The rheology of NFC suspensions that exhibited different microstructures and colloidal stability, namely TEMPO and enzymatic NFC suspensions, was investigated at the macro and mesoscales using a ...transparent Couette rheometer combined with optical observations and ultrasonic speckle velocimetry (USV). Both NFC suspensions showed a complex rheology, which was typical of yield stress, non-linear and thixotropic fluids. Hysteresis loops and erratic evolutions of the macroscale shear stress were also observed, thereby suggesting important mesostructural changes and/or inhomogeneous flow conditions. The
in situ
optical observations revealed drastic mesostructural changes for the enzymatic NFC suspensions, whereas the TEMPO NFC suspensions did not exhibit mesoscale heterogeneities. However, for both suspensions, USV measurements showed that the flow was heterogeneous and exhibited complex situations with the coexistence of multiple flow bands, wall slippage and possibly multidimensional effects. Using USV measurements, we also showed that the fluidization of these suspensions could presumably be attributed to a progressive and spatially heterogeneous transition from a solid-like to a liquid-like behavior. As the shear rate was increased, the multiple coexisting shear bands progressively enlarged and nearly completely spanned over the rheometer gap, whereas the plug-like flow bands were eroded.
The coexistence of multiple flow bands, wall slippage and possible multidimensional effects in enzymatic and TEMPO cellulose nanofibril suspensions under shear were revealed using optical and ultrasonic speckle velocimetry (USV) rheometry techniques.
Socio-economic status (SES) is a strong determinant of eating behavior and the obesity risk.
To determine which eating and lifestyle behaviors mediate the association between SES and obesity.
We ...performed a case-control study of 318 obese people and 371 non-obese people in northern France. Ten eating behavior traits were assessed using the Three-Factor Eating Questionnaire Revised 21-Item and an eating attitude questionnaire (on plate size, the number of servings, reasons for stopping eating and the frequency of eating standing up, eating in front of the television set (TV) and eating at night). The SES score (in three categories) was based on occupation, education and income categories. Mediation analysis was performed using the test of joint significance and the difference of coefficients test.
The age- and gender-adjusted obesity risk was higher for individuals in the low-SES groups (odds ratio (OR) (95% confidence interval (CI)=1.82 (1.48-2.24), P<0.0001). Additional servings were associated with a higher obesity risk (OR=3.43, P<0.0001). Cognitive restraint (P<0.0001) and emotional eating (P<0.0001) scores were higher in obese participants than in non-obese participants but did not depend on SES. Of the 10 potential factors tested, eating off a large plate (P=0.01), eating at night (P=0.04) and uncontrolled eating (P=0.03) significantly mediated the relationship between SES and obesity.
Our results highlighted a number of obesogenic behaviors among socially disadvantaged participants: large plate size, uncontrolled eating and eating at night were significant mediators of the relationship between SES and the obesity risk.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This study aimed at investigating how ice templating conditions affected the shrinkage, the microstructure, and the mechanical properties of cellulose nanofibril (NFC) foams. Enzymatic and ...TEMPO-oxidized NFC foams were fabricated using two solidification techniques, i.e., quenching NFC suspensions in temperature-controlled baths, and mechanical stirring of NFC suspensions during solidification followed by quenching, prior to freeze-drying. Foams prepared by direct quenching using stabilized TEMPO-oxidized NFC suspensions exhibited higher specific mechanical properties and more regular anisotropic cells with unimodal size than enzymatic NFC foams. In addition, NFC concentration and NFC morphology severely affected the foam shrinkage and the geometry of foam cells. Controlling the solidification had also a drastic effect on the foam microstructure, e.g. foams prepared by mechanical stirring and quenching exhibited bimodal cell size and enhanced mechanical properties. The compressive behavior of foams showed successive elastic, strain-hardening and densification regimes with auxetic effects and strain localization. Both the elastic modulus and the yield stress were power-law functions of the foam relative density whose exponents reached unusual high values for enzymatic NFC foams, potentially because of their chaotic microstructures. The evolution of a typical TEMPO-oxidized NFC foam was studied under compression using X-ray microtomography, unveiling deformation mechanisms at the cell scale.
