47 research outputs found
Theory of viscoelastic adhesion and friction
No full textWe present a novel theory of the adhesive contact of linear viscoelastic materials sliding at constant velocity against rough substrates. Despite the non-conservative behaviour of the system, the closure equation, needed to calculate the unknown size of the contact area, can be rigorously formulated in the form of a local energy balance. The results highlight three main peculiar features of the contact, which are strictly ascribable to the interplay of adhesion and viscoelasticity. First, a velocity dependent pull-off force is predicted, whose maximum value occurs at intermediate sliding velocity. Second, the energy release rates G1 and G2 at the contact trailing and leading edges respectively, present a non-monotonic dependence on the indenter sliding velocity. Third, the velocity dependence of the hysteretic friction mu is significantly altered and presents a friction peak much more pronounced compared to the adhesiveless viscoelastic case. Theoretical predictions are in very good agreement with existing experimental data.(c) 2022 Elsevier Ltd. All rights reserved
Enhancement of adhesion strength in viscoelastic unsteady contacts
No full textWe present a general energy approach to study the unsteady adhesive contact of viscoelastic materials. Under the assumption of infinitely short-range adhesive interactions, we exploit the principle of virtual work to generalize Griffith's local energy balance at contact edges to the case of a non-conservative (viscoelastic) material, subjected to a generic contact time-history. We apply the proposed energy balance criterion to study the approach-retraction motion of a rigid sphere in contact with a viscoelastic half-space. A strong interplay between adhesion and viscoelastic hysteretic losses is reported which can lead to strongly increased adhesion strength, depending on the loading history. Specifically, two different mechanisms are found to govern the increase of pull-off force during either approach-retraction cycles and approach - full relaxation - retraction tests. In the former case, hysteretic losses occurring close to the circular perimeter of the contact play a major role, significantly enhancing the energy release rate. In the latter case, instead, the pull-off enhancement mostly depends on the glassy response of the whole (bulk) material which, triggered by the fast retraction after relaxation, leads to a sort of 'frozen' state and results in a flat-punch-like detachment mechanism (i.e., constant contact area). In this case, the JKR theory of adhesive contact cannot be invoked to relate the observed pull-off force to the effective adhesion energy, i.e. the energy release rate G, and strongly overestimates it. Therefore, a rigorous mathematical procedure is also proposed to correctly calculate the energy release rate in viscoelastic dissipative contacts
Adhesive contact mechanics of viscoelastic materials
No full textIn this study, we propose a theory of rough adhesive contact of viscoelastic materials in steady-state sliding. By exploiting a boundary formulation based on Green's function approach, the unknown contact domain is calculated by enforcing the local energy balance at the contact edge, thus considering also the non-conservative work of internal stresses which is directly related to the odd part of the Green's function. Theoretical predictions indicate that viscoelasticity may enhance the adhesive performance depending on the sliding velocity, thus leading to larger contact area and pull-off force compared to the equivalent adhesive elastic case The interplay between viscoelasticity and adhesion also affects the overall friction. Indeed, at low velocity, friction is strongly enhanced compared to the adhesiveless viscoelastic case, mainly due to the small-scale viscoelastic hysteresis induced by the adhesive neck close to the contact edge At higher velocity, the effect of viscoelastic hysteresis occurring at larger scales (bulk material) leads to even higher friction. Under these conditions, in the presence of adhesion, the small-scale and large-scale viscoelastic contributions to friction cannot be separated. Finally, in contrast with usual predictions for crack propagation/healing in infinite systems, we found a non-monotonic trend of the energy release rates at the trailing and leading contact edges, which is consistent with the finiteness of the contact length. All the presented results are strongly supported by existing experimental evidences
Lymphangiogenèse et activité biologique du facteur de croissance vasculaire endothélial-C
No full textLe facteur de croissance vasculaire endothélial (vascular endothelial growth factor (VEGF))-C est un nouveau membre de la famille des VEGFs, facteurs de croissance polypeptidiques qui jouent un rôle clé dans la physiologie et la pathologie de plusieurs aspects du système cardiovasculaire, y compris la vasculogenèse, l’hématopoïèse, l’angiogenèse et la perméabilité vasculaire. Les signaux des VEGFs sont transmis dans les cellules endothéliales grâce à des récepteurs de type tyrosine kinase, les VEGFRs, dont on connaît actuellement trois exemplaires, exprimés par les cellules endothéliales et les précurseurs hématopoïétiques. Comparé au premier VEGF décrit, le VEGF-A ; qui est un mitogène spécifique des cellules endothéliales et facteur angiogénique de premier ordre, le VEGF-C semble avoir un rôle principal dans le développement du système lymphatique. Ceci reflète le fait que, si VEGF-A est un ligand pour VEGFR-1 et VEGFR-2, VEGF-C exerce son activité biologique par l’intermédiaire du récepteur-3, dont l’expression, au cours du développement, va se restreindre aux vaisseaux lymphatiques et à certaines veines. Cependant, la capacité du VEGF-C de se lier aussi au VEGFR-2 et de l’activer est probablement responsable de son pouvoir d'induire une réponse angiogénique dans certains modèles, ce qui le rend intéressant du point de vue des manipulations expérimentales ou cliniques ayant pour but de bloquer ou de stimuler l’angiogenèse. Dans cet article nous discutons brièvement les connaissances actuelles sur l’activité biologique de VEGF-C, en mettant en évidence que celle-ci, comme dans le cas d’autres facteurs angiogéniques, est profondément influencée par l’activité d’autres régulateurs angiogéniques présents dans le microenvironnement des cellules endothéliales
