ESPCI web site
Séminaire café 2025
| 2 janvier | Christmas holidays |
| 9 janvier | PhD student : Elias Lundheim & Jules Tampier |
| 16 janvier | Mina Jafari, Doctorante à l’Institut d’Alembert Theoretical and Numerical Modeling of Hydro-Sedimentary Flows for Geophysical Applications Hydro-sedimentary avalanches, such as debris flows, are an increasing risk in mountainous regions, exacerbated by climate change and soil degradation. These flows, composed of water and sediments, pose significant threats to terrestrial and marine ecosystems, human health, and infrastructure. However, modeling these complex, multiphase events at large scales is challenging due to their non-linear, non-Newtonian nature. This work introduces a bi-layer simulation framework using the shallow water approximation, implemented in the Basilisk framework, to capture the dynamics of such events from a broader perspective. The approach integrates non-Newtonian rheological models, including visco-plastic and granular models, to describe the sedimentary nature of debris flows in a continuous manner. By superposing a water layer on top of these complex sediment layers, we investigate the impact of precipitation on the dynamics of a dry sedimentary bed, muddy flow, or other phases of debris flows. Preliminary results will focus on the interaction between layers with different rheologies, examining their influence on key parameters such as velocity, flow thickness, and run-out distance. The model is validated against both new and existing analytical solutions. |
| 23 janvier | Basile Dhote, Doctorant au laboratoire FAST Orientation de flotteurs déformables dans un champs de vagues We investigate the slow, second order motion of thin flexible floating strips drifting in surface gravity waves. We introduce a diffractionless model (Froude-Krylov approximation) that neglects viscosity, surface tension, and radiation effects. This model predicts a mean yaw moment that favors a longitudinal orientation of the strip, along the direction of wave propagation, and a small reduction in the Stokes drift velocity. Laboratory experiments with thin rectangular strips of polypropylene show a systematic rotation of the strips towards the longitudinal position, in good agreement with our model. We finally observe that the mean angular velocity towards the stable longitudinal orientation decreases as the strip length increases, an effect likely due to dissipation, which is not accounted for in our inviscid model. |
| 30 janvier | Basile Poujol, Doctorant au laboratoire LMD Convective clouds as hydrodynamic dipoles Tropical convective clouds tend to spontaneously organize into various geometric patterns at the mesoscale (10-100 km). This organization affects the Earth’s radiative budget and is a significant source of uncertainty in climate change projections. While most studies on the organization of convection have focused on thermodynamic causes of convective aggregation, here we explore a simple model in which convective clouds are regarded as hydrodynamic dipoles. We study the circulation around convective clouds, compare it against satellite and in situ observations, and finally explore the potential impacts of this circulation on the interaction between different clouds and the subsequent convective organization. |
| 6 février | Andrew Schulz - Haptic Intelligence Department, Max Planck Institute for Intelligent Systems Metamaterials Investigations of Biological Composites in the Elephant Trunk Abstract : Elephant trunks have been seen bulldozing trees, throwing lions several meters into the air, and picking up a tortilla chip without breaking. These trunks comprise the same foundational structural proteins, collagen and keratin, that include human fingernails, skin, and muscles. In the elephant trunk, these proteins form complex biological metamaterials. We define a biological metamaterial as a complex organization at the micro or meso-scale that drives macro-scale functional benefits. Elephant skin has metamaterial properties that assist with the duality of mechanical extension while maintaining a protective sheet around these inner muscles that provide the muscular hydrostat namesake. This presentation will discuss some complex metamaterials inside the elephant trunk and how they can lead to mechanical, sensing, or medical benefits. We discuss some tools that can be used to investigate metamaterial mechanics in biological tissues ranging from soft collagenated tissue to heavily keratinized materials. We look forward to what techniques could be used to investigate metamaterials, including muscular hydrostats, and expand to biological composites. |
| 13 février | Laureano Ortellado - CNRS & UGA, LIPhy, Grenoble Fracture of Polymeric Double Networks via Coarse-Grained Molecular Dynamics Simulations. Recently, the synthesis of polymeric multi-network systems has shown promise in creating networks with remarkably enhanced mechanical properties [1]. In these systems, a first isotropically pre-stretched brittle network is coupled to a second or more floppy ductile networks that only break at later stages. The synergistic interaction between these networks significantly increases the overall toughness [2]. However, the physical mechanisms underlying this enhancement remain poorly understood, which led to a trial-and-error synthesis approach to optimize the mechanical properties of these new polymeric materials. In this study, we employ coarse-grained molecular dynamics simulations to investigate the mechanical response of double networks at the monomeric level, from synthesis to uniaxial stretch testing. By tuning the synthesis parameters we are able to replicate the key features of the experimental response. The detailed insights provided by numerical simulations allow us to quantify several quantities inaccessible experimentally. Our results demonstrate that double networks exhibit more delocalized and isotropic stress relaxation after a bond scission event compared to single networks, preventing early damage. Furthermore, we show how changes in the initial structure and preparation protocol affects the mechanical response of the polymeric double network, providing insights for the synthesis of novel high-performance polymeric networks. [1] Gong, J. P., et al., Advanced materials 15 14 (2003) 1155-1158. [2] Slootman, J., et al., PNAS 119 13 (2022) e2116127119. |
| 20 février | Lars Kürten - Gulliver, ESPCI Does the nucleation discrepancy disappear for particle-resolved data ? Crystal nucleation and self-assembly are the underlying mechanisms of a variety of natural phenomena and technical applications. Consequently, tremendous scientific effort has been devoted to understanding these fundamental processes over the last few decades. Despite continuous progress in this field, a massive disagreement persists between nucleation rates measured experimentally and those predicted by computer simulations. Predicting nucleation is an immense problem, with discrepancies in a number of systems, including Argon [1] and metallic and molecular systems [2]. In the most studied system, colloidal hard spheres, the discrepancy is many orders of magnitude. In contrast to the experiments conducted in the last millennium, we, for the first time, investigated density matched systems in the regime of the discrepancy at the single-particle level, with confocal imaging to calculate nucleation barriers from the particle-resolved data. Advances in structural analysis methods enabled us to precisely map our results to hard sphere simulations and compare nucleation barriers for identical state points. With our method, the nucleation barriers show reasonable agreement, leading to the disappearance of the discrepancy. [1] Sinha, et al., J. Chem. Phys. 132 (2010), 064304 [2] G.C. Sosso, et al., Chem. Rev. 116 (2016), 7078-7116 [3] C.P. Royall, et al., Rev. Mod. Phys. 96 (2024), 045003 |
| 27 février | Renaud Baillou - Departament de Fisica de la Materia Condensada, Universitat de Barcelona Attractive Colloidal Ice : Crystallization and Geometric Frustration Artificial colloidal ice (ACI) is an Ising-like experimental model system in which paramagnetic beads are confined in a 2D lattice of elongated PDMS wells, individually placed using optical tweezers. The interactions between beads are mediated by a tunable magnetic field, and an annealing process drives the system toward a configuration that minimizes its magnetic energy. While repulsive interactions have been extensively studied, I numerically explore the uncharted regime of attractive interactions. In simple geometries, attraction promotes a crystalline state characterized by an "all-in/all-out" alternation. However, in a newly introduced _pentaheptite_ geometry, frustration emerges, preventing global ordering. |
