Publications
459 results found
Profito FJ, Vladescu SC, Reddyhoff T, et al., 2024, Numerical and experimental investigation of textured journal bearings for friction reduction, Tribology International, Vol: 195, ISSN: 0301-679X
This work investigates the mechanisms for friction reduction in textured journal bearings. The measured friction of laser-etched connecting-rod big-end shells was compared with a non-textured reference under engine-like conditions. Experiments covered several working conditions, and the friction and lubricant film temperatures were measured for three texture configurations. The origin of the verified friction reduction was explored using a numerical model that simulates the hydrodynamic lubrication and global thermal effects. The simulation results accurately matched the friction measurements, revealing that the microscopic mechanisms of reduction in lubricant shear stress and effective viscosity due to texture-induced cavitation act simultaneously to reduce the friction coefficient for the textured shells. The findings have practical applications for the optimal design of textured hydrodynamic bearings.
Maffioli L, Ewen JP, Smith ER, et al., 2024, TTCF4LAMMPS: A toolkit for simulation of the non-equilibrium behaviour of molecular fluids at experimentally accessible shear rates, Computer Physics Communications, Vol: 300, ISSN: 0010-4655
We present TTCF4LAMMPS, a toolkit for performing non-equilibrium molecular dynamics (NEMD) simulations to study the fluid behaviour at low shear rates using the LAMMPS software. By combining direct NEMD simulations and the transient-time correlation function (TTCF) technique, we study the fluid response to shear rates spanning 15 orders of magnitude. We present two examples for simple monatomic systems: one consisting of a bulk liquid and another with a liquid layer confined between two solid walls. The small bulk system is suitable for testing on personal computers, while the larger confined system requires high-performance computing (HPC) resources. We demonstrate that the TTCF formalism can successfully detect the system response for arbitrarily weak external fields. We provide a brief mathematical explanation for this feature. Although we showcase the method for simple monatomic systems, TTCF can be readily extended to study more complex molecular fluids. Moreover, in addition to shear flows, the method can be extended to investigate elongational or mixed flows as well as thermal or electric fields. The high computational cost needed for the method is offset by the two following benefits: i) the cost is independent of the magnitude of the external field, and ii) the simulations can be made highly efficient on HPC architectures by exploiting the parallel design of the algorithm. We expect the toolkit to be useful for computational researchers striving to study the nonequilibrium behaviour of fluids under experimentally-accessible conditions. Program summary: Program title: TTCF4LAMMPS CPC Library link to program files: https://doi.org/10.17632/hh2rkcxbrf.1 Developer's repository link: https://github.com/edwardsmith999/TTCF4LAMMPS Licensing provisions: GNU General Public License 3 Programming language: Python 3 Nature of problem: Measuring the nonequilibrium behaviour of bulk and confined fluids under experimentally accessible strain rates in non-equilibrium molec
Weiand E, Koenig PH, Rodriguez-Ropero F, et al., 2024, Boundary lubrication performance of polyelectrolyte–surfactant complexes on biomimetic surfaces, Langmuir, 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.
Lasen M, Dini D, Schwingshackl CW, 2024, Experimental control of frictional contact behaviour via piezoelectric actuation, Mechanical Systems and Signal Processing, Vol: 211, ISSN: 0888-3270
Assembled structures in complex machinery usually have many joints, used to reduce assembly time and manufacturing complexity, to facilitate maintenance, ensure sealing and provide overall structural stiffness. Joints in these structures can change the stiffness and introduce friction damping and hence they impact the dynamic behaviour of the overall assembly. Currently, the joint impact on the structure is mainly considered only as defined at the design stage; however, attempts are underway to use the frictional interfaces to improve and control the overall dynamic performance during operation. This paper explores the possibility of actively manipulating the interface geometry of a frictional joint to influence its stiffness and energy dissipation capabilities. A proof of concept experimental campaign, of a novel concept that changes the hysteretic behaviour and frequency responses in dry friction contacts by means of a series of piezoelectric actuators will be presented and discussed. The experimental results are compared against simulation results obtained using a finite element model. The investigation shows that the concept is feasible and that it effectively changes the contact conditions, by changing the hysteresis loops and influencing the frequency responses from hammer test.
