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International audience; This opinion paper offers a scientific view on the current debate of the place of biodegradable plastics as part of the solution to deal with the growing plastic pollution in the world's soil, aquatic, and marine compartments. Based on the current scientific literature, we focus on the current limits to prove plastic biodegradability and to assess the toxicity of commercially used biobased and biodegradable plastics in natural environments. We also discuss the relevance of biodegradable plastics for selected applications with respect to their use and end of life. In particular, we underlined that there is no universal biodegradability of plastics in any ecosystem, that considering the environment as a waste treatment system is not acceptable, and that the use of compostable plastics requires adaptation of existing organic waste collection and treatment channels.

International audience; Both an experimental and a theoretical investigation of the fracture propagation mechanisms acting at the process zone scale in glassy polymers are presented. The main aim is to establish a common modeling for different kinds of glassy polymers presenting either steady-state fracture propagation or stick-slip fracture propagation or both, depending on loading conditions and sample shape. On the experimental point of view, new insights are provided by in-situ AFM measurements of the viscoplastic strain fields acting within the micrometric process zone in a brittle epoxy resin, which highlight an extremely slow unexpected steadystate regime with finite plastic strains of about 30% around a blunt crack tip, and accompanied by propagating shear lips. On the theoretical point of view, we apply to glassy polymers some recently developed models for describing soft dissipative fracture that are pertinent with the observed finite strains. We propose a unified modeling of the fracture energy for both the steady-state and stick-slip fracture propagation based on the evaluation of the energy dissipation density at a characteristic strain rate induced in the process zone by the competition between the crack propagation velocity and the macroscopic sample loading rate.

International audience; The amount and characteristics of plastic waste in the environment are closely monitored with the regular expansion of their collection to smaller debris sizes and new compartments, such as atmosphere, soil and even food. These data, subject to constant re-evaluation, are necessary to assess the threat that plastic particles pose. In this article, we take stock of the most recent measurements of their size distribution on the ocean surface. We confront the observed size distribution to the commonly invoked degradation modes of plastics and propose a scenario that accounts for the quite unusual features of the size distribution of floating debris. We conclude that delamination alone could be a major mechanism explaining the large number of small microplastics (

International audience; Plastic pollution has become a worldwide concern. It was demonstrated that plastic breaks down to nanoscale particles in the environment, forming so-called nanoplastics. It is important to understand their ecological impact, but their structure is not elucidated. In this original work, we characterize the microstructure of oceanic polyethylene debris and compare them to the nonweathered objects. Cross-sections are analysed by several emergent mapping techniques. We highlight deep modifications of the debris within a layer a few hundred microns thick. The most 2 intense modifications are macromolecule oxidation and a considerable decrease in the molecular weight. The adsorption of organic pollutants and trace metals is also confined to this outer layer. Fragmentation of the oxidized layer of the plastic debris is the most likely source of nanoplastics. Consequently nanoplastic chemical nature differ greatly from plastics.

Les comportements humains et l’utilisation intensive du plastique associes a une faible performance des systemes de gestion ont engendre une accumulation massive de dechets plastiques dans le milieu marin ou ils representent 50 a 80 % de l’ensemble des dechets. Leur distribution, leurs comportements en mer, leur degradation et leurs impacts decoulent directement de leur composition et proprietes d’usage. Cette infographie fait un bilan des connaissances et decrit les principaux enjeux scientifiques, environnementaux et socio-economiques, ainsi que les possibles solutions necessaires a la gestion d’un probleme environnemental devenu global.

International audience; An original setup combining a very stable loading stage, an atomic force microscope and an environmental chamber, allows to obtain very stable sub-critical fracture propagation in oxide glasses under controlled environment, and subsequently to finely characterize the nanometric roughness properties of the crack surfaces. The analysis of the surface roughness is conducted both in terms of the classical root mean square roughness to compare with the literature, and in terms of more physically adequate indicators related to the self-affine nature of the fracture surfaces. Due to the comparable nanometric scale of the surface roughness, the AFM tip size and the instrumental noise, a special care is devoted to the statistical evaluation of the metrologic properties. The 2 roughness amplitude of several oxide glasses was shown to decrease as a function of the stress intensity factor, to be quite insensitive to the relative humidity and to increase with the degree of heterogeneity of the glass. The results are discussed in terms of several modeling arguments concerning the coupling between crack propagation, material's heterogeneity, crack tip plastic deformation and water diffusion at the crack tip. A synthetic new model is presented combining the predictions of a model by Wiederhorn et al. [1] on the effect of the material's heterogeneity on the crack tip stresses with the self-affine nature of the fracture surfaces.

International audience

Human behavior and the intensive use of plastic, combined with poor performance of management systems have led to a massive accumulation of plastic debris in the marine environment, accounting for 50 to 80 % of all marine litter. Their distribution, their fate, their degradation and their impacts stem directly from their composition and use properties. This article reviews knowledge and describes the main scientific, environmental and socio-economic issues, as well as the possible solutions needed to manage an environmental problem that has become global., Les comportements humains et l’utilisation intensive du plastique associés à une faible performance des systèmes de gestion ont engendré une accumulation massive de déchets plastiques dans le milieu marin où ils représentent 50 à 80 % de l’ensemble des déchets. Leur distribution, leurs comportements en mer, leur dégradation et leurs impacts découlent directement de leur composition et propriétés d’usage. Le présent article fait un bilan des connaissances et décrit les principaux enjeux scientifiques, environnementaux et socio-économiques, ainsi que les possibles solutions nécessaires à la gestion d’un problème environnemental devenu global.

