Duration: 3 years.
Desired starting period: 09/2024

Description:

Understanding the relationship between microscopic structure of materials and their mechanical properties is of paramount importance in many applications, underpinning the rational design of novel materials with tailored properties. For instance, the elastic modulus of elastomers is related to their mesh size, whereas in crystals it is related to the crystalline structure and interactions. Such a simple relationship does not hold in complex, network-forming materials such as polymer composites or colloidal gels. In these cases, the ability of a given structure to bear mechanical stress stems not only from its local conformation, but also from the topology of the network as a whole, which dictates how stress is distributed in the material. In addition, a given structure with floppy mechanical properties at rest can be rigidified by an applied deformation. As a consequence, structural predictors of material properties are particularly challenging to obtain. 

The impact of network topology on local stress distribution is even more severe when the material is deformed beyond its linear regime. Under nonlinear deformations, portions of the material under increasingly large stresses undergo plastic rearrangements that alter the local microstructure and entail a global redistribution of the applied stress. Predicting a priori when and where these plastic rearrangements would occur is the holy grail of material science, yet it is beyond our current capabilities. The growing need of developing novel, sustainable materials to meet the most demanding applications calls for innovative approaches to relating the structure of complex materials to their final properties.

This project aims at leveraging weakly nonlinear rheology coupled to light scattering to locate the regions of the material where the stress concentrates. In the weakly nonlinear regime, only a very small fraction of the stress-bearing structure in the sample is stressed beyond the yield point. Upon yielding, the local structure is rearranged, entailing non-affine displacements that can be probed accurately with space-resolved dynamic light scattering. In turn, the impact of this local plastic event can be measured by monitoring rheological observables such as the linear elastic modulus. Repeating this weakly-nonlinear deformation several times, we aim at mapping the distribution of yielding events, material-science analogues of seismic maps, and relating them to the underlying microstructure of the materials.

To fully exploit the power of this multiscale experimental approach, we aim at developing model composite materials featuring a colloidal gel network interpenetrated with a hydrogel network. We plan to leverage the strong optical contrast of colloidal particles to detect the nonaffine deformation of the colloidal gel network. To sense the stress and strain state of the hydrogel network, we plan to integrate mechanochrome molecules as crosslinking points for the hydrogel, and to detect their position and brightness using fluorescence microscopy. This approach will enable the characterization of microscopic dynamics, of mesoscale stresses and strains, and of the macroscopic mechanical response of the samples, thereby allowing us to bridge the scale between microstructure and rheology. This approach will be later on extended to more complex materials of industrial relevance.