Bastola A, McCarron R, Shipway P, et al., 2024, Experimental and numerical investigations of sliding wear behaviour of an Fe-based alloy for PWR wear resistance applications, Wear, Vol: 540-541, ISSN: 0043-1648
The excellent wear and corrosion resistance of Co-based alloys make them desirable for tribological applications in the nuclear industry. However, neutron activation of the Co-based alloys leads to significant occupational radiation doses. An alternative Fe-based alloy called RR2450 was developed by Rolls-Royce plc to replace these alloys. This paper presents the first comprehensive study evaluating the sliding wear resistance of RR2450 alloy in representative PWR conditions. The sliding counterface of RR2450 balls was a Co-based alloy, Haynes 25 discs. Four tests were performed at temperatures up to 80 °C, and 12 tests were performed up to 200 °C. Results showed wear performance of RR2450 balls degraded at higher loads and temperatures, with temperature having a significant role. Microstructural investigation revealed voids and hard silicide phases, negatively impacting wear resistance. Nonetheless, the wear performance of RR2450 was similar to a Co-based alloy, a=Stellite 20, at nuclear reactor conditions. Two wear tests with uneven wear tracks were selected for 3D finite element analysis. The wear simulation procedure is based on Archard's wear equation and is implemented in a commercial FE package, ABAQUS. The FEA method was used to capture the wear on both surfaces, and was shown to predict nominal wear profiles. These comparisons with experiments show that the FEA results can provide representative wear profiles when wear depths and widths are asymmetrical and irregular.
Yu X, Xu Y, Morales-Espejel G, et 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.
Knudsen PA, Heyes DM, Niss K, et al., 2024, Invariant dynamics in a united-atom model of an ionic liquid., J Chem Phys, Vol: 160
We study a united-atom model of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl)sulfonylamide to determine to what extent there exist curves in the phase diagram along which the microscopic dynamics are invariant when expressed in dimensionless, or reduced, form. The initial identification of these curves, termed isodynes, is made by noting that contours of reduced shear viscosity and reduced self-diffusion coefficient coincide to a good approximation. Choosing specifically the contours of reduced viscosity as nominal isodynes, further simulations were carried out for state points on these, and other aspects of dynamics were investigated to study their degree of invariance. These include the mean-squared displacement, shear-stress autocorrelation function, and various rotational correlation functions. These were invariant to a good approximation, with the main exception being rotations of the anion about its long axis. The dynamical features that are invariant have in common that they are aspects that would be relevant for a coarse-grained description of the system; specifically, removing the most microscopic degrees of freedom in principle leads to a simplification of the potential energy landscape, which allows for the existence of isodynes.
Bhamra JS, Everhard EM, Bomidi JAR, et 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
Renso F, Giacopini M, Bertocchi E, et al., 2024, Numerical modelling of the cavitation damage in the conrod big end bearing of a high-performance internal combustion engine, Pages: 506-516
In this contribution a complementarity formulation for the solution of the elastohydrodynamic problem in the presence of cavitation is employed to investigate the tribological behaviour of the conrod big end bearing in a high-performance internal combustion engine. The continuous effort towards higher engine efficiencies, poses new challenges related to the increased specific loads to which engine components are subjected. In particular, the connecting rod big end bearing is subjected to both high loads and high relative velocity of the mating surfaces. Therefore, its tribological behaviour plays a crucial role. In fact, on one side, possible asperity contact pressures can produce wear of the interested components, and on the other side, a parallel possible cavitation of the lubricant can additionally damage the mating interfaces. Unfortunately for quantifying the cavitation damage, a universally established theory does not exist, even if it is well accepted that it is related to the sudden rapid implosion of the vapour bubbles near the surface. The precise investigation of this damage mechanism is usually neglected in big end bearing analysis since the implosion of the bubbles is difficult to quantify and it is not a standard output of any commercial software. Thus, in this work, a quantitative index previously proposed is reviewed and adopted to quantify the cavitation damage in a connecting rod big end bearing.