International audience; We analyze stress distributions in a two-dimensional bidisperse cemented granular packing for a broad range of the values of particle-size ratio, the volumes of large and small particles, and the amount of cementing matrix. In such textured porous materials, the stress concentration, which controls the fracture and fragmentation of the material under tensile loading or in grinding processes, reflects not only the porosity but also the contact network of the particle phase and the resulting stress chains. By means of peridynamic simulations under tensile loading, we show how both the texture and stress distribution depend on size ratio, volume ratio, and the amount of the cementing matrix. In particular, the volume fraction of the class of small particles plays a key role in homogenizing stresses across the system by reducing porosity. Interestingly, the texture controls not only the porosity but also the distribution of pores inside the system with its statistical variability, found to be strongly correlated with the homogeneity of stresses inside the large particles. The most homogeneous stress distribution occurs for the largest size ratio and largest volume fraction of small particles, corresponding to the lowest pore size dispersion and the cushioning effect of small particles and its similar role to the binding matrix for stress redistribution across the packing. At higher porosity, the tensile stresses above the mean stress fall off exponentially in all phases with an exponent that strongly depends on the texture. The exponential part broadens with decreasing matrix volume fraction and particle-size ratio. These correlations reveal the strong interplay between size polydispersity and the cohesive action of the binding matrix for stress distribution, which is significant for the behavior of textured materials in grinding operations.

International audience; The increase of plastics and microplastics in the environment is a major environmental challenge. Still, little is known about the degradation kinetics of macroplastics into smaller particles, under the joint actions of micro-organisms and physico-chemical factors, like UV or mechanical constraints. In order to gain insight into (bio)-degradation in various media, we perform accelerated erosion experiments by using a well-known enzymatic system. We show that the microstructure of semi-crystalline polymers plays a crucial role in the pattern formation at their surface. For the first time, the release of fragments of micrometric size is evidenced, through a mechanism that does not involve fracture propagation. A geometric erosion model allows a quantitative understanding of erosion rates and surface patterns, and provides a critical heterogeneity size, parting two types of behavior: spherulites either released, or eroded in situ. This new geometric approach could constitute a useful tool to predict the erosion kinetics and micro-particle generation in various media.

International audience; Biodegradable plastics appear as one promising means to help solving the increasing issue of environmental pollution by plastics. The present study aims at comparing the biodegradation mechanisms of two promising biodegradable plastics, PHBV Poly(3hydroxybutyrate-co-3-hydroxyvalerate) and PBSA Poly(butylene succinate-co-adipate) with the objective to provide a better understanding of the mechanisms involved and identify the most relevant indicators to follow biodegradation. For this purpose, the progress of the biodegradation process was monitored under controlled composting conditions at the laboratory scale at 58°C using several methodological approaches for evaluating polymer degradation. Indicators of the extent of material disappearance based on respirometry and

National audience; La pollution des océans par les déchets plastiques est devenue un problème environnemental majeur résultant de son accumulation dans les environnements terrestre et marin. Lorsqu'ils sont mal gérés, les plastiques pénètrent dans le milieu aquatique où ils subissent une dégradation et une fragmentation en microplastiques désormais omniprésents dans tous les milieux aquatiques (Law, 2017). Outre le fait qu'il est impossible d'éliminer les microplastiques du milieu marin, leur impact sur l’environnement est plus important. Diverses études ont montré que de nombreux types d'organismes marins ingéraient des microplastiques, ce qui entraînait des effets néfastes à plusieurs niveaux de la chaîne alimentaire et des écosystèmes marins (Rochman et al., 2016 ; Chae et al., 2017). On soupçonne également que les microplastiques, qui constituent un nouvel habitat pour les micro-organismes, sont des vecteurs de bactéries potentiellement pathogènes (Kirstein et al.,2016 ; Dussud et al.,2018).Le devenir des polymères dans le milieu aquatique dépend à la fois de phénomènes abiotiques (UV, stress mécanique) et biotiques, dus à la colonisation du plastique par des micro-organismes marins (bactéries, phytoplancton, champignons, etc.). Une des principales étapes de la biodégradation est la constitution d'un biofilm et la réduction de la longueur des chaînes de polymère via des exo-enzymes produites par des bactéries issues du biofilm. Une fois que les chaînes de polymère sont suffisamment courtes, elles peuvent être assimilées par les bactéries (Ennouri et al., 2017). Alors que les phénomènes abiotiques entraînent l’endommagement et la fragmentation d’un polymère par des mécanismes d’oxydation et d’hydrolyse, la création de défauts structurels et la propagation de fractures, il est généralement admis que seuls les phénomènes biotiques conduiront à la biodégradation complète d’un polymère, c’est-à-dire à sa conversion en biomasse, eau et CO2. En milieu marin, de nombreuses questions demeurent quant à la cinétique relative de la dégradation abiotique et biotique et à leur impact respectif en termes de fragmentation (Shah et al., 2008). Par exemple, plusieurs articles (Ter Halle et al., 2016 ; Cozar et al., 2018) ont récemment rapporté que la distribution en taille des particules collectées dans l'océan entre 5 mm et quelques centaines de microns ne semble pas correspondre à un processus de fragmentation monocinétique.Les profils d'érosion des polymères semi-cristallins ont fait l'objet d'études approfondies en laboratoire dans des conditions enzymatiques ou bactériennes et divers profils de dégradation ont été observés, leur apparition est principalement liée à la différence de cinétique d'érosion entre les régions cristallines et amorphes (Morse et al., 2011 ; Martinez-Tobon et al., 2018). À ce jour, il y a beaucoup moins d'études sur la manière dont l'évolution des patterns de surface influencera à son tour le processus d'érosion, et pourra conduire à la fracture ou à la génération de fragments.Afin d'étudier le processus d'érosion enzymatique, nous avons utilisé le système bien connu PDLLA / protéinase K (Yamashita et al., 2005). Etant particulièrement intéressés par le rôle des hétérogénéités à l’échelle de quelques nanomètres à quelques micromètres, nous avons utilisé un polymère de composition chimique donnée (PDLLA, 1,7% de D-mer, Mn = 95 kg / mol, indice de polydispersité I = 1,63) et de morphologie contrôlée par traitement thermique.

