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.