Publications
456 results found
Ewen J, Gao H, Mueser M, et al., 2019, Shear heating, flow, and friction of confined molecular fluids at high pressure, Physical Chemistry Chemical Physics, Vol: 21, Pages: 5813-5823, ISSN: 1463-9076
Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.
Yu M, Arana C, Evangelou S, et al., 2019, Quarter-car experimental study for series active variable geometry suspension, IEEE Transactions on Control Systems Technology, Vol: 27, Pages: 743-759, ISSN: 1063-6536
In this paper, the recently introduced series active variable geometry suspension (SAVGS) for road vehicles is experimentally studied. A realistic quarter-car test rig equipped with double-wishbone suspension is designed and built to mimic an actual grand tourer real axle, with a single-link variant of the SAVGS and a road excitation mechanism implemented. A linear equivalent modeling method is adopted to synthesize an H-infinity control scheme for the SAVGS, with the geometric nonlinearity compensated. Simulations with a theoretical nonlinear quarter-car indicate the SAVGS potential to enhance suspension performance, in terms of ride comfort and road holding. Practical features in the test rig are further considered and included in the nonlinear model to compensate the difference between the theoretical and testing behaviors. Experiments with a sinusoidal road, a smoothed bump and hole, and a random road are performed to evaluate the SAVGS practical feasibility and performance improvement, the accuracy of the model, and the robustness of the control schemes. Compared with the conventional passive suspension, ride comfort improvements of up to 41% without any deterioration of the suspension deflection are demonstrated, while the SAVGS actuator power is kept very low, at levels below 500 W.
Vladescu S-C, Putignano C, Marx N, et al., 2019, The percolation of liquid through a compliant seal - an experimental and theoretical study, Journal of Fluids Engineering, Vol: 141, Pages: 031101-031101, ISSN: 0098-2202
Hu S, Reddyhoff T, Wen J, et al., 2019, Characterization and simulation of bi-Gaussian surfaces induced by material transfer and additive processes, Tribology International, ISSN: 0301-679X
Dzepina B, Balint D, Dini D, 2019, A phase field model of pressure-assisted sintering, Journal of the European Ceramic Society, Vol: 39, Pages: 173-182, ISSN: 0955-2219
The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.
Ma S, Scaraggi M, Yan C, et al., 2019, Bio-inspired 3D printed locomotion devices based on anisotropic friction, Small, Vol: 15, Pages: 1802931-1802931, ISSN: 1613-6810
Anisotropic friction plays a key role in natural systems, particularly for realizing the purpose of locomotion and strong attachment for the survival of organisms. Of particular interest, here, is the observation that friction anisotropy is promoted numerous times by nature, for example, by wild wheat awn for its targeted and successful seed anchorage and dispersal. Such feature is, however, not fully exploited in man‐made systems, such as microbots, due to technical limitations and lack of full understanding of the mechanisms. To unravel the complex dynamics occurring in the sliding interaction between anisotropic microstructured surfaces, the friction induced by asymmetric plant microstructures is first systematically investigated. Inspired by this, anisotropic polymer microactuators with three‐dimensional (3D) printed microrelieves are then prepared. By varying geometric parameters, the capability of microactuators to generate strong friction anisotropy and controllable motion in remotely stretched cylindrical tubes is investigated. Advanced theoretical models are proposed to understand and predict the dynamic behavior of these synthetic systems and to shed light on the parameters and mechanisms governing their behavior. Finally, a microbot prototype is developed and cargo transportation functions are successfully realized. This research provides both in‐depth understanding of anisotropic friction in nature and new avenues for developing intelligent actuators and microbots.