International audience; Pollution of the ocean by plastic litter has become a major environmental problem resulting from its accumulation in terrestrial and marine environments. When mismanaged, plastics enter the aquatic environment where they undergo degradation and fragmentation into microplastics that are now ubiquitous in all aquatic compartments. In addition to the fact that microplastics are impossible to remove from the marine environment, they are even more damaging than the macro waste. Various studies have shown that microplastics are ingested by many types of marine organisms leading to adverse effects at several levels of the food chain and of the marine ecosystems. It is also suspected that microplastics, that constitute a new habitat for micro-organisms, are vectors for potentially pathogenic bacteria.The fate of polymers in the aquatic environment depends both on abiotic phenomena (UV, mechanical stress), and on biotic ones, due to the colonization of plastics by marine micro-organisms (bacteria, phytoplankton, fungi, etc.). A primary step for bio-degradation is the constitution of a biofilm and reduction of the polymer chain length via exo-enzymes produced by bacteria from the biofilm. Once polymer chains are short enough, they can be assimilated by bacteria. While abiotic phenomena lead to the damage and fragmentation of a polymer by oxidation and hydrolysis mechanisms, creation of structural defects and fracture propagation, it is generally admitted that only biotic phenomena will result into the complete bio-degradation of a polymer, i.e. its conversion into biomass, water and CO2. In the marine environment, many questions remain about the relative kinetics of abiotic and biotic degradation and their respective impact in terms of fragmentation. For instance, several papers have recently reported that the size distribution of particles collected in the ocean between 5mm and a few hundreds of microns, does not seem to correspond to a single-kinetic fragmentation process.The erosion patterns of semi-crystalline polymers have been extensively studied in laboratory under enzymatic or bacterial conditions and various degradation patterns have been observed whose occurrence is mainly linked to the difference in the erosion kinetics between crystalline and amorphous regions. To date, there are much less studies addressing how the evolution of these surface patterns will in turn influence the erosion process, lead to fracture and potential fragments generation.In order to study the enzymatic erosion process, we used the well-known model system PDLLA/proteinase K. Being specifically interested in the role of heterogeneities at the scale of a few nanometers to a few micrometers, we used a polymer of a given chemical composition (PDLLA, 1.7% of D-mer, Mn = 95 kg/mol, polydispersity index I=1.63) and monitored its morphology through its change in crystallinity ratio, everything else remaining constant.Three types of samples were studied: 100% amorphous (A), semi-crystalline with 5% (SC5) and 35% (SC35) crystallinity.The samples morphologies were characterized through DSC, polarized optical microscopy (POM) and SEM. Enzymatic erosion kinetics were measured through weight loss experiments for the 3 polymers and the erosion patterns were observed over time through atomic force microscopy (AFM) and SEM. In order to interpret the results, we combined a simple two-phase geometric erosion model with the well-known Michaelis-Menten model for enzymatic kinetics. Our geometric erosion model is based on the evolution of the erosion front with time induced by the erosion rate difference between crystalline and amorphous regions. This new model accounts very well for the experimental results. Moreover, we observed a morphology-dependent release of fragments, which the model is also able to predict. In particular, one observes the release of spherulites as long as they are smaller than a critical size determined in the model. Some important consequences relevant for the understanding of the formation of micro-plastics in the ocean can be drawn from these experiments.

International audience; Pollution of the ocean by plastic litter has become a major environmental problem resulting from its accumulation in terrestrial and marine environments. When mismanaged, plastics enter the aquatic environment where they undergo degradation and fragmentation into microplastics that are now ubiquitous in all aquatic compartments. In addition to the fact that microplastics are impossible to remove from the marine environment, they are even more damaging than the macroscopic waste.The fate of polymers in the aquatic environment depends both on abiotic phenomena (UV, mechanical stress), and on biotic ones, due to the colonization of plastics by marine micro-organisms. A primary step for bio-degradation is the constitution of a biofilm and reduction of the polymer chain length via exo-enzymes produced by bacteria from the biofilm. Once polymer chains are short enough, they can be assimilated by bacteria. While abiotic phenomena lead to the damage and fragmentation of a polymer by oxidation and hydrolysis mechanisms, creation of structural defects and fracture propagation, it is generally admitted that only biotic phenomena will result into the complete bio-degradation of a polymer, i.e. its conversion into biomass, water and CO2. In the marine environment, many questions remain about the relative kinetics of abiotic and biotic degradation and their respective impact in terms of fragmentation. For instance, several papers have recently reported that the size distribution of particles collected in the ocean between 5mm and a few hundreds of microns, does not seem to correspond to a single-kinetic fragmentation process.In order toWe studiedy the enzymatic erosion process in semi-crystalline polymersand to understand the potential fracture and fragments generation in relation to the formation of erosion patterns in semi-crystalline polymers, we used the well-known model system PDLLA/proteinase K. Being specifically interested in the role of heterogeneities at the scale of a few nanometers to a few micrometers, we used a polymer of a given chemical composition and monitored its morphology through its change in crystallinity ratio, everything else remaining constant. We used the well-known model system PDLLA/proteinase KEnzymatic erosion kinetics were measured through weight loss experiments and erosion patterns were observed over time through atomic force microscopy (AFM) and SEM. In order to interpret the results, we combined a simple two-phase geometric erosion model with the well-known Michaelis-Menten model for enzymatic kinetics. Our geometric erosion model is based on the evolution of the erosion front with time induced by the erosion rate difference between crystalline and amorphous regions. This new model accounts very well for the experimental results and unexpectedly predicts that after a lag time, the final erosion rate will be the one of the fastest eroding phase. Moreover, we observed a morphology-dependent release of fragments, which the model is also able to predict. In particular, one observes the release of spherulites as long as they are smaller than a critical size determined in the model. Some important consequences relevant for the understanding of the formation of micro-plastics in the ocean can be drawn from these resultsexperiments.

