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 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)