Abstract
In this thesis, we introduce a "multi-scale" characterization of soft matter based on novel setups that couple macroscopic measurements (temperature, force, deformation) to measurements of the microscopic dynamics. In a first series of experiments, we use a recently introduced optical method, space-resolved diffusion wave spectroscopy (PCI-DWS), to follow the microscopic dynamics of fat materials during a temperature ramp. This allows us to detect phase transitions and to localize them in space for heterogeneous samples. In a second series of experiments, the same optical method is coupled to mechanical measurements performed on a commercial universal traction machine. For a semi-crystalline polymer, we measure by PCI-DWS the mesoscopic deformation field under tension, with no need to pre-treat the sample surface as in conventional imaging methods. For the same polymer, we measure the microscopic dynamics during tensile stress relaxation tests, both in the linear and non-linear regime. We find a remarkably simple relationship between the microscopic dynamics and the macroscopic stress relaxation, and propose a simple model to rationalize it. In the last part of the thesis, we have designed and implemented a setup prototype to measure simultaneously the microscopic dynamics (by PCI-DWS), the force and the deformation during tensile tests on elastomers. Thanks to this apparatus, we unveil dynamic precursors preceding by thousands of seconds anymicroscopic sign of the incipient material failure.