Résumé:

Brittle and ductile fracture processes in solids have been extensively studied. By contrast, referring to fracture in transient networks is conceptually challenging, as this class of materials is viscoelastic and intrinsically self-healing. There is nevertheless some experimental evidence of brittle crack propagation in self-assembled networks. However, experimental evidence and theoretical understanding of a brittle-to-ductile transition in transient networks is still lacking. We use a Hele-Shaw cell, where a transient network confined between two glass plates is pushed by a low viscous oil injected at a fixed rate, Q. We show that depending on Q, two classes of instability develop: a standard Saffman-Taylor fingering instability or a fracturing instability. By combining the displacement fields of the viscoelastic material around the crack or finger, as obtained by particle image velocimetry, with a morphology analysis, we clearly identify the fingering to fracturing transition. In addition, our analysis allows one to discriminate slow cracks where the viscoelasticity of the network matters and fast cracks dominated by elasticity. We use this approach to investigate the fracture mechanisms in a novel class of transient networks, made of surfactant micelles of tunable morphology reversibly linked by block copolymers. By coupling rheology and time-resolved X-ray measurements, we have proposed the fluctuations of the degree of alignment of the micelles under shear as a probe to identify a fracture process, and have evidenced a brittle-to-ductile transition in transient gels, as the morphology of the micelles varies, thus suggesting a parallel between the fracture of solids and the fracture under shear of viscoelastic fluids. The measurement of birefringence of the samples at the vicinity of the crack allows us to relate the crack properties to the structural properties of the complex fluids and to identify the mechanisms at play in a brittle to ductile transition in transient networks.

Ref Arxiv: 1212.3945
DOI: 10.1103/PhysRevE.87.032306
WoS: 000316393300007
Ref. & Cit.: NASA ADS
13 Citations
Résumé:

We study the freezing kinetics of colloidal polycrystals made of micelles of Pluronic F108, a thermosensitive copolymer, to which a small amount of silica nanoparticles of size comparable to that of the micelles are added. We use rheology and calorimetry to measure Tc, the crystallization temperature, and find that Tc increases with the heating rate \dot{T} used to crystallize the sample. To rationalize our results, we first use viscosity measurements to establish a linear mapping between temperature T and the effective volume fraction, {\phi}, of the micelles, treated as hard spheres. Next, we reproduce the experimental \dot{T} dependence of the crystallization temperature with numerical calculations based on standard models for the nucleation and growth of hard spheres crystals, classical nucleation theory and the Johnson-Mehl-Avrami-Kolmogorov theory. The models have been adapted to account for the peculiarities of our experiments: the presence of nanoparticles that are expelled in the grain boundaries, and the steady increase of T and hence {\phi} during the experiment. We moreover show that the polycrystal grain size obtained from the calculations is in good agreement with light microscopy data. Finally, we find that the {\phi} dependence of the nucleation rate for the micellar polycrystal is in remarkable quantitative agreement with that found in previous experiments on colloidal hard spheres. These results suggests that deep analogies exist between hard-sphere colloidal crystals and Pluronics micellar crystals, in spite of the difference in particle softness. More generally, our results demonstrate that crystallization processes can be quantitatively probed using standard rheometry.

Commentaires: Journal: Phys. Rev. E 87, 032306 (2013)

Résumé:

