Processes such as embryogenesis, tissue repair, and cancer metastasis rely on the movement of large groups of cells through changes in group morphogenesis or collective migration. These processes involve both molecular and mechanical interactions between cells and their environment. Several efforts have been made to model such interactions [1]. In this presentation, I will demonstrate how studying tissue dynamics through a coarse-grain approach, considering tissues as active polar fluids, helps us understand their physical properties. We will examine two specific examples: first, the case of Neural Crest cell colonies, which migrate during embryogenesis to form head tissue, and how proliferation in these colonies affects their migration [2,3]; second, the case of mouse embryos changing their morphology to spread and attach to the substrate, and how the mother's age can influence this process [4]. [1] R. Alert and X. Trepat. Physical models of collective cell migration. Ann. Rev. 11: 77-LF1 (2020) [2] E.H. Barriga, K. Franze, et al. Tissue stiffening coordinates morphogenesis by triggering collective cell migration in vivo. Nature 554: 523-527 (2018) [3] C.L. Marchant, A.N. Malmi-Kakkada, et al. Cell clusters softening triggers collective cell migration in vivo. Nat. Mat. 21: CR14-CR23 (2022) [4] C. Pérez-González, R. Alert, et al. Active wetting