Résumé
Material failure is ubiquitous on length scales ranging from a few nanometers, as in fracture of atomic or molecular systems, up to geological scales, as in earthquakes. The detection of any precursors that may point to incipient failure is the Holy Grail in many disciplines, from material science to engineering and geology. Material failure is particularly relevant to soft matter systems (colloids, emulsions, polymers etc.), which are an ideal benchmark to investigate how mechanical stress impacts condensed matter. Indeed, soft matter is very susceptible to even modest mechanical loads and most soft materials can be conveniently investigated by powerful optical methods such as microscopy or light scattering.We have built up a shear-cell that allows us to couple rheology to light scattering measurements on soft solids. In this talk I will focus on a fractal colloidal gel produced by in-situ aggregation of silica nanoparticles in water and show how reversible and irreversible (microscopic and macroscopic) deformations, build up when the gel is submitted to a shear stress. Thanks to our set-up, we are able to elucidate the physical processes at play at the onset of yielding.