Résumé:

Pendant drop experiments performed with a model physical gel (made from oil in water microemulsion droplets reversibly linked together by triblock copolymers), exhibit a very peculiar filament rupture corresponding to highly brittle failure of a viscoelastic fluid. Indeed, we show that a large class of viscoelastic fluids, i.e., transient networks, are brittle according to the Griffith’s theory of solid fracture. However, contrary to solids, cracks are intrinsic to the material arising from the equilibrium nature of the fluid microstructure. The brittleness of these fluids comes from thermal fluctuations of bonds distribution. In this approach, the rupture stress is predicted to be on the order of the Young modulus, in very good agreement with experimental values. The fracture propagation has been tracked by high speed videomicroscopy. Analysis of the time evolution of the fracture profile shows that the fracture is purely elastic and reversible without any significant bulk and interfacial viscous disspation . This behavior is well explained by the viscoelastic trumpet model of de Gennes. The velocity of the fracture’s propagation is measured and compared to the predictions of a simple microscopic model.