Transport processes generating irreversibility at the macroscale, the hydrodynamic equations must include these dissipative effects to be complete. The hydrodynamic modes stem from the fundamental conservation laws and from the breaking of continuous symmetries. A key question is to deduce the dissipative hydrodynamic equations ruling all these modes from the microscopic Hamiltonian dynamics of the particles with the methods of non-equilibrium statistical mechanics. In this presentation, a first-principle derivation based on a local-equilibrium approach is introduced. As an illustrative example, the crystalline solid is considered. The breaking of the three-dimensional continuous group of spatial translations implies the existence of three slow modes in addition to the five slow modes arising from the fundamental conservation laws of mass, energy, and linear momentum. Based on the results of the local-equilibrium approach, the equilibrium and nonequilibrium properties relevant to the dissipative hydrodynamics of the hard-sphere crystal are obtained with molecular dynamics simulations. Finally, the application of the method to amorphous solids and active matter systems is discussed.

The study of hydrodynamics at increasingly small scales raises fundamental questions about the validity of classical hydrodynamics hypothesis.
Particularly, the sharp interface hypothesis becomes increasingly difficult to verify for nanofluidic systems where the typical size approaches the interface thermal fluctuations length scale. More generally the interplay between macroscopic hydrodynamics and statistical physics at the interface scale is expected to have considerable influence for multi-scales problems. For example, phase-field simulations predict that the viscous dissipation singularity at a moving contact is regularised by evaporation-condensation mechanisms at the interface level.

Following that, here we present an experimental and numerical investigation of the spreading of a sessile droplet for near-critical two-phases liquid mixture. The Tanner's law regime is robustly verified even for droplets close to the critical temperature showing the universality of the viscous spreading. However the system at the droplet scale is thermodynamically unstable and the droplet also shrinks in size. The mechanisms behind this shrinking analogous to evaporation are then explored for spherical aerosols droplets showing gravity and curvature induced diffusion mechanisms. The coupling between spreading and evaporation is then discussed to give a complete picture of the sessile droplet dynamics.

We will then discuss, despite more briefly, an evaporation-driven colloidal skin formation inside an evaporating droplet deposited on superhydrophobic surfaces. The particle's assembly dynamics is explored using molecular dynamics simulations introducing the effect of droplet's evaporation where the skin formation for high Peclet. Strikingly the skin-bulk transition zone can be modeled through a viscous-diffusion balance and shows analogue properties of a phases separating interface but here with a length scale determined by the Peclet number.

Dans cet exposé, je présenterai la méthode des éléments finis (MEF) en électromagnétisme, en m'attardant sur notamment ses spécificités par rapport à d'autres méthodes numériques (e.g. différence finies, méthodes de Fourier, de Mie...) et ainsi que sur les cas typiques où cette méthode est pertinente.
Je présenterai la méthode du point de vue discret dans le cas 1D dont l'intérêt est principalement didactique puisqu'on peut assembler le système discret "à la main" et résoudre un premier problème de diffraction : le slab. J'aborderai la question de recherche de fréquences propres et de modes de systèmes optiques.
Je montrerai l'utilisation des logiciels libres Gmsh et GetDP ainsi que leur interface ONELAB développés par nos collègues de l'Université de Liège et détaillerai l'implémentation d'un problème de diffraction en 2D et en 3D.
Enfin, je tenterai de dresser un comparatif entre ces logiciels libres et des logiciels propriétaires bien connus.

The study of the yielding transition of amorphous materials is a complex phenomenon that can manifest with profoundly different macroscopic features in different circumstances.

While some materials yield abruptly, in a way often referred to as brittle, others exhibit an extended nonlinear deformation regime characterized by plasticity, a phenomenology typically called ductile failure. For one given material, yielding can be either ductile or brittle depending on the deformation rate and/or on the temperature. More recently, a series of computer simulation works suggested that, even under the same testing conditions (rate and temperature), ductile or brittle failure may be observed for the same material, depending on the previous mechanical or thermal history. Despite huge progress in this field, a comprehensive understanding of the transition between ductile and brittle yielding is still missing, mostly because stringent experimental validations of the ideas proposed in the literature are still scarce.

In this seminar I will present recent research project addressing two important questions related to the ductile to brittle transition:

1 - What is the interplay between the microscopic dynamic transition, the formation of shear bands and the macroscopic weakening of a Yield Stress Fluid?

2 - Can we use mechanical conditioning to effectively prepare the same YSF in different states with different yielding properties?

To address the first question, I will present results of simultaneous experimental study of the (a) rheological response of soft glasses, (b) mesoscopic deformation fields, and (c) microscopic plastic dynamics. Both homogeneous yielding and shear banding cases are studied [[1]].

For the second question, we combined experimental rheology and particle-dynamic simulations, with the aim to rationalize the role of internal stress distribution on material’s yielding properties, memory, ageing and annealing [[2]].


[1] Paolo Edera, Matteo Brizioli, Mahnoosh Madani, Elie N’Gouamba, Philippe Coussot, Veronique Trappe, George Petekidis, Fabio Giavazzi, Roberto Cerbino. Yielding under the microscope: a multi-scale perspective on brittle and ductile behaviors in oscillatory shear. https://arxiv.org/abs/2402.00221

[2] Paolo Edera, Minaspi Bantawa, Stefano Aime, Roger Bonneacaze, Michel Coitre. Tuning residual stress, directional memory and aging in soft glassy materials. https://arxiv.org/abs/2402.08293

In this talk I will summarize how duality symmetries of supergravity models can potentially lead to global symmetries. These are associated to deformation classes of spaces with non-trivial duality bundle and classified by so-called bordism groups. Since global symmetries are not allowed for consistent theories of quantum gravity, there need to exist certain objects that break these global symmetries. We will discuss S-duality for type IIB supergravity as well as U-duality for maximal 8d supergravities and explore the necessary symmetry-breaking objects in the various setups.

Elaborating on the recent developments concerning Feynman integrals' calculus and Scattering Amplitudes' evaluation,
I introduce the Intersection Theory for twisted de Rham co-homology, and discuss how Intersection Numbers rule the vector-space structure of special functions, such as Euler-Mellin integrals, appearing in Mathematics and Physics.
Applications in Electromagnetism, Quantum Mechanics,
QFT and Cosmology point to the crucial impact of Intersection Theory on the theory of Fundamental Interactions.

TBA