Spagnoli A, Terzano M, Dini D, 2019, Mixed-mode crack propagation during needle penetration for surgical interventions, 25th International Conference on Fracture and Structural Integrity, Publisher: ELSEVIER SCIENCE BV, Pages: 775-780, ISSN: 2452-3216
Tan Z, Dini D, Rodriguez y Baena F, et al., 2018, Composite hydrogel: A high fidelity soft tissue mimic for surgery, Materials and Design, Vol: 160, Pages: 886-894, ISSN: 0264-1275
Accurate tissue phantoms are difficult to design due to the complex non-linear viscoelastic properties of real soft tissues. A composite hydrogel, resulting from a mix of poly(vinyl) alcohol and phytagel, is able to reproduce the viscoelastic responses of different soft tissues due to its compositional tunability. The aim of this work is to demonstrate the flexibility of the composite hydrogel in mimicking the interactions between surgical tools and various soft tissues, such as brain, lung and liver. Therefore compressive stiffness, insertion forces and frictional forces were used as matching criteria to determine the hydrogel compositions for each soft tissue. A full map of the behaviour of the synthetic material is provided for these three characteristics and the compositions found to best match the mechanical response of brain, lung and liver are reported. The optimised hydrogel samples are then tested and shown to mimic the behaviour of the three tissues with unprecedented fidelity. The effect of each hydrogel constituent on the compressive stiffness, needle insertion and frictional forces is also detailed in this work to explain their individual contributions and synergistic effects. This study opens important opportunities for the realisation of surgical planning and training devices and tools for in-vitro tissue testing.
Ewen J, Heyes D, Dini D, 2018, Advances in nonequilibrium molecular dynamics simulations of lubricants and additives, Friction, Vol: 6, Pages: 349-386, ISSN: 2223-7704
Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.
Tajabadi-Ebrahimi M, Dini D, Balint DS, et al., 2018, Discrete crack dynamics: a planar model of crack propagation and crack-inclusion interactions in brittle materials, International Journal of Solids and Structures, Vol: 152-153, Pages: 12-27, ISSN: 0020-7683
The Multipole Method (MPM) is used to simulate the many-body self-consistentproblem of interacting elliptical micro-cracks and inclusions in single crystals. Acriterion is employed to determine the crack propagation path based on the stressdistribution; the evolution of individual micro-cracks and their interactions withexisting cracks and inclusions is then predicted using what we coin the DiscreteCrack Dynamics (DCD) method. DCD is fast (semi-analytical) and particularlysuitable for the simulation of evolving low-speed crack networks in brittle orquasi-brittle materials. The method is validated against finite element analysispredictions and previously published experimental data.
Verschueren J, Gurrutxaga-Lerma B, Balint D, et al., 2018, Instabilities of high speed dislocations, Physical Review Letters, Vol: 121, ISSN: 0031-9007
Despite numerous theoretical models and simulation results, a clear physical picture of dislocations traveling at velocities comparable to the speed of sound in the medium remains elusive. Using two complementary atomistic methods to model uniformly moving screw dislocations, lattice dynamics and molecular dynamics, the existence of mechanical instabilities in the system is shown. These instabilities are found at material-dependent velocities far below the speed of sound. We show that these are the onset of an atomistic kinematic generation mechanism, which ultimately results in an avalanche of further dislocations. This homogeneous nucleation mechanism, observed but never fully explained before, is relevant in moderate and high strain rate phenomena including adiabatic shear banding, dynamic fracture, and shock loading. In principle, these mechanical instabilities do not prevent supersonic motion of dislocations.
Yu M, Arana C, Evangelou S, et al., 2018, Parallel active link suspension: a quarter-car experimental study, IEEE-ASME Transactions on Mechatronics, Vol: 23, Pages: 2066-2077, ISSN: 1083-4435
In this paper, a novel electro-mechanical active suspension for cars, the Parallel Active Link Suspension (PALS), is proposed and then experimentally studied. PALS involves the introduction of a rotary-actuator-driven rocker-pushrod mechanism in parallel with the conventional passive suspension assembly, to exert an additional controlled force between the chassis and the wheel. The PALS geometric arrangement is designed and optimized to maximize the rocker torque propagation onto the tire load increment. A quarter car test rig with double wishbone suspension is utilized for the PALS physical implementation. Based on a linear equivalent model of the PALS quarter car, a conservative and an aggressive robust H∞ control schemes are synthesized separately to improve the ride comfort and the road holding, with different levels of control effort allowed in each of the control schemes. Simulations with a theoretical nonlinear model of the PALS quarter car are performed to evaluate the potential in suspension performance enhancement and power demand in the rocker actuator. Experiments with a harmonic road, a smoothed bump and hole, and swept frequency are conducted with the quarter car test rig to validate the practical feasibility of the novel PALS, the ride comfort enhancement, as well as the accuracy of the theoretical model and of a further nonlinear model in which practical features existing in the test rig are identified and included.