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•Cellulose nanofibril (NFC) foams with various structural and mechanical properties were fabricated using cryodesiccation.•Two solidification techniques of NFC suspensions were used: direct quenching and mechanical stirring followed by quenching.•Solidification, type and concentration of NFC suspensions affected both foam microstructure and mechanical performances.•3D X-ray microtomography images of in situ compression experiment unveiled the deformation mechanisms of foam cells.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Processing cellulose nanofibril (CNF) hydrogels with a high concentration is a solution to reduce logistics costs and drying energy and to produce CNF-based materials with good dimensional stability. ...However, the rheology of concentrated and highly concentrated CNF hydrogels is poorly understood due to the difficulties to characterise them using standard shear rheometers. In this study, enzymatic CNF hydrogels in the concentrated and highly concentrated regimes (3–13.6 wt%) were subjected to lubricated compression at various strain rates. At low strains, compression curves exhibited a linear regime. At higher strains and low strain rates, a heterogeneous and marked hardening of stress levels was observed and accompanied with a two-phase flow with significant fluid segregation and network consolidation. At high strain rates, a homogeneous and incompressible one-phase plateau-like regime progressively established. In this regime, a yield stress was measured and compared with literature data, showing a good agreement with them.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
TEMPO-oxidized cellulose nanofibril (CNF) hydrogels or cellulose nanocrystal (CNC) hydrogels can now be obtained at high concentrations (>10 wt%) and used to fabricate biobased materials and ...structures. Thus, it is required to control and model their rheology in process-induced multiaxial flow conditions using 3D tensorial models. For that purpose, it is necessary to investigate their elongational rheology. Thus, concentrated TEMPO-oxidized CNF and CNC hydrogels were subjected to monotonic and cyclic lubricated compression tests. These tests revealed for the first time that the complex compression rheology of these two electrostatically stabilised hydrogels combines viscoelasticity and viscoplasticity. The effect of their nanofibre content and aspect ratio on their compression response was clearly emphasised and discussed. The ability of a non-linear elasto-viscoplastic model to reproduce the experiments was assessed. Even if some discrepancies were observed at low or high strain rates, the model was consistent with the experiments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this study, we characterized and modeled the rheology of TEMPO-oxidized cellulose nanofibril (NFC) aqueous suspensions with electrostatically stabilized and unflocculated nanofibrous structures. ...These colloidal suspensions of slender and wavy nanofibers exhibited a yield stress and a shear thinning behavior at low and high shear rates, respectively. Both the shear yield stress and the consistency of these suspensions were power-law functions of the NFC volume fraction. We developed an original multiscale model for the prediction of the rheology of these suspensions. At the nanoscale, the suspensions were described as concentrated systems where NFCs interacted with the Newtonian suspending fluid through Brownian motion and long range fluid-NFC hydrodynamic interactions, as well as with each other through short range hydrodynamic and repulsive colloidal interaction forces. These forces were estimated using both the experimental results and 3D networks of NFCs that were numerically generated to mimic the nanostructures of NFC suspensions under shear flow. They were in good agreement with theoretical and measured forces for model colloidal systems. The model showed the primary role played by short range hydrodynamic and colloidal interactions on the rheology of NFC suspensions. At low shear rates, the origin of the yield stress of NFC suspensions was attributed to the combined contribution of repulsive colloidal interactions and the topology of the entangled NFC networks in the suspensions. At high shear rates, both concurrent colloidal and short (in some cases long) range hydrodynamic interactions could be at the origin of the shear thinning behavior of NFC suspensions.
