Crack initiation and propagation have been previously studied in our group for model complex fluids using pendant drop experiments [1]. We will present here results obtained in a radial Hele-Shaw cell obtained when a low viscosity oil pushes a more viscous complex fluid, resulting in instabilities of the oil/gel interface. We use as complex fluid, we use a soft transient network gel made of an oil in water microemulsion reversibly link by telechelic polymers. We find a discontinuous transition between the standard Saffman-Taylor fingering instability and a fracturing instability as the oil injection rate increases. We show that the transition towards fracturing is driven by the rheological properties of the gel ahead of the finger tip. By analyzing the displacement field of the gel in the vicinity of the finger or crack, we show that in the fingering regime, the oil gently pushes the gel, whereas in the fracturing regime, the crack tears apart the gel, resulting in a strong drop of the gel velocity ahead of the crack tip as compared to the tip velocity. We find a universal behavior for the whole displacement field of a gel around a crack, which is drastically different from that around a finger, and reveals the solid-like behavior of the gel at short time. We will also present preliminary results obtained with another class of transient gels with tunable morphology for which rheology data suggest a brittle to ductile transition [4]. Thanks to the anisotropy of these gels, birefringence measurements can be performed , allowing one to relate the crack properties to the structural properties of the complex fluid in the vicinity of the cracks. [1] H. Tabuteau, S. Mora, M. Ciccotti, C. Hui, C. Ligoure, Soft Matter 7, 9474 (2011) [2] C. Ligoure, S. Mora, Rheologica Acta 52, Issue 2, 91-114 (2013) [3] H. Zhao, Maher JV., Phys.Rev. E 47, 4278 (1993) [4] L. Ramos, A. Laperrousaz, P. Dieudonné, C. Ligoure, Phys.Rev.Lett. 107, 148302 (2011)

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.

We present a comprehensive review of the cur- rent state of fracture phenomena in transient networks, a wide class of viscoelastic fluids. We will first define what is a fracture in a complex fluid and recall the main structural and rheological properties of transient networks. Secondly, we review experimental reports on fractures of transient networks in several configurations: shear-induced fractures, fractures in Hele-Shaw cells, and fracture in extensional geometries (filament-stretching rheometry and pendant drop experiments), including fracture propaga- tion. The tentative extension of the concepts of brittleness and ductility to the fracture mechanisms in transient net- works is also discussed. Finally, the different and appar- ently contradictory theoretical approaches developed to interpret fracture nucleation will be addressed and con- fronted to experimental results. Rationalized criteria to dis- criminate the relevance of these different models will be proposed.

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.

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.

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