Novel thermally conductive coating for cotton fabrics based on reduced graphene oxide decorated with in situ synthesized silver nanoparticles
No full textThe application of thermally conductive materials as coating on fiber surfaces represents an innovative technology solution for conveying heat dissipation capability to IR-opaque textiles. In this work, a sustainable and scalable approach to manufacture a hybrid nanocomposite coating for cotton, formed by Reduced Graphene Oxide (RGO) sheets functionalized by histidine (His) and decorated by Ag nanoparticles (NPs), is reported for increasing thermal conductivity of cotton fabrics. Tens nm in size Ag NPs were synthesized, in situ, at the coordinating sites of the His-RGO modified cotton impregnated by H2O/CH3OH solutions of the AgNO3 precursor, under UV-light exposure, without using chemical reductants. The physical chemical properties of the nanocomposite modified fabrics were comprehensively investigated, integrating chemical, structural and morphological analysis, with characterizations of their thermal, electrical, oxygen permeability, surface wettability and mechanical properties. Thermal conductivity of cotton was measured by Differential Scanning calorimetry (DSC) technique, which was here validated by Transient Plane Source (TPS) method, assessing the effectiveness of DSC in measuring thermal conductivity of textiles. The resulting coating exhibits a thermal conductivity, which was twice as high as untreated cotton, maintaining its breathability, increasing its flexibility, while simultaneously reducing its wettability. This notable enhancement can be attributed to the synergistic effect of the conductive Ag nanostructures formed among the His-RGO sheets within the nanocomposite, and it matches the thermal conductivity achieved by current state-of-the-art methods, while offering additional advantages of being more eco-friendly, scalable, and sustainable. The reported characterization of the structural properties of the achieved coating opens the venue to interesting perspectives towards its application in passive conducting cooling textiles for personal thermal comfort management
Lymphangiogenèse et activité biologique du facteur de croissance vasculaire endothélial-C
No full textReduced graphene oxide-histidine-silver nanocomposite coating with antibacterial activity for abdominal biomedical textiles
No full textThe synthesis and characterization of a novel antibacterial hybrid nanocomposite composed of reduced graphene oxide (RGO) functionalized with histidine and decorated with Ag nanoparticles (NPs) is reported. The material was developed via an in situ colloidal approach, performed in water at ice bath temperature. Histidine was selected as molecular linker due to its unique physicochemical properties: (i) ability to exfoliate RGO and enhance its dispersion in aqueous media, (ii) pi-pi aromatic stacking interactions with the graphene basal plane, and (iii) presence of imidazole and amine groups that provide coordination sites for Ag+ ions, enabling controlled nucleation and anchoring of monodispersed Ag NPs. The nanocomposite structure was finely tuned by systematically optimizing synthetic parameters, and each parameter was evaluated through morphological and spectroscopic characterization to assess NP size distribution and anchoring density. The nanocomposite was applied as coating on cotton fabrics and tested for the antibacterial activity against Escherichia coli (E. coli), following the ISO 20743:2021 quantitative standard protocol. The nanocomposite demonstrated superior and more stable antibacterial performance compared to Ag NP coatings synthesized with comparable size and surface properties. MTS assays confirmed that the modified textiles did not significantly impair cell viability. Given its colloidal stability, sustained antibacterial efficacy, and safety profile, the nanocomposite represents a robust platform for functional coatings in biomedical textiles, wound dressings and surgical fabrics, particularly suited to abdominal and intestinal procedures where E. coli contamination is a major concern
Smart IoT system empowered by customized energy-aware wireless sensors integrated in graphene-based tissues to improve workers thermal comfort
No full textThermal stress is a factor that must be considered, particularly in working environments where severe microclimatic conditions can lead to a loss of well-being or, in the worst-case scenario, to worker health damage. When it comes to thermal comfort, the best conditions for ensuring worker activity, productivity, and well-being include not only the proper design of spaces and cooling/heating systems, but also constant, real-time monitoring of the worker's physical condition. This work aims to present a complete system comprising various components capable not only of ensuring the worker's thermal comfort through the use of innovative textiles for optimal heat exchange between the individual and the environment, but also of continuously monitoring certain physiological parameters useful in determining whether the worker is subject to thermal stress. The system developed also allows the worker to be warned in the event of thermal stress and provides control and analysis tools to safety operators. This work, in particular, provides ample space for the description and validation of the textile component, in addition to presenting the system architecture and describing the main components. Finally, the wireless sensors designed to monitor physiological parameters are described and validated, and a brief functional validation of some of the architecture's software components is presented
