Séminaire Programme Gradué Chimie PSL

Micromechanics of gel rupture

Abstract: This thesis is a study on observing fracture precursors and their nucleation under stress in a model protein gel of casein (yogurt). Fracture is a phenomenon that we encounter in our daily life and are often eager to avoid. This year marks a century since Griffith gave the criteria for crack propagation. Although since then, significant progress has been made in its understanding, fracture prediction still remains elusive due to the multi-scale nature of this phenomenon. As such, finding the precursor: the structural origin to fracture can help in linking the different stages of fracture and predict the inevitable failure of materials. However, observing these precursors is not easy, especially in disordered systems. The soft matter with a large macrostructural size provides a gateway, as the structural heterogeneities for these systems are of the scale which can be visualized by microscopy. We work on one such highly heterogeneous system of casein gel. The rheology of such soft systems requires a highly sensitive and precise experimental setup. We have developed a cantilever-based apparatus for the rheology of soft solids and optimized the physical chemistry of our gel to have in-situ gelation. We have demonstrated successfully that macroscopic shear rheology coupled with microscopic observation can help in visualizing damage accumulation before fracture. Specifically, for experiments under constant subcritical stress, we are able to observe precursor strand breaking events that nucleate and lead to the catastrophic failure of the material. We are able to obtain Spatio-temporal statistics on these plastic events and observe that damage and crack nucleation starts to occur in these systems during the primary creep regime of these systems which is macroscopically linear viscoelastic in nature.