Stefano AIME

Open positions :

PhD Thesis: "Break to map: revealing stress-bearing structures through weakly nonlinear rheology"

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.

Research projects

Rheo-{Optics, X-ray scattering}

Linking mechanics to microstructure requires measuring both at once, and few commercial instruments allow this. We therefore develop combined setups and the data analysis methods that go with them. These include a stress-controlled shear cell for small-angle light scattering [S. Aime et al., Rev. Sci. Instrum. (2016)], and a new-generation, two-axes shear cell that extends these measurements to more complex deformation protocols [C. Marraffa and S. Aime, Rheol. Acta (2025)], combined with advanced methods to reconstruct three dimensional motion using dynamic speckle holography [S. Aime et al., Phys. Rev. Lett (2021)]. We combine these lab-scale measurements with experimental campaigns at large scattering facilities [Chèvremont et al., Phys. Rev. Materials (2026)]. Using combined rheo-scattering setups to extract shear-induced dynamics is far from trivial. To this end, the development of new instruments goes together with the development of new methods for extracting non-affine dynamics from scattering experiments under shear [S. Aime and L. Cipelletti, Soft Matter (2019)].

In collaboration with: L. Cipelletti & L. Ramos (Montpellier), T. Gibaud (ENS Lyon), M. Cloitre, F. Tournilhac & P. Edera (ESPCI)

Structure and rheology of colloidal and polymeric networks

In colloidal gels and polymeric networks, the multi-scale, hierarchical microstructure produces a complex linear and nonlinear rheology. Stress is distributed in a spatially heterogeneous way that depends on the network topology, so the relationship between microstructure and stress is nonlocal, and the consequences of non-affine rearrangements are far from trivial.

We have mostly studied this in colloidal gels, whose structure arises from the kinetics of particle aggregation [E. Julien et al., Langmuir (2024)] and can be finely tuned through [D. Mangal et al., Soft Matter (2024)]. In these systems, we have linked power-law viscoelasticity to the fractal structure [S. Aime et al., J. Rheol. (2018)], and we have showed that delayed failure under load is accompanied by microscopic precursors at a length scale set by the mesh size of the gel network [S. Aime et al., PNAS (2018)], suggesting that the microscopic origin of failure is the loss of connectivity between adjacent clusters.

These results extend to polymeric networks, where we relate mechanical properties to connectivity arising from phase separation [S. Aime et al., J. Rheol. (2017)], dynamic bonds [J. Crozzolo et al., ACS Appl. Polym. Mater. (2022)], or a combination of both [H. Sun et al., ACS Mater. Lett. (2025)].

Current work extends these questions to double networks and polymer/colloid composites. The aim is to map where stress is carried and where yielding starts, and so address fatigue and failure in materials of practical relevance.

In collaboration with: L. Cipelletti & L. Ramos (Montpellier), D. Weitz (Harvard), O. Lev & S. Rubinstein (Jerusalem), E. del Gado (Georgetown), T. Gibaud (ENS Lyon), F. Lechenault (ENS Paris), R. Nicolay, N. Van Zee, F. Tournilhac & P. Edera (ESPCI)

Nonlinear rheology of soft glasses and dense suspensions

Dense, often homogeneous packings of soft elements have a much simpler microstructure than networks, which in principle should give a simpler link between microscopic dynamics and rheology. This holds only partly. Our experiments in microgels, emulsions, and other soft colloids showed that yielding is a transition between two distinct dynamical states, a slow solid-like one and a faster one driven by plastic rearrangements. We rationalized it with a lattice model that gives a unified state diagram, in which distance from a critical point decides whether yielding looks abrupt or gradual [S. Aime et al., Nat. Phys (2023)]. Studied in detail, this model predicts that a spatial correlation length emerges from the coupling between disorder and the bifurcating dynamics [S. Aime and D. Truzzolillo, Phys. Rev. Mater (2025)].

At the macroscopic scale, bifurcating dynamics give rise to residual stresses, encoding memory of the material's deformation history [P. Edera et al., Phys. Rev. X (2025)]. The relationship between the microscopic and macroscopic manifestation of yielding [T. Divoux et al., Soft Matter (2024)] is a topic of active research in our group.

In collaboration with: D. Truzzolillo, L. Ramos & L. Cipelletti (Montpellier), D. Pine (NYU), G. Zanchetta (Milano), K. Martens & R. Mari (Grenoble), F. Blanc (Nice), G. Ovarlez (Bordeaux), P. Edera & M. Cloitre (ESPCI)

Rheology and dynamics of micron-scale systems

When the sample is only micrometers across, or when the relevant physics sits at an interface [A. Saad et al., Langmuir (2022); Z. Wu et al., Lab Chip (2025)], rheology works differently. Surface and bulk contributions are hard to separate, and mesoscopic scales are no longer small compared to the sample. Yet, measuring dynamics and rheology of micron-sized objects is much more challenging than doing so on large bulk samples. To this end, we developed a multispeckle dynamic light scattering technique able to finely resolve microscopic motion inside a protein condensate, and we found that liquid-solid transition is initiated at the droplet interface [Y. Shen et al., PNAS (2023)].

Measuring the consequence of these confinement-dominated processes on droplet viscoelasticity is also challenging, since most well-established techniques probe rather slow timescales, and their low throughput struggles with highly heterogeneous populations. Microfluidic channels are a natural tool here because their geometry sets the forcing applied to each transported object. Our recent technique, Rheofluidics, uses it to measure the frequency-dependent viscoelasticity of single droplets, microgels, and vesicles at high throughput [M. Milani et al., arXiv (2026)].

In collaboration with: D. Weitz (Harvard), T. Knowles (Cambridge), J. Frataccioli (ENS Paris), J. McGraw, M. Milani, A. Lindner (ESPCI Paris)

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