Virtually all real-life crystalline materials, metal or ceramics, are in fact aggregates of crystalline grains. Grain-boundaries (GBs), the two-dimensional lattice defects that separate the different grains of a crystal, control the mechanical properties of polycrystalline materials. Although GB motion is known to play important roles in plastic deformation, the microscopic origin of the plasticity of polycrystalline materials is still largely unknown, because of the limitations of available experimental tools to record, during deformation, the dynamics of the process with a nanometer resolution. To overcome these limitations, we have designed a colloidal analogue of atomic polycrystals. In the first part of my talk I will present our experimental system, obtained by doping a copolymer micellar crystal with nanoparticles. Nanoparticles act as impurities, and as such, they segregate in the GBs of the colloidal polycrystal, allowing their visualization by imaging or scattering techniques. I will show that the microstructure of the polycrystal can be tuned by independently varying the nanoparticle volume fraction and the crystallization rate, and that our findings can be quantified using standard models for the nucleation and growth of crystalline materials. The second part of my talk will be devoted to our preliminary results on the plasticity of the colloidal polycrystals. We perform multispeckle time-resolved dynamic light scattering measurements on the samples submitted to cyclic shear deformations using a novel light scattering apparatus specifically designed to access the dynamics of the network of GBs. Plasticity is quantified by analyzing the correlation of the scattered intensity measured after a given number of deformation cycles. We demonstrate that the shear-induced GB dynamics at the origin of plasticity is aging in a length-scale dependence manner. We extract a characteristic length above which the GB dynamics is stationary, which is found to increase with the shear amplitude. Our data suggest a hierarchical organization of the GB network under shear and provides a novel framework to understand the plasticity of polycrystals.

Résumé:

Crack initiation and propagation have been previously studied in our group for model complex fluids using pendant drop experiments [1].We will present here experimental results obtained in a radial Hele Shaw cell, where crack propagation can be better controlled and followed over longer time, once initiated by pushing the complex fluids with a low viscous oil. We use self-assembled transient networks with tunable morphology for which rheology data suggest a brittle to ductile transition [2]. We characterize the crack morphology and speed thanks to imaging with a high speed camera and couple these data to the complex fluids displacement fields around the crack as obtained by particle image velocimetry (fig. 1). Using crossed polarizers, we moreover measure the birefringence of the samples under flow (fig. 2) allowing us to relate the crack properties to the structural properties of the complex fluids and to identify the process zone at the vicinity of the crack tip. Thanks to our space and time-resolved measurements, the mechanisms at play in a brittle to ductile transition in complex fluids could be elucidated.

Résumé:

Nous étudions la nucléation et la propagation de fractures ainsi que d'autres types d'instabilités de fluides viscoélastiques modèles dans une cellule de Hele-Shaw sous injection radiale par des techniques de visualisation résolues en temps. Nous tenterons de comprendre les relations entre la structures des fluides sous écoulement, l'apparition, et la nature des instabilités.

Ref HAL: hal-00762379_v1
Ref Arxiv: 1212.1064
DOI: 10.1002/adfm.201200666
WoS: 000312300600006
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
32 Citations
Résumé:

We use hexagonal mesophases made of oil-swollen surfactant-stabilized tubes arranged on a triangular lattice in water and doped with metallic salts as templates for the radiolytic synthesis of nanostructures. The nanostructures formed in this type of soft matrix are bimetallic palladium-platinum porous nanoballs composed of 3D-connected nanowires, of typical thickness 2.5 nm, forming hexagonal cells. We demonstrate using electron microscopy and small-angle X-ray scattering that the pore size of the nanoballs is directly determined by the diameter of the oil tube of the doped mesophases, which we have varied in a controlled fashion from 10 to 55 nm. Bimetallic nanostructures comprising various proportions of palladium and platinum can be synthesized. Their alloy structure was evidenced by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and high-angular dark field scanning transmission electron microscopy experiments. Our templating approach allows therefore the synthesis of bimetallic nanoballs of tunable porosity and composition.

Commentaires: Journal: Advanced Functional Materials 22, 4900 (2012)

Ref HAL: hal-00760633_v1
DOI: 10.1039/c2nj40342k
WoS: 000308825300031
Exporter : BibTex | endNote
30 Citations
Résumé:

Very long and thin monocrystalline Pd nanowires were synthesized by decomposition using hydrazine vapour treatment of Pd2(DBA)3 confined in the oil phase of a hexagonal mesophase. The mesophase is composed of toluene-swollen tubes, doped with the Pd precursor, stabilized by a monolayer of surfactant and cosurfactant molecules, and arranged on a triangular lattice in water. Our synthesis is selective and the experiments show that both the confinement and the nature of the surfactant are essential to obtain these Pd nanowires.