Gattinoni C, Ewen JP, Dini D, 2018, Adsorption of Surfactants on α-Fe<sub>2</sub>O<sub>3</sub>(0001): A Density Functional Theory Study, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 20817-20826, ISSN: 1932-7447
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- Citations: 29
Vakis A, Yastrebov V, Scheibert J, et al., 2018, Modeling and simulation in tribology across scales: An overview, Tribology International, Vol: 125, Pages: 169-199, ISSN: 0301-679X
This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
Yu M, Evangelou S, Dini D, 2018, Control design for a quarter car test rig with parallel active linkSuspension, 2018 Annual American Control Conference (ACC), ISSN: 2378-5861
In this paper, a recently proposed novel vehicle suspension of Parallel Active Link Suspension (PALS) is adapted on a quarter car test rig. Control strategies with the PALS are studied and synthesized for ride comfort and road holding performance enhancement. A linear equivalent model of the PALS-retrofitted quarter car is derived, with geometric nonlinearity compensated. A linear control scheme is then synthesized, with an outer-loop H-infinity control and an inner-loop actuator torque tracking. Nonlinear simulations with the model of the PALS-retrofitted quarter car test rig are performed over typical road profiles, including 2 Hz harmonic road, smoothed bump and hole, and ISO random road. Results are discussed to evaluate the potential of the PALS-retrofitted quarter car test rig in ride comfort and road holding performance enhancement, as well as the power consumption in the actuator.
Shen L, Denner F, Morgan N, et al., 2018, Capillary waves with surface viscosity, Journal of Fluid Mechanics, Vol: 847, Pages: 644-663, ISSN: 0022-1120
Experiments over the last 50 years have suggested a tentative correlation between the surface (shear) viscosity and the stability of a foam or emulsion. We examine this link theoretically using small-amplitude capillary waves in the presence of a surfactant solution of dilute concentrations where the associated Marangoni and surface viscosity effects are modelled via the Boussinesq-Scriven formulation. The resulting integro-differential initial value problem is solved analyticallyand surface viscosity is found to contribute an overall damping effect on the amplitude of the capillary wave with varying degrees depending on the lengthscale of the system.Numerically, we find the critical damping wavelength to increase for increasing surface concentration but the rate of increase remains different for both the surface viscosity and the Marangoni effect.
Vidotto M, Dini D, Momi ED, 2018, Effective Diffusion and Tortuosity in Brain White Matter., 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Pages: 4901-4904, ISSN: 1557-170X
Patients affected by glioblastomas have a very low survival rate. Emerging techniques, such as convection enhanced delivery (CED), need complex numerical models to be effective; furthermore, the estimation of the main parameters to be used to instruct constitutive laws in simulations represents a major challenge. This work proposes a new method to compute tortuosity, a key parameter for drug diffusion in fibrous tissue, starting from a model which incorporates the main white matter geometrical features. It is shown that tortuosity increases from 1.35 to 1.85 as the extracellular space width decreases. The results are in good agreement with experimental data reported in the literature.
