Colloidal Yielding: tunable glasses to explore plasticity from macroscopic to microscopic length-scales.

Upon external mechanical loading, amorphous solids initially deform elastically, but for large enough applied stress they may fracture or flow as a plastic fluid. This transition, known as yielding, is one of the most fascinating open problems in condensed matter statistical physics. In fact, the exact nature of the transition is still unclear: some systems show abrupt (brittle) yielding (e.g., in oxide glasses) while others, such soft gels or foams, display ductility with a smooth and gradual increase of plastic deformation. A unique picture of the processes at play is yet missing and recent models and theories claim experimental support. Here I will use a use a novel colloidal glass to explore the yielding transition and its properties. Colloidal glasses are controlled through their volume fraction: it plays the same role of temperature in molecular ones, but it is much more difficult to precisely tune. I will overcome this difficulty exploiting a mixture of nanoparticles and non-colloidal polymer mesogels. The mesogels swell/shrink as a function of temperature, controlling the available ‘free’ volume for the (temperature-insensitive) nanoparticles. This approach allows for an unprecedented precise control of the volume fraction in a colloidal glass without changing particle-particle interaction or their size (e.g., in microgels). The versatility of the system will be exploited to investigate plastic activity for different level of glass equilibration and external applied stresses thanks to cutting-edge experiments involving rheology, time- and space- resolved visible scattering and synchrotron-based X-ray photon correlation. These approaches will be combined to obtain a comprehensive picture of the processes at play, allowing for a ground-breaking study from inter-particle distances up to macroscopic sizes. The yielding transition will then be tackled from a new point of view, potentially elucidating its nature and confronting emerging theories.