as the level of bleaching damage increases. By selecting surfaces with specific damage ratios, we obtain contact angles from the MD simulations that are in good agreement with experiments for both solvents on virgin and bleached human hairs. To negate the possible effects of microscale curvature and roughness of real hairs on wetting, we also conduct additional experiments using biomimetic surfaces that are co-functionalised with fatty acids and sulfonate groups. In both the MD simulations and experiments, the cosine of the water contact angle increases linearly with the sulfonate group surface coverage with a similar slope. We expect that the proposed systems will be useful for future molecular dynamics si

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