Yuan T, Shen L, Dini D, 2024, Porosity-permeability tensor relationship of closely and randomly packed fibrous biomaterials and biological tissues: Application to the brain white matter., Acta Biomater, Vol: 173, Pages: 123-134
The constitutive model for the porosity-permeability relationship is a powerful tool to estimate and design the transport properties of porous materials, which has attracted significant attention for the advancement of novel materials. However, in comparison with other materials, biomaterials, especially natural and artificial tissues, have more complex microstructures e.g. high anisotropy, high randomness of cell/fibre dimensions/position and very low porosity. Consequently, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive. To fill this gap, we start a mathematical derivation from the fundamental brain white matter (WM) formed by nerve fibres. This is augmented by a numerical characterisation and experimental validations to obtain an anisotropic permeability tensor of the brain WM as a function of the tissue porosity. A versatile microstructure generation software (MicroFiM) for fibrous biomaterial with complex microstructure and low porosity was built accordingly and made freely accessible here. Moreover, we propose an anisotropic poro-hyperelastic model enhanced by the newly defined porosity-permeability tensor relationship which precisely captures the tissues macro-scale permeability changes due to the microstructural deformation in an infusion scenario. The constitutive model, theories and protocols established in this study will both provide improved design strategies to tailor the transport properties of fibrous biomaterials and enable the non-invasive characterisation of the transport properties of biological tissues. This will lead to the provision of better patient-specific medical treatments, such as drug delivery. STATEMENT OF SIGNIFICANCE: Due to the microstructural complexity, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive, which hinders our way of tuning the fluid transport property of the biomaterials by directly programming their microstructure. The same pr
Heyes DM, Dini D, Pieprzyk S, et al., 2023, Harmonic models and molecular dynamics simulations of isomorph behavior of Lennard-Jones fluids: Excess entropy and high temperature limiting behavior., J Chem Phys, Vol: 159
Henchman's approximate harmonic model of liquids is extended to predict the thermodynamic behavior along lines of constant excess entropy ("isomorphs") in the liquid and supercritical fluid regimes of the Lennard-Jones (LJ) potential phase diagram. Simple analytic expressions based on harmonic cell models of fluids are derived for the isomorph lines, one accurate version of which only requires as input parameters the average repulsive and attractive parts of the potential energy per particle at a single reference state point on the isomorph. The new harmonic cell routes for generating the isomorph lines are compared with those predicted by the literature molecular dynamics (MD) methods, the small step MD method giving typically the best agreement over a wide density and temperature range. Four routes to calculate the excess entropy in the MD simulations are compared, which includes employing Henchman's formulation, Widom's particle insertion method, thermodynamic integration, and parameterized LJ equations of state. The thermodynamic integration method proves to be the most computationally efficient. The excess entropy is resolved into contributions from the repulsive and attractive parts of the potential. The repulsive and attractive components of the potential energy, excess Helmholtz free energy, and excess entropy along a fluid isomorph are predicted to vary as ∼T-1/2 in the high temperature limit by an extension of classical inverse power potential perturbation theory statistical mechanics, trends that are confirmed by the MD simulations.
Afferrante L, Violano G, Dini D, 2023, How does roughness kill adhesion?, Journal of the Mechanics and Physics of Solids, Vol: 181, ISSN: 0022-5096
It is well-known that adhesion is strongly influenced by surface roughness. Nevertheless, the literature currently contains an ongoing debate regarding which roughness scales are primarily responsible for adhesion loss. In this study, we aim to contribute to this debate by conducting numerical simulations on self-affine fractal profiles with varying fractal dimensions. Our results reveal that the long-wavelength portion of the roughness spectrum plays a crucial role in killing adhesion when considering profiles with Hurst exponent H>0.5. Conversely, for profiles with H<0.5, results show a different trend, indicating that adhesive stickiness is also influenced by short wavelength roughness. These findings are corroborated by our recent experimental observations. In such case, adhesive hysteresis and pull-off force exhibit a continuous decrease with increasing roughness scales. However, for H>0.5, the pull-off force converges towards a finite value as the magnification increases.