International audience; Contact-resonance atomic force microscopy (CR-AFM) is of great interest and very valuable for a deeper understanding of the mechanics of biological materials with moduli of at least a few GPa. However, sample surfaces can present a high topography range with significant slopes, where the local angle can be as large as ± 50°. The non-trivial correlation between surface slope and CR-frequency hinders a straightforward interpretation of CR-AFM indentation modulus measurements on such samples. We aim to demonstrate the significant influence of the surface slope on the CR-frequency that is caused by the local angle between sample surface and the AFM cantilever and present a practical method to correct the measurements. Based on existing analytical models of the effect of the AFM set-up's intrinsic cantilever tilt on CR-frequencies, we compute the non-linear variation of the first two (eigen)modes CR-frequency for a large range of surface angles. The computations are confirmed by CR-AFM experiments performed on a curved surface. Finally, the model is applied to directly correct contact modulus measurements on a durum wheat starch granule as an exemplary sample.

International audience; Fracture propagation is inherently a multiscale problem, involving the coupling of many length scales from sample dimension to molecular level. Fracture mechanics provides a valuable link between the macroscopic scale of the structural loading of the samples and the scale of the process zone for brittle materials. Modeling the toughness of materials requires yet an investigation at scales smaller than this process zone, which is nanometric in oxide glasses and micrometric in polymer glasses.We present here the important insights that have been obtained through an in situ experimental investigation of the strain fields in the micrometric neighborhood of a propagating crack. We show the richness of atomic force microscopy combined with digital image correlation although it limits the observations to the external surface of the sample and to very slow crack propagation (below nm/s). For oxide glasses, this novel technique provided enlightening information on the nanoscale mechanisms of stress corrosion during subcritical crack propagation (Ciccotti, J Phys D Appl Phys 42:214006, 2009; Pallares et al., Corros Rev 33(6):501–514, 2015), including the relevance of crack tip plasticity (Han et al., EPL 89:66003, 2010), stress-induced ion exchange processes (Célarié et al., J Non-Cryst Solids 353:51–68, 2007), and capillary condensation in the crack tip cavity (Grimaldi et al., Phys Rev Lett 100:165505, 2008; Pallares et al., J Am Ceram Soc 94:2613–2618, 2011). An extension of this technique has recently been developed for glassy polymers (George et al., J Mech Phys Solids 112:109–125, 2018), leading to novel insights on the transition between crazing and shear yielding mechanisms and to promising new ways to link the toughness properties to the time-dependent large strain material properties of these nominally brittle materials.

International audience; Light transmission through wheat (T. aestivum L.) grain longitudinal cross sections of different thickness was used to study the endosperm microstructure and was shown to strictly follow a Beer-Lambert law allowing a non ambiguous quantification of the endosperm vitreousness. Therefore similar samples obtained from near-isogenic lines differing by hardness and grown in two distinct environments affecting their vitreousness were analyzed and confirmed the relationship between light transmission and the endosperm microstructure. In each sample, moreover analysis of light transmission within the different grain parts highlighted the greater compactness of the central endosperm cheeks in comparison with the distal and the proximal regions. These results helps a better understanding of the endosperm microstructure.

International audience; A novel experimental technique based on in situ AFM monitoring of the mechanisms of damage and the strain fields associated to the slow steady-state propagation of a fracture in glassy polymers is presented. This micron-scale investigation is complemented by optical measurements of the sample deformation up to the millimetric macroscopic scale of the sample in order to assess the proper crack driving conditions. These multi-scale observations provide important insights towards the modeling of the fracture toughness of glassy polymers and its relationship with the macromolecular structure and non-linear rheological properties. This novel technique is first tested on a standard PMMA thermoplastic in order to both evaluate its performance and the richness of this new kind of observations. Although the fracture propagation in PMMA is well known to proceed through crazing in the bulk of the samples, our observations provide a clear description and quantitative evaluation of a change of fracture mechanism towards shear yielding fracture accompanied by local necking close to the free surface of the sample, which can be explained by the local change of stress triaxiality. Moreover , this primary surface necking mechanism is shown to be accompanied by a network of secondary grooves that can be related to surface crazes propagating towards the interior of the sample. This overall scenario is validated by post-mortem fractographic investigations by scanning electron microscopy.

International audience

Plastic pollution is widespread in ocean ecosystems worldwide, but it is unknown if plastic offers a unique habitat for bacteria compared to communities in the water column and attached to naturally-occurring organic particles. The large set of samples taken during the Tara-Mediterranean expedition revealed for the first time a clear niche partitioning between free-living (FL), organic particle-attached (PA) and the recently introduced plastic marine debris (PMD). Bacterial counts in PMD presented higher cell enrichment factors than generally observed for PA fraction, when compared to FL bacteria in the surrounding waters. Taxonomic diversity was also higher in the PMD communities, where higher evenness indicated a favorable environment for a very large number of species. Cyanobacteria were particularly overrepresented in PMD, together with essential functions for biofilm formation and maturation. The community distinction between the three habitats was consistent across the large-scale sampling in the Western Mediterranean basin. 'Plastic specific bacteria' recovered only on the PMD represented half of the OTUs, thus forming a distinct habitat that should be further considered for understanding microbial biodiversity in changing marine ecosystems.

International audience

International audience; An original method based on atomic force microscopy (AFM) in contact mode was developed to abrade progressively the surface of tablets made of starch or gluten polymers isolated from wheat. The volume of the material removed by the tip was estimated from the analysis of successive topographic images of the surface, and the shear force was measured by keeping a constant normal force. Our data together with a simple tribological model provide clear evidence for a higher hardness and shear strength of starch compared to gluten. Gluten appears to have mechanical properties close to soft materials, such as talc, whereas starch displays higher hardness close to calcite. Our results are in a better agreement with structural properties of gluten (complex protein network) and starch (granular and semi-cristalline structure) than earlier studies by micro-indentation. This work shows that the AFM scratching method is relevant for the characterization of any polymer surface, in particular in application to materials made of different polymers at the nano-scale.