Tan Z, Forte A, Rodriguez y Baena F, et al., 2018, Needle-tissue interactions during convection enhanced drug delivery in neurosurgery, International Conference on BioTribology
Kanca Y, Milner P, Dini D, et al., 2018, Tribological evaluation of biomedical polycarbonate urethanes against articular cartilage, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 82, Pages: 394-402, ISSN: 1751-6161
This research investigated the in-vitro wear and friction performance of polycarbonate urethane (PCU) 80A as they interact with articular cartilage, using a customised multidirectional pin-on-plate tester. Condyles were articulated against PCU 80A discs (Bionate® I and Bionate® II) (configuration 1) and the results arising from these tests were compared to those recorded during the sliding of PCU pins against cartilage plates (configuration 2). Configuration 1 produced steadily increasing coefficient of friction (COF) (up to 0.64 ± 0.05) and had the same trend as the cartilage–on–stainless steel articulation (positive control). When synovial fluid rather than bovine calf serum was used as lubricant, average COF significantly decreased from 0.50 ± 0.02–0.38 ± 0.06 for condyle–on–Bionate® I (80AI) and from 0.41 ± 0.02–0.24 ± 0.04 for condyle–on–Bionate® II (80AII) test configurations (p < 0.05). After 15 h testing, the cartilage–on–cartilage articulation (negative control) tests showed no cartilage degeneration. However, different levels of cartilage volume loss were found on the condyles from the positive control (12.5 ± 4.2 mm3) and the PCUs (20.1 ± 3.6 mm3 for 80 AI and 19.0 ± 2.3 mm3 for 80AII) (p > 0.05). A good correlation (R2 =0.84) was found between the levels of average COF and the volume of cartilage lost during testing; increasing wear was found at higher levels of COF. Configuration 2 showed low and constant COF values (0.04 ± 0.01), which were closer to the negative control (0.03 ± 0.01) and significantly lower than configuration 1 (p < 0.05). The investigation showed that PCU is a good candidate for use in hemiarthroplasty components, where only one of the two articulating surfaces is replaced, as long as the synthetic material is implanted in a region where migrating cartilage contact is achieved. Bio
Heyes D, Dini D, Smith E, 2018, Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model, Journal of Chemical Physics, Vol: 148, ISSN: 0021-9606
The viscoelastic behavior of sheared fluids is calculated by Non-Equilibrium Molecular Dynamics(NEMD) simulation, and complementary analytic solutions of a time-dependent extension of Eyring’smodel (EM) for shear thinning are derived. It is argued that an “incremental viscosity,”ηi, or IV whichis the derivative of the steady state stress with respect to the shear rate is a better measure of the physicalstate of the system than the conventional definition of the shear rate dependent viscosity (i.e., the shearstress divided by the strain rate). The stress relaxation function,Ci(t), associated withηiis consistentwith Boltzmann’s superposition principle and is computed by NEMD and the EM. The IV of the Eyringmodel is shown to be a special case of the Carreau formula for shear thinning. An analytic solutionfor the transient time correlation function for the EM is derived. An extension of the EM to allow forsignificant local shear stress fluctuations on a molecular level, represented by a gaussian distribution,is shown to have the same analytic form as the original EM but with the EM stress replaced by its timeand spatial average. Even at high shear rates and on small scales, the probability distribution functionis almost gaussian (apart from in the wings) with the peak shifted by the shear. The Eyring formulaapproximately satisfies the Fluctuation Theorem, which may in part explain its success in representingthe shear thinning curves of a wide range of different types of chemical systems.
Yu M, Evangelou SIMOS, Dini DANIELE, 2018, Chassis Leveling Control with Parallel Active Link Suspension, 14th International Symposium on Advanced Vehicle Control
Ewen JP, Kannam S, Todd B, et al., 2018, Slip of alkanes confined between surfactant monolayers adsorbed on solid surfaces, Langmuir, Vol: 34, Pages: 3864-3873, ISSN: 0743-7463
The slip and friction behaviour of n-hexadecane, confined between organic friction modifier (OFM) surfactant films adsorbed on hematite surfaces, have been studied using nonequilibrium molecular dynamics (NEMD) simulations. The influence of OFM type and coverage, as well as the applied shear rate and pressure have been investigated. A measurable slip length is only observed for OFM films with a high surface coverage, which provide smooth interfaces between well-defined OFM and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes towards a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behaviour is well described using the extended molecular kinetic theory (MKT) slip model. This study provides a more detailed understanding of the slip of alkanes on OFM monolayers. It also suggests that high coverage OFM films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant.