Modelling physical and mechanical properties of fibrous materials requires a relevant description of their microstructures, e.g. the descriptors of fibres and fibre–fibre contacts. In this work, a ...method is proposed to identify fibres with complex cross sections and their contacts from 3D images of disordered fibrous media, obtained from X-ray microtomography. The image analysis procedure first consists in obtaining a map of the local fibre orientation. Fibres are then detected by deleting regions of high local orientation gradients. Therewith, dilatation operations using slender orientated structuring elements are performed to retrieve deleted fibre parts. The procedure thus provides 3D images with labelled fibres and fibre–fibre contacts. Its relevance is assessed by analysing the microstructures of three typical fibrous media used in short fibre reinforced polymer composites or as paper materials: mats of (i) mono-disperse copper fibres with circular cross sections, (ii) glass fibres bundles, (iii) wood fibres.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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•Highly concentrated hydrogels with cellulose nanofibrils or nanocrystals were ice-templated to produce lightweight architected biosourced foams.•X-Ray nanotomography and SEM images ...showed that microstructures of foams switch from anisotropy to isotropy while increasing the nanofiber concentration or aspect ratio.•Compression results showed that the specific mechanical properties of the foams make them as relevant cellular materials for structural applications.
Ice-templated cellular materials made of entangled networks of slender cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs) exhibit excellent specific physical properties but their mechanical properties are still insufficient: these biosourced systems cannot be used as structural materials, e.g., as cores of composite sandwich structures. To overcome this limitation, we ice-templated hydrogels with high concentrations of CNFs and CNCs by using a specific setup that enabled their unidirectional solidification. The microstructure and the mechanical properties of the freeze-dried foams were investigated using SEM, X-Ray nanotomography and compression tests, respectively. Increasing the content of nanofibers yielded to drastic shifts of (i) the foam microstructures from highly anisotropic (columnar with CNFs, lamellar with CNCs) to more isotropic ones (ii) the mechanical properties towards isotropy together with a significant increase of the foam stiffness, yield stress and absorbed energy. In addition, due to the high aspect ratio of the CNFs, CNF foams with elevated relative densities exhibit noteworthy specific mechanical properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Foldable composite structures are bellows-like structures that can be stored in a flat state and deployed to a finite volume after folding. These composite structures are being increasingly demanded ...for numerous engineering applications including aerospace, aeronautics, robotics and medical. In this study, we report an original processing route for obtaining foldable composite laminates. These composite laminates were obtained by selectively curing pre-impregnated materials that were made of three layers of plain weave fabrics impregnated with an UV-curable epoxy-acrylate resin system. After selective curing, the composite laminates were made of rigid domains (cured zones) connected by flexible domains (uncured zones) that played the role of hinges upon folding. The selective curing of the photocurable resin system was performed using two different approaches. The first approach consisted in depositing UV-blocking masks designed numerically onto the surface of pre-impregnated materials before placing them inside the chamber of an UV-curing machine. The second one consisted in using a Digital Light Processing (DLP) 3D printing machine. The UV-curable epoxy acrylate resin exhibited fast curing kinetics characterized by a gelation time on the order of 1 s. Hence, both types of approaches enabled foldable composite laminates with very short time cycles (≈ 5 to 10 s) to be obtained. The as-obtained pre-impregnated sheets were folded into 3D composite structures and then irradiated with UV-light. This original fabrication method is versatile enough to provide a wide diversity of composite part geometries that are promising for many engineering applications. The observations of the microstructure of processed samples revealed that the reinforcement fabrics were rather well impregnated by the epoxy acrylate resin. These observations also tended to show that both types of approaches enabled a complete and efficient in-depth curing of the resin system.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The present study provides original experimental data concerning the evolution of the microstructure of a bundle of fibres during its deformation. For that purpose, a model saturated fibre bundle was ...processed and was subjected to a compression loading by using a specially designed micro-compression rheometer which was mounted on a synchrotron X-ray microtomograph. Thus, 3D images of the evolving fibrous microstructure could be obtained. Results first show that the compression induced both the bundle consolidation and liquid phase migration. Secondly, (i) the position, the orientation, the displacement and the deformation of each fibre together with (ii) the position and the evolution of each fibre-fibre contact were followed during the compression. The
in situ tracking of these microstructure descriptors allows the consolidation micro-mechanisms to be analysed and provides useful information for theoretical or numerical models used to predict fibre bundle deformation during processing of fibre bundle reinforced polymer composites.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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