International audience; Wheat flours are essential ingredients of daily food products like bread, cookies or pastries. Their quality depends on the milling process and mechanical strength of wheat grains. Although it is well known that the strength and rupture of grains are strongly controlled by the endosperm microstructure, the respective roles of the starch and polymer volume fractions and their adhesion are not yet fully understood. This typical biological microstructure can be modeled as a cemented granular material, where the two size populations of starch granules (large:A-type, small:B-type) are the particles, and the protein matrix, which partially fills the space between granules, plays the role of a cement. This structural model of wheat endosperm is used, together with mechanical characteristics of starch and proteins obtained by means of Atomic Force Microscopy (AFM) measurements, to simulate the mechanical behavior and breakage of wheat endosperm in milling process. We find that the porosity outweighs the effect of other parameters for the elastic modulus, which declines as a nearly linear function of porosity. We also show that the tensile strength is an increasing function of the amount and connectivity of starch granules with increasing concentration of stresses along chains of granules. This effect is more significant at low porosity where stress distribution is mainly controlled by the contact network between starch granules. This effect explains why the protein content is not fully correlated to vitreousness, and samples of similar protein content can be different in vitreosity. Finally, we find that the starch-granule adhesion strongly affects the tensile strength whereas the effect of starch volume fraction appears mainly at high interface adhesion, which is the case of hard type wheat grains.

Crack tips in silica glass in moist atmosphere are filled with an equilibrium liquid condensation of a few hundred nanometers length. Not only does this local environment affect the chemistry of slow crack propagation by stress corrosion, but it also has an important mechanical effect due to its highly negative Laplace pressure. The present article presents an original technique for measuring the physical properties of the liquid condensation in terms of the Laplace pressure and critical condensation distance. This is achieved by combining in situ atomic force microscopy measurements of the condensate length and optical determination of the crack closure threshold in a double cleavage drilled compression specimen.

Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100 nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10^-9 to 10^-10 m/s). These observations are based on in-situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip., Accepted in JNCS. In press

The increase in computational power has made it possible to simulate the behaviour of granular materials in realistic situations. However there is a lack of knowledge of the values of the particle properties that are needed as input parameters to computer simulations for many materials used in the nuclear industry. The Atomic Force Microscope (AFM) is a good candidate tool for the measurement of individual particle properties. In the study presented here, an AFM has been used to measure the interaction forces (normal or lateral) between particles and to characterise the topography of the contact areas, in the case of both model systems (spherical glass particles and planar mica surfaces) and industrially relevant systems involving irregular uranium oxide particles. It will be shown how these experiments provide useful information for the analysis of the flow properties of a powder. The possible use of this experimental data in the simulation of bulk powder flowing behaviour will be discussed.

Fracture propagation involves the coupling of many length scales ranging from the sample loading geometry to the molecular level. In brittle materials, the length scales of the damage process zone are reduced to a submicrometric scale and the coupling with the macroscopic scale is expected to be the domain of linear elastic fracture mechanics. However, although 2D elastic analyses are generally adequate to describe the sample deformation at macroscopic scales, local investigations of failure mechanisms at the sample-free surface require the use of 3D mechanical tools due to the crack front local curvature and to the corner point singularities at the intersection between the crack front and the external surfaces of the sample. We present here a thorough multiscale investigation of the slow crack growth of a sharp crack in oxide glasses in the stress-corrosion regime, combining experimental and numerical analyses of the displacement fields from the millimeter scale to the nanoscale range. The principal aim of the study is identifying the length and time scales of the mechanisms of damage and interaction between water and glass, which have been the subject of an extensive debate in the last decades.

Specific AFM analytical methods were tested to the surface evaluation of wheat powders at micrometric scale in dry conditions. The objective was to evaluate the adhesion properties of particles based on the main wheat endosperm components (starch, protein, and arabinoxylan) that are characterized by large diversity in terms of chemical composition and particle characteristics. Experiments were conducted by using the force spectroscopy mode to determine interactions generated by the surfaces of particles that were glued on AFM tips, and two reference flat and smooth surfaces, a glass and a polysine slide. Particular care has been devoted to characterize the surfaces in contact since local contact radius is a key parameter in the interpretation of the interactions. The statistical analysis of the normalized adhesion forces have shown the sensitivity of the AFM technique for hydrophobic wheat components and their correlation with the surface tension of the particles.

The opening profile of the cracks produced in the Double Cleavage Drilled Compression (DCDC) specimens for brittle materials is investigated. The study is achieved by combining Finite Element simulations of a DCDC linear elastic medium with experimental measurements by crack opening interferometry on pure silica glass samples. We show that the shape of the crack can be described by a simple expression as a function of the geometrical parameters of the sample and the external loading conditions. This result can be used to measure accurately in real time relevant quantities during DCDC experiments, such as the crack length or the stress intensity factor applied to the specimen., Accepted for publication in the International Journal of Fracture

The roughness of fracture surfaces has been shown to exhibit self-affne scale invariance for a wide variety of materials and loading conditions. The range of scales over which this regime extends remains a matter of debate, together with the universality of the associated exponents. The topography of these surfaces is however often investigated with a contact probe that is larger than the micro-structure. In this case, we show that the correlation function of the roughness and the corresponding Hurst exponent $\zeta$ can only be measured down to a length scale $\Delta xc$ which depends on the probe size $R$, on $\zeta$ and on the surface topothesy $l$, and exhibit spurious behavior at smaller scales. First, we derive the dependence of $\Delta xc$ on these parameters from a simple scaling argument. Then we study this dependence numerically and verify our theoretical prediction. Finally, we establish the relevance of this analysis from AFM measurements on an experimental glass fracture surface and provide a metrological procedure for roughness measurements., Comment: 4 pages, 4 figures, submitted to PRL

We study the equilibrium properties of a liquid phase condensed at the nanoscale between the surfaces of a sharp crack in fused silica in a moist controlled atmosphere. The extension of the condensed phase along the fracture is measured by in situ atomic force microscopy phase imaging and it is shown to be determined by a critical distance between the opposite crack surfaces, which is an increasing function of humidity. The present technique is very promising for measuring the properties of confined liquids at the nanoscale as well as for modeling the physics and chemistry of slow crack propagation in glasses.

International audience; The slow advance of a crack in sodo-silicate glasses was studied at nanometer scale by in-situ atomic force microscopy (AFM) in a well-controlled atmosphere (N2 and H2O). An enhanced diffusion of sodium ions in the stress-gradient field at the sub-micrometric vicinity of the crack tip was revealed through several effects: growth of nodules in height images, changes in the AFM tip–sample energy dissipation as detected in phase images. Ex-situ chemical micro-analyses completed the AFM measurements. The nodules patterns revealed a dewetting phenomenon evidenced by “breath figures”, i.e. analog to the fogging that occurs when a vapour condenses onto a 'cold' surface [D. Beysens et al., Phys. Rev. Let. 57, 1433 (1986)]. These experimental results were explained by a two-step process: i) a fast migration of sodium ions towards the fracture surfaces as proposed by Langford et al. [J. Mat. Res. 6, 1358 (1991)], ii) a slow backwards diffusion of the cations as evidenced in these AFM experiments (typical time: few minutes). Measurements of the diffusion coefficient of that relaxing process were done at room temperature. Our results strengthen the theoretical concept of a near-surface structural relaxation due to the stress-gradient at the vicinity of the crack tip. Raman and SIMS studies revealed that nodules – for samples studied after exposition to common air - are preferentially covered by an organic overlayer of a carboxylate salt with a long aliphatic chain. The catalytic role of sodium ions in that chemical process is suspected.

Crack propagation in oxide glasses at low crack velocities is controlled by stress corrosion. Proper knowledge of the crack tip chemical environment is thus crucial to understand the slow fracture process of these materials. The formation of a liquid condensate in the confined area of the crack tip is theoretically expected and is classically mentioned to explain specific behavior during the crack propagation. Since this condensate is of the nanometer scale and cannot be traced post mortem, it is extremely difficult to observe. This paper reports the experimental evidences of the presence of a liquid condensate at the tip of a crack propagating by stress corrosion in silica glass. The observation has been performed in situ under carefully controlled atmosphere through phase imaging by atomic force microscopy (AFM). The evolution of the condensate has been followed as a function of the relative humidity. The impact of those evidences of the liquid condensate will be discussed and information on its size or evolution kinetic will be reported.

The wheat grain is a natural composite material of worldwide importance. The major part of the grain is the starchy endosperm. To obtain food products, such as flour, the endosperm’s compact structure needs to be disintegrated, which is achieved by milling the grains under high forces. The quantity and quality of the milling products notably depend on the fragmentation behaviour of the endosperm.Due to the endosperm’s composite nature, this behaviour depends strongly on the mechanical properties of its components and their interaction. The main components of the endosperm are carbohydrates and proteins. The carbohydrates are deposited as starch in the form of granules of micro-meter size, whereas proteins form a network (gluten), which surrounds the starch granules. The interactions between starch and proteins is believed to be influenced by certain non-gluten proteins (puroindolines), whose presence and allelic state are genetically controlled. If puroindoline genes are present in the wild-type form, grain hardness is low, which have been related to low starch-protein adhesion. The complete absence of puroindolines in the durum wheat species leads to very high grain hardness and indicates a strong adhesion.The aim of this thesis was to investigate the biomechanics of wheat grain fractionation with a focus on the role of the starch granules therein, which was pursued with a multi-disciplinary approach. Different size scales were considered, from the micro meter-sized structures of starch and protein, the complexity of their arrangement in the endosperm, up to the millimeter-sized grains. In this work, grain-scale milling experiments were combined with nano-mechanical measurements by atomic force microscopy (AFM) and numerical simulations.The milling behaviour of a transgenic durum wheat line, which contained puroindoline genes, was determined by grain scale milling experiments and compared to the milling behavior of non-modified durum wheat. A significant change of milling behavior of the transformed durum wheat grains was observed in terms of milling energy, flour yield and starch damage, which was solely attributable to the presence of puroindolines. The observed changes were consistent with the hypothesis of a lower adhesion between starch granules and protein matrix due to the presence of puroindolines and confirmed the significant effect of puroindolines on the fragmentation behaviour, independent of the grain’s genetic background.The change of fragmentation behaviour is a result of modifications of the mechanical properties of the endosperm’s components and/ or their interaction. Such modifications can be investigated by AFM nano-mechanical measurements. Based on previous work illuminating the global nano-mechanical properties of starch and gluten, contact-resonance AFM (CR-AFM) was applied to obtain maps of the nano-mechanical properties inside the grains. Due to the high topography variations of grain section surfaces and the non-trivial correlation between surface slope and contact resonance-frequency, which hindered a straight-forward interpretation of CR-AFM measurements, a practical method based on existing analytical models of the cantilever vibration was developed to correct the measurements. CR-AFM studies of the endosperm were then focused specifically on the mechanical properties of starch granules and the link to starch structure, and applied to the study of starches from wheat in comparison to plants from different botanical origin (other cereals and legumes).Finally, the role of starch granules, their size distribution, and mechanical properties on endosperm fragmentation was analysed by parametric numerical studies. The influence of the bi-modal size distribution of granules on the mesoscale mechanical properties was shown, as well as the governing role of granule toughness and interface adhesion on the granule damage.; La culture du blé est l’une des plus importantes au monde. Le grain de blé est un matériau composite naturel dont la majeure partie est constituée d’albumen amylacé formé d’un assemblage compact de granules d’amidons (glucides) enchâssés dans une matrice protéique (gluten). Pour obtenir des produits comme la farine, la structure de l'albumen doit être fragmentée en broyant les grains sous des fortes contraintes. La quantité et la qualité des produits obtenus dépendent du comportement de l’albumen à la fragmentation. En raison de sa nature composite, le comportement rhéologique du grain est tributaires des propriétés mécaniques des phases qui le composent (granules, gluten, pores), de leurs interactions, ainsi que de leur distribution spatiale. Les granules d’amidons sont de formes relativement sphériques et de tailles micrométriques, tandis que les protéines sont organisées en un réseau entourant les granules. L'interaction entre l'amidon et ce réseau protéique est influencée par certaines protéines, les puroindolines, dont la présence et le type d’allèle sont contrôlées génétiquement. Si les gènes codant pour les puroindolines sont présents sous forme sauvages, la dureté meunière, c’est à dire l’aptitude à la fragmentation du grain est faible. L’origine de ce comportement est liée à une adhérence limitée entre matrice protéique et amidon. L'absence totale de puroindolines chez le blé dur conduit au contraire à une dureté très élevée des grains et à une forte adhérence. L'objectif de cette thèse est d'étudier, à partir d’une approche multidisciplinaire, la biomécanique du fractionnement du grain de blé en mettant l'accent sur le rôle des granules d'amidon. Des échelles de taille différentes sont considérées : échelle micrométrique du granule et de la matrice protéique; agencement complexe de ces composants dans l'albumen et échelle millimétrique du grain. Ainsi, des expériences de broyage à l'échelle du grain ont été combinées avec des mesures nano-mécaniques par microscopie à force atomique (AFM) et des simulations numériques.Le comportement au broyage a été étudié en utilisant un micro-moulin instrumenté. Une comparaison a été effectuée entre des essais réalisés sur une variété de blé dur et sur la même variété dans laquelle ont été introduits les gènes codant pour les puroindolines. Un changement significatif du comportement mécanique des grains transformés, attribuable uniquement à la présence de puroindolines, a été observé - en termes d'énergie consommée, - de productivité en farine et - de taux d'amidon endommagé. Ces changements sont compatibles avec l'hypothèse d'une faible adhérence, entre granules d'amidon et matrice protéique, induite par la présence des puroindolines et montrent l'effet significatif de celles-ci sur le comportement à la fragmentation. Ces modifications de comportement mécanique peuvent être étudiées par des mesures AFM nano-mécaniques. Pour compléter des travaux antérieurs ayant permis la mesure des propriétés de l'amidon et du gluten, une méthode basée sur des mesures AFM en mode résonance de contact (CR-AFM) a été développée. Celle-ci permet de cartographier les propriétés directement à l'intérieur des granules d’amidon et prend en compte à travers un modèle théorique les variations importantes de topographie observées dans les sections de grains. Ces études CR-AFM de l'albumen ont ensuite porté sur les propriétés mécaniques des granules d'amidon d'origines botaniques différentes (céréales et légumineuses).Enfin, le rôle de la distribution bimodale en taille des granules d'amidon sur la fragmentation de l'albumen a été précisé à partir d’une étude numérique paramétrique détaillée. Les propriétés mécaniques élastiques et à la rupture ont été analysées en détail, ainsi que le rôle dominant de la ténacité des granules et de l'adhérence à l'interface sur l’endommagement de l’amidon.