Ewen JP, Kannam S, Todd B, et al., 2018, Slip of hexadecane on organic friction modifier monolayers, APS March Meeting 2018, Publisher: American Physical Society, ISSN: 0003-0503
Menga N, Carbone G, Dini D, 2018, Do uniform tangential interfacial stresses enhance adhesion?, Journal of the Mechanics and Physics of Solids, Vol: 112, Pages: 145-156, ISSN: 0022-5096
We present theoretical arguments, based on linear elasticity and thermodynamics, to show that interfacial tangential stresses in sliding adhesive soft contacts may lead to a significant increase of the effective energy of adhesion. A sizable expansion of the contact area is predicted in conditions corresponding to such scenario. These results are easily explained and are valid under the assumptions that: (i) sliding at the interface does not lead to any loss of adhesive interaction and (ii) spatial fluctuations of frictional stresses can be considered negligible. Our results are seemingly supported by existing experiments, and show that frictional stresses may lead to an increase of the effective energy of adhesion depending on which conditions are established at the interface of contacting bodies in the presence of adhesive forces.
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Ferretti A, Giacopini M, Mastrandrea L, et al., 2018, Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling, WCX World Congress Experience
© 2018 SAE International. All Rights Reserved. Bearings represent one of the main responsible of friction losses in internal combustion engines and their lubrication performance has a crucial influence on the operating condition of the engine. In particular, the conrod small-end bearing is one of the most critical engine parts from a tribological point of view since limited contact surfaces have to sustain high inertial and combustion forces. In this contribution an analysis is performed of the tribological behaviour of the lubricated contact between the piston pin and the conrod small-end of a high performance motorbike engine. An algorithm is employed based on a complementarity formulation of the cavitation problem. A comparison between two different approaches to simulate the asperity contact problem is performed, the former based on the standard Greenwood-Tripp theory and the latter based on a complementarity formulation of the asperity contact problem. A model validation is performed by comparing the results with those obtained adopting the commercial software AVL Excite Power Unit. Similar results are obtained from both the approaches, if a proper calibration of the model input data is performed. However, a remarkable sensitivity is highlighted of the results obtained using the Greenwood/Tripp model to the adjustment parameters. The realistic (engineering) difficulty in defining and identifying the roughness data and their purely statistical nature returns results that may be afflicted by a dose of uncertainty. Considering that results of such simulations usually offer guidelines for a correct design of the coupling, further investigations are suggested to identify a relationship between simply available roughness data and model input, starting from a direct experimental measurements of real roughness profiles.
Zhang J, Wong J, Dini D, et al., 2018, Mechanochemical film formation by ZDDP, 6th Asia International Conference on Tribology (ASIATRIB), Publisher: MALAYSIAN TRIBOLOGY SOC-MYTRIBOS, Pages: 138-139
Lu J, Reddyhoff T, Dini D, 2017, 3D Measurements of Lubricant and Surface Temperatures Within an Elastohydrodynamic Contact, Tribology Letters, Vol: 66, ISSN: 1023-8883
We present an infrared microscopy technique, capable of measuring the temperature of both the bounding surfaces and the oil film in an elastohydrodynamic contact. This technique can, for the first time, spatially resolve the oil film temperature in three dimensions. The contact is produced by loading a steel ball against a sapphire disc, and the film is viewed using an infrared microscope focussing through the disc. Two band pass filters are used to isolate the radiation from the oil film, and Planck’s law is applied to data obtained at a known temperature as part of the calibration procedure. The proposed technique requires the emissivity of the oil film to be measured, which is acquired in situ and is shown to vary strongly as a function of thickness and temperature. The technique is validated under pure rolling conditions, when the temperature of the oil film is equal to the controlled lubricant reservoir temperature, and also compared to an equation commonly used to predict average film temperatures, confirming the value of the unknown constant. The technique is then used to gain insights into the thermal/rheological behaviour within a contact. This is important since the temperature of elastohydrodynamic contacts is critical in determining friction and hence the efficiency of machine components and this technique enables much needed validation and provides input data for CFD and numerical simulations.