S Bhamra J, P Ewen J, Ayestarán Latorre C, et 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.
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
Massocchi D, Chatterton S, Lattuada M, et al., 2023, Effect of Friction Reducers with Unreinforced PEEK and Steel Counterparts in Oil Lubrication, Lubricants, Vol: 11
The increasing adoption of PEEK (polyetheretherketone) in many industrial applications has promoted intense research to optimize its lubrication along with the development of friction reducers (FRs), additives that help in reducing fuel consumption and, consequently, CO2 emissions. In this study, the effect of FRs in improving the lubrication of PEEK–steel couplings was evaluated and their mechanism studied using the Mini Traction Machine (MTM) tribometer. Different types of FRs (such as Molybdenum dithiocarbamate, glycerol monooleate, amine and polymeric derivatives) and coupling combinations (steel/steel, steel/PEEK and PEEK/steel) were considered. The oil samples were evaluated as fresh and after a rubbing time considering different operative conditions (from high to low T, fixed load and type of contact motion), and a measurement of the tribofilm was acquired. The experimental campaign showed a ranking among FRs friction-reducing behavior and, in some cases, a synergistic effect was noted between the tribofilm containing the friction modifier and the PEEK surface. Comparing the top performing FRs with reference oil showed a reduction in friction of 22%, 21% and 37%, respectively, in steel–steel, PEEK–steel and steel–PEEK couplings, while in the standard steel–steel coupling, two out of four FRs did not reduce the friction. After conditioning in the presence of PEEK, all friction-modifier additives reduced the friction effectively. This demonstrates the promising performance of PEEK, its compatibility with friction-reducing additives, and its applicability to sliding machine parts in order to improve efficiency and thus reduce CO2 emissions.
Ntioudis S, Ewen JP, Dini D, et al., 2023, A hybrid off-lattice kinetic Monte Carlo/molecular dynamics method for amorphous thin film growth, Computational Materials Science, Vol: 229, ISSN: 0927-0256
The ability to understand and model the growth of amorphous thin films on solid surfaces is essential to a wide range of industrial applications, from the deposition of wear-resistant coatings to the production of solar cells. Here, a three-dimensional (3D) hybrid off-lattice kinetic Monte Carlo/molecular dynamics (kMC/MD) algorithm is developed to study the growth of thin amorphous films on solid substrates with atomistic resolution over timescales of tens of seconds. We use this method to study the growth of polyphosphate films from tricresyl phosphate (TCP) molecules on an iron substrate. Molecular adsorption/desorption, bond breaking/formation processes, and diffusion of iron ions through the film are simulated in the kMC stage and the film is relaxed during the MD stage. The kMC/MD method is approximately eleven orders of magnitude faster than equivalent reactive force field (ReaxFF) MD simulations. The simulated film growth rate and topology agree well with experimental results and the chemical structure of the film is consistent with previous molecular dynamics simulations of iron polyphosphates. The newly-developed hybrid kMC/MD methodology can be adapted to yield important insights into thin film growth for several other potential applications.