The wheat grain is a natural composite material of worldwide importance. The major part of the grain is the starchy endosperm. To obtain food products, such as flour, the endosperm’s compact structure needs to be disintegrated, which is achieved by milling the grains under high forces. The quantity and quality of the milling products notably depend on the fragmentation behaviour of the endosperm.Due to the endosperm’s composite nature, this behaviour depends strongly on the mechanical properties of its components and their interaction. The main components of the endosperm are carbohydrates and proteins. The carbohydrates are deposited as starch in the form of granules of micro-meter size, whereas proteins form a network (gluten), which surrounds the starch granules. The interactions between starch and proteins is believed to be influenced by certain non-gluten proteins (puroindolines), whose presence and allelic state are genetically controlled. If puroindoline genes are present in the wild-type form, grain hardness is low, which have been related to low starch-protein adhesion. The complete absence of puroindolines in the durum wheat species leads to very high grain hardness and indicates a strong adhesion.The aim of this thesis was to investigate the biomechanics of wheat grain fractionation with a focus on the role of the starch granules therein, which was pursued with a multi-disciplinary approach. Different size scales were considered, from the micro meter-sized structures of starch and protein, the complexity of their arrangement in the endosperm, up to the millimeter-sized grains. In this work, grain-scale milling experiments were combined with nano-mechanical measurements by atomic force microscopy (AFM) and numerical simulations.The milling behaviour of a transgenic durum wheat line, which contained puroindoline genes, was determined by grain scale milling experiments and compared to the milling behavior of non-modified durum wheat. A significant change of milling behavior of the transformed durum wheat grains was observed in terms of milling energy, flour yield and starch damage, which was solely attributable to the presence of puroindolines. The observed changes were consistent with the hypothesis of a lower adhesion between starch granules and protein matrix due to the presence of puroindolines and confirmed the significant effect of puroindolines on the fragmentation behaviour, independent of the grain’s genetic background.The change of fragmentation behaviour is a result of modifications of the mechanical properties of the endosperm’s components and/ or their interaction. Such modifications can be investigated by AFM nano-mechanical measurements. Based on previous work illuminating the global nano-mechanical properties of starch and gluten, contact-resonance AFM (CR-AFM) was applied to obtain maps of the nano-mechanical properties inside the grains. Due to the high topography variations of grain section surfaces and the non-trivial correlation between surface slope and contact resonance-frequency, which hindered a straight-forward interpretation of CR-AFM measurements, a practical method based on existing analytical models of the cantilever vibration was developed to correct the measurements. CR-AFM studies of the endosperm were then focused specifically on the mechanical properties of starch granules and the link to starch structure, and applied to the study of starches from wheat in comparison to plants from different botanical origin (other cereals and legumes).Finally, the role of starch granules, their size distribution, and mechanical properties on endosperm fragmentation was analysed by parametric numerical studies. The influence of the bi-modal size distribution of granules on the mesoscale mechanical properties was shown, as well as the governing role of granule toughness and interface adhesion on the granule damage., La culture du blé est l’une des plus importantes au monde. Le grain de blé est un matériau composite naturel dont la majeure partie est constituée d’albumen amylacé formé d’un assemblage compact de granules d’amidons (glucides) enchâssés dans une matrice protéique (gluten). Pour obtenir des produits comme la farine, la structure de l'albumen doit être fragmentée en broyant les grains sous des fortes contraintes. La quantité et la qualité des produits obtenus dépendent du comportement de l’albumen à la fragmentation. En raison de sa nature composite, le comportement rhéologique du grain est tributaires des propriétés mécaniques des phases qui le composent (granules, gluten, pores), de leurs interactions, ainsi que de leur distribution spatiale. Les granules d’amidons sont de formes relativement sphériques et de tailles micrométriques, tandis que les protéines sont organisées en un réseau entourant les granules. L'interaction entre l'amidon et ce réseau protéique est influencée par certaines protéines, les puroindolines, dont la présence et le type d’allèle sont contrôlées génétiquement. Si les gènes codant pour les puroindolines sont présents sous forme sauvages, la dureté meunière, c’est à dire l’aptitude à la fragmentation du grain est faible. L’origine de ce comportement est liée à une adhérence limitée entre matrice protéique et amidon. L'absence totale de puroindolines chez le blé dur conduit au contraire à une dureté très élevée des grains et à une forte adhérence. L'objectif de cette thèse est d'étudier, à partir d’une approche multidisciplinaire, la biomécanique du fractionnement du grain de blé en mettant l'accent sur le rôle des granules d'amidon. Des échelles de taille différentes sont considérées : échelle micrométrique du granule et de la matrice protéique; agencement complexe de ces composants dans l'albumen et échelle millimétrique du grain. Ainsi, des expériences de broyage à l'échelle du grain ont été combinées avec des mesures nano-mécaniques par microscopie à force atomique (AFM) et des simulations numériques.Le comportement au broyage a été étudié en utilisant un micro-moulin instrumenté. Une comparaison a été effectuée entre des essais réalisés sur une variété de blé dur et sur la même variété dans laquelle ont été introduits les gènes codant pour les puroindolines. Un changement significatif du comportement mécanique des grains transformés, attribuable uniquement à la présence de puroindolines, a été observé - en termes d'énergie consommée, - de productivité en farine et - de taux d'amidon endommagé. Ces changements sont compatibles avec l'hypothèse d'une faible adhérence, entre granules d'amidon et matrice protéique, induite par la présence des puroindolines et montrent l'effet significatif de celles-ci sur le comportement à la fragmentation. Ces modifications de comportement mécanique peuvent être étudiées par des mesures AFM nano-mécaniques. Pour compléter des travaux antérieurs ayant permis la mesure des propriétés de l'amidon et du gluten, une méthode basée sur des mesures AFM en mode résonance de contact (CR-AFM) a été développée. Celle-ci permet de cartographier les propriétés directement à l'intérieur des granules d’amidon et prend en compte à travers un modèle théorique les variations importantes de topographie observées dans les sections de grains. Ces études CR-AFM de l'albumen ont ensuite porté sur les propriétés mécaniques des granules d'amidon d'origines botaniques différentes (céréales et légumineuses).Enfin, le rôle de la distribution bimodale en taille des granules d'amidon sur la fragmentation de l'albumen a été précisé à partir d’une étude numérique paramétrique détaillée. Les propriétés mécaniques élastiques et à la rupture ont été analysées en détail, ainsi que le rôle dominant de la ténacité des granules et de l'adhérence à l'interface sur l’endommagement de l’amidon.

This work deals with the slow crack growth in glass polymer materials at room temperature, namely PMMA and stoechiometric epoxy-amine DGEBA-IPD resin as archetypes of thermoplastic (TP) and thermoset (TS) respectively. TS/TP blends developing phases separation in nano-domains of thermoplastic during the cure process have also been studied since they are used as matrix with a complex morphology in carbon fibers reinforced composites materials for aircrafts. An innovative experimental setup has been developed for in situ investigation of slow crack growth by combining optical measurements, in situ atomic force microscopy (AFM) and a Double Cleavage Drilled Compression (DCDC) sample. By this way, a steady state crack propagation regime has been highlighted for a first time in thermosets (pure and TS/TP blends) with crack speed ranging from pm/s to nm/s. Thus crack speed and stress intensity factor (SIF) diagrams were established for each material. Furthermore, in situ AFM images allowed characterizing deformations at the process zone scale by combining topographic imaging with Digital Images Correlation (DIC) technique. Finally, we developed a model for predicting fracture energy for steady state propagation and stick-slip regime, based on the visco-plastic strain fields measured round the process zone, the local strain rate, and the constitutive laws of materials measured at the macroscopic scale., L’étude porte sur la propagation lente de fissure à température ambiante dans deux classes de polymères à l’état vitreux : le PMMA et la résine époxy-amine stœchiométrique DGEBA-IPD comme prototypes respectifs de thermoplastique (TP) et de thermodurcissable (TD). Des mélanges TD/TP présentant une séparation de phase de nano-domaines de thermoplastique dans une phase dominante de thermodurcissable ont été aussi explorés en vue de leur intérêt en tant que matrice à morphologie complexe utilisée dans les composites à renfort de fibre de carbone destinés à l’aéronautique. Un montage expérimental original a été développé pour le suivi in situ de propagation lente de fissures, combinant une caméra optique, un microscope à force atomique (AFM) et un échantillon sollicité en géométrie Double Cleavage Drilled Compression (DCDC). La possibilité d’accéder ainsi à une gamme de vitesses de propagation entre le pm/s et le nm/s a permis de mettre en évidence pour la toute première fois un régime de propagation stationnaire dans les thermodurcissables (résines pures et mélanges TD/TP) analogue au comportement établi pour les thermoplastiques, et de le caractériser en termes d’une loi cinétique reliant la vitesse de propagation au facteur d’intensité des contraintes (SIF). L’analyse in situ AFM a de plus permis de caractériser les différentes modalités de déformation à l’échelle de la zone de process en combinant l’imagerie topographique et des techniques de corrélation d’images numériques (DIC). Un modèle de prédiction des énergies de rupture en propagation stationnaire ainsi qu’en régime de stick-slip a été développé, en intégrant les champs de déformation visco-plastiques mesurés dans la zone de process, la vitesse de déformation locale, et la loi de comportement des matériaux mesurée à l’échelle macroscopique.