Tan Z, Parisi C, Di Silvio L, et al., 2017, Cryogenic 3D printing of super soft hydrogels, Scientific Reports, Vol: 7, ISSN: 2045-2322
Conventional 3D bioprinting allows fabrication of 3D scaffolds for biomedical applications. In this contribution we present a cryogenic 3D printing method able to produce stable 3D structures by utilising the liquid to solid phase change of a composite hydrogel (CH) ink. This is achieved by rapidly cooling the ink solution below its freezing point using solid carbon dioxide (CO2) in an isopropanol bath. The setup was able to successfully create 3D complex geometrical structures, with an average compressive stiffness of O(1) kPa (0.49 ± 0.04 kPa stress at 30% compressive strain) and therefore mimics the mechanical properties of the softest tissues found in the human body (e.g. brain and lung). The method was further validated by showing that the 3D printed material was well matched to the cast-moulded equivalent in terms of mechanical properties and microstructure. A preliminary biological evaluation on the 3D printed material, coated with collagen type I, poly-L-lysine and gelatine, was performed by seeding human dermal fibroblasts. Cells showed good attachment and viability on the collagen-coated 3D printed CH. This greatly widens the range of applications for the cryogenically 3D printed CH structures, from soft tissue phantoms for surgical training and simulations to mechanobiology and tissue engineering.
Shen L, Denner F, Morgan N, et al., 2017, Marangoni effect on small-amplitude capillary waves in viscous fluids, Physical Review E, Vol: 96, Pages: 053110-053110, ISSN: 1539-3755
We derive a general integro-differential equation for the transient behavior of small-amplitude capillary waves on the planar surface of a viscous fluid in the presence of the Marangoni effect. The equation is solved for an insoluble surfactant solution in concentration below the critical micelle concentration undergoing convective-diffusive surface transport. The special case of a diffusion-driven surfactant is considered near the the critical damping wavelength. The Marangoni effect is shown to contribute to the overall damping mechanism, and a first-order term correction to the critical wavelength with respect to the surfactant concentration difference and the Schmidt number is proposed.
Putignano, Dini D, 2017, Soft matter lubrication: does solid viscoelasticity matter?, ACS Applied Materials and Interfaces, Vol: 9, Pages: 42287-42295, ISSN: 1944-8244
Classical lubrication theory is unable to explain a variety of phenomena and experimental observations involving soft viscoelastic materials, which are ubiquitous and increasingly used in e.g. engineering and biomedical applications. These include unexpected ruptures of the lubricating film and a friction–speed dependence, which cannot be elucidated by means of conventional models, based on time-independent stress–strain constitutive laws for the lubricated solids. A new modeling framework, corroborated through experimental measurements enabled via an interferometric technique, is proposed to address these issues: Solid/fluid interactions are captured thanks to a coupling strategy that makes it possible to study the effect that solid viscoelasticity has on fluid film lubrication. It is shown that a newly defined visco-elasto-hydrodynamic lubrication (VEHL) regime can be experienced depending on the degree of coupling between the fluid flow and the solid hysteretic response. Pressure distributions show a marked asymmetry with a peak at the flow inlet, and correspondingly, the film thickness reveals a pronounced shrinkage at the flow outlet; friction is heavily influenced by the viscoelastic hysteresis which is experienced in addition to the viscous losses. These features show significant differences with respect to the classical elasto-hydrodynamic lubrication (EHL) regime response that would be predicted when solid viscoelasticity is neglected. A simple yet powerful criterion to assess the importance of viscoelastic solid contributions to soft matter lubrication is finally proposed.
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