Yu M, Evangelou S, Dini D, 2023, Advances in active suspension systems for road vehicles, Engineering, ISSN: 2095-8099
Active suspension systems (ASSs) have been proposed and developed for a few decades, and nowadays again become a thriving topic in both academia and industry, due to the high demand in driving comfort and safety, and the compatibility with vehicle electrification and autonomy. Existing review papers on ASSs are mainly about dynamics modelling and robust control, however, the gap between academic research outcomes and industrial application requirements is not yet bridged, hindering most ASS research knowledge from transferring to vehicle companies. This paper comprehensively reviews advances in ASSs for road vehicles, focusing on hardware structures and control strategies. Particularly, state-of-the-art ASSs that have been recently adopted in production cars are detailed, including representative solutions of Mercedes Active Body Control and Audi Predictive Active Suspension; novel concepts that could become alternative candidates are also introduced, including the Series Active Variable Geometry Suspension, and the Active Wheel Alignment System. The ASSs with compact structure, small mass increment, low power consumption, high frequency response, acceptable economic costs and high reliability are more likely to be adopted by car manufacturers. In terms of control strategies, future ASSs not only aim to stabilize the chassis attitude and attenuate the chassis vibration, moreover, but also cooperate with other bodies (e.g., steering and braking) and sensors (e.g., camera) within a car, and even with high-level decision (e.g., reference driving speed) in the overall transportation system – these strategies will be compatible with the rapidly developed electric and autonomous vehicles.
Ardah S, Profito FJ, Reddyhoff T, et al., 2023, Advanced modelling of lubricated interfaces in general curvilinear grids, TRIBOLOGY INTERNATIONAL, Vol: 188, ISSN: 0301-679X
Fatti G, Ciniero A, Ko H, et 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
Weiand E, Rodriguez-Ropero F, Roiter Y, et 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
Kew B, Holmes M, Liamas E, et al., 2023, Transforming sustainable plant proteins into high performance lubricating microgels, NATURE COMMUNICATIONS, Vol: 14
Ciniero A, Fatti G, Marsili M, et al., 2023, Defects drive the of PTFE: An ab-initio, NANO ENERGY, Vol: 112, ISSN: 2211-2855
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Cam MY, Giacopini M, Dini D, et al., 2023, A numerical algorithm to model wall slip and cavitation in two-dimensional hydrodynamically lubricated contacts, TRIBOLOGY INTERNATIONAL, Vol: 184, ISSN: 0301-679X
Lalli NS, Shen L, Dini D, et al., 2023, The stability of magnetic soap films, Physics of Fluids, Vol: 35, Pages: 1-16, ISSN: 1070-6631
Although previous studies have shown that a magnetic field can drastically alter drainage in soap films containing particles responsive to a magnetic field, which we refer to as magnetic soap films, it is yet to be understood whether a magnetic field may be able to control the rate of drainage and film stability. Furthermore, the effect of a magnetic field on key drainage mechanisms, such as marginal regeneration, is unknown. An experimental investigation involving interferometry is conducted here to develop further understanding of the behavior of horizontal soap films containing magnetite nanoparticles. Three scenarios are considered: soap films, magnetic soap films, and magnetic soap films in an inhomogeneous magnetic field. In each of the three scenarios, high-resolution images capturing the time evolution of each film are acquired, and the lifetime of each film is measured. In addition, a measure of the rate of drainage and the velocities of thin patches of fluid arising from marginal regeneration are evaluated. The results suggest that a magnetic field may be able to have either a stabilizing or destabilizing effect on magnetic soap films, depending on their composition. Furthermore, applying a magnetic field to magnetic soap films alters the trajectory of thin patches of fluid arising from marginal regeneration. This study reveals how a magnetic field can be used in conjunction with magnetic particles to control the stability of soap films, which opens up the possibility for new technologies that require a fine control of film stability.
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.
Weiand E, Ewen JP, Roiter Y, et 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.
Rahman MR, Shen L, Ewen JP, et al., 2023, Non-equilibrium molecular simulations of thin film rupture, JOURNAL OF CHEMICAL PHYSICS, Vol: 158, ISSN: 0021-9606
Bartolo MK, Newman S, Dandridge O, et 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
Heyes DM, Dini D, Pieprzyk S, et al., 2023, Departures from perfect isomorph behavior in Lennard-Jones fluids and solids, JOURNAL OF CHEMICAL PHYSICS, Vol: 158, ISSN: 0021-9606
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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