Self-healing slip pulses and the friction of gelatin gels

We present an extensive experimental study and scaling analysis of friction of gelatin gels on glass. At low driving velocities, sliding occurs via propagation of periodic self-healing slip pulses whose velocity is limited by collective diffusion of the gel network. Healing can be attributed to a frictional instability occurring at the slip velocity $v = V_c$. For $v > V_c$, sliding is homogeneous and friction is ruled by the shear-thinning rheology of an interfacial layer of thickness of order the (nanometric) mesh size, containing a semi-dilute solution of polymer chain ends hanging from the network. Inspite of its high degree of confinement, the rheology of this system does not differ qualitatively from known bulk ones. The observed ageing of the static friction threshold reveals the slow increase of adhesive bonding between chain ends and glass. Such structural ageing is compatible with the existence of a velocity-weakening regime at velocities smaller than $V_c$, hence with the existence of the healing instability.Comment: 9 pages, 16 figure

Frictional dissipation of polymeric solids vs interfacial glass transition

We present single contact friction experiments between a glassy polymer and smooth silica substrates grafted with alkylsilane layers of different coverage densities and morphologies. This allows us to adjust the polymer/substrate interaction strength. We find that, when going from weak to strong interaction, the response of the interfacial junction where shear localizes evolves from that of a highly viscous threshold fluid to that of a plastically deformed glassy solid. This we analyse as resulting from an interaction-induced ``interfacial glass transition'' helped by pressure

Self healing slip pulses along a gel/glass interface

We present an experimental evidence of self-healing shear cracks at a gel/glass interface. This system exhibits two dynamical regimes depending on the driving velocity : steady sliding at high velocity (> Vc = 100-125 \mu m/s), caracterized by a shear-thinning rheology, and periodic stick-slip dynamics at low velocity. In this last regime, slip occurs by propagation of pulses that restick via a ``healing instability'' occuring when the local sliding velocity reaches the macroscopic transition velocity Vc. At driving velocities close below Vc, the system exhibits complex spatio-temporal behavior.Comment: 4 pages, 6 figure

Solid Friction from stick-slip to pinning and aging

We review the present state of understanding of solid friction at low velocities and for systems with negligibly small wear effects. We first analyze in detail the behavior of friction at interfaces between wacroscopic hard rough solids, whose main dynamical features are well described by the Rice-Ruina rate and state dependent constitutive law. We show that it results from two combined effects : (i) the threshold rheology of nanometer-thick junctions jammed under confinement into a soft glassy structure (ii) geometric aging, i.e. slow growth of the real arrea of contact via asperity creep interrupted by sliding. Closer analysis leads to identifying a second aging-rejuvenation process, at work within the junctions themselves. We compare the effects of structural aging at such multicontact, very highly confined, interfaces with those met under different confinement levels, namely boundary lubricated contacts and extended adhesive interfaces involving soft materials (hydrogels, elastomers). This leads us to propose a classification of frictional junctions in terms of the relative importance of jamming and adsoprtion-induced metastability.Comment: 28 page

Dynamique de demixtion d'un melange binaire au voisinage du point critique: effets d'un ecoulement

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 82482 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Ionic control of crack propagation in biopolymer hydrogels

International audienceAlginate hydrogels stabilized by ionic interactions, fracture via crosslink unzipping and chain pull-out. It is found that, allowing non-binding ions to diffuse from a drop of brine into the crack tip region, leads to a strong acceleration of the crack. This is ascribed to the exchange between binding (Ca 2+) cations and non-binding ones, which facilitates the opening of unit chelating cages. The resulting lowering of the effective energy barrier is found merely entropic. The ion-exchange can be modelled as a rate-limited kinetic process, the order of which is fixed by the electroneutrality requirement, as checked by comparing the effect of monovalent and divalent cations. Although the embrittlement induced by a ionic shock could hinder the use of alginate gels in physiological environments where structural integrity and load-bearing capacity are required, it however can be thought as a powerful analytic tool for studying the nature and spatial extent of the dissipative mechanism at work when fracturing ultra-tough double network (including a ionic one) gels

Propriétés physiques d'hydrogels de gélatine réticulés par voie enzymatique

Les hydrogels sont constitués de réseaux élastiques de polymères hydrophiles qui emprisonnent une grande quantité d eau. La nature des liens, physique ou chimique, qui existent entre les chaînes du réseau définit en partie leurs propriétés mécaniques. Un enjeu actuel important pour les applications envisagées en médecine régénérative est le renforcement mécanique des gels qu on peut obtenir par mélange des deux types de liens afin de profiter des avantages de leurs réseaux respectifs. La gélatine, un biopolymère, est un bon système modèle pour répondre à cet objectif, car elle offre différentes voies de réticulation : physique, chimique et mixte. Cette thèse présente dans un premier temps une étude extensive de la formation d hydrogels chimiques de gélatine à l aide d une transglutaminase microbienne. Etonnamment ces gels sont turbides, contrairement aux gels physiques qui sont transparents. Nous avons mené une caractérisation détaillée de la cinétique de réticulation et de la turbidité dans ces systèmes et nous avons expliqué ce phénomène par un mécanisme d auto- mplification des inhomogénéités de concentration de polymères. Nous avons identifié les paramètres permettant de contrôler le niveau de turbidité et même de la supprimer en utilisant une gélatine plus chargée. Dans un deuxième temps, nous avons développé de façon contrôlée des systèmes mixtes transparents, où des liens chimiques et physiques sont introduits successivement. Nous avons montré que même une très faible quantité de liaisons covalentes permet d obtenir un gel mixte ultra-résistant à la coupure. Nous avons ainsi mis au point une gamme de systèmes modèles qui ouvrent de nombreuses perspectives pour étudier quantitativement la relation entre propriétés structurelles et mécanisme de renforcement mécanique des gels.Hydrogels are elastic networks of hydrophilic polymers which trap a large amount of water. The nature of the cross-links, physical or chemical, between the network chains, determines in part their mechanical properties. An important current challenge for the regenerative medicine applications is the mechanical toughening of gels which can be obtained by mixing both types of bonds in order to take advantage of their respective networks. Gelatin, a biopolymer, is a good model system to meet this objective, as it offers different crosslinking routes: physical, chemical and mixed. This thesis presents firstly an extensive study of chemically crosslinked gelatin hydrogels with a microbial transglutaminase. Surprisingly these gels are turbid unlike physical ones that are transparent. We conduct a detailed characterization of the reaction kinetics and turbidity in these systems and explain this phenomenon by a self-amplification mechanism of polymer concentration inhomogeneities. We identify the parameters which control the level of turbidity and are even able to suppress it with the use of a more charged gelatin. In a second step we have developed, in a controlled way, transparent mixed systems, where chemical and physical bonds are successively introduced. We show that even a very small amount of covalent bonds is sufficient to make a mixed gel ultra-resistant to cutting. Theses model systems offer many perspectives to study quantitatively the relationship between structural properties and toughening mechanism of gels.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Dynamique de fracture d'un hydrogel thermoréversible de biopolymères

Nous avons étudié la dynamique de fracture d'hydrogels thermoréversibles de biopolymères : les gels de gélatine. Nous avons montré, par des expériences de fracture en mode I, quasi-stationnaire et dans un régime subsonique, que, contrairement à la fracture des gels chimiques et des élastomères, la propagation de la fracture dans les gels de gélatine n'implique pas la scission des chaînes : elles sont extraites entièrement à travers le réseau en tête de fracture, la dissipation provenant simplement de leurs frottements dans le solvant. Nous avons pu produire un modèle simple de type > qui permet de rendre compte des ordres de grandeur mesurés ainsi que de prédire des lois d'échelle vérifiées expérimentalement.En parallèle, nous avons étudié le faciès des surfaces créées par la propagation de la fracture. Nous avons montré qu'il n'existe pas de lois d'échelle comme celles observées dans d'autres matériaux ductiles ou fragiles mais que la micro-rugosité présente une hauteur RMS croissante avec la vitesse de fracture, observation jamais rapportée auparavant. Nous avons mis en évidence une vitesse critique en-dessous de laquelle des défauts macroscopiques apparaissent, défauts précédemment observés par Gent et al. dans les élastomères et décrit exhaustivement par Sekimoto et al. dans les gels de polyacrylamide. Nous avons pu expliquer la hauteur caractéristique de ces défauts en prenant en compte que ces matériaux très déformables présentent le phénomène d'émoussage de la fracture (>). Nous observons par ailleurs que ces défauts et la micro-rugosité présentent une anisotropie selon un angle > indépendant de la vitesse de fracture et des caractéristiques du gel de gélatine.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Cooperative Effect of Stress and Ion Displacement on the Dynamics of Cross-Link Unzipping and Rupture of Alginate Gels

International audienceWe study the effect of nonbinding Na(+) ions on the kinetics of rupture of alginate gels cross-linked by Ca(2+). Wetting a crack tip with a saline solution at physiological concentrations is found to be able to induce a quasi-instantaneous, 10-fold velocity jump. This effect is analyzed with a phenomenological model for the rate-dependent fracture energy in physical gels, extended here to account for the role of ions on the rate of cross-link ``unzipping''. Ionic interaction is found to act cooperatively with mechanical tension, leading to an enhanced rate of rupture. The kinetics turns out to be second order in counterion concentration. The definition of the reference state requires to take into account counterion condensation due to long-range interactions in the polyelectrolyte gel. Surprisingly, the contribution of the Na(+) ions to the free energy of the activated state is essentially entropic, suggesting that the displacement of Ca(2+) is primarily a steric process, electrostatic interactions being reduced to the constraint of charge conservation. This phenomenon may have important consequences on the rate of degradation of alginate based scaffolds for in vivo tissue regeneration

Élasticité et rhéologie d'une interface macroscopique (du piégeage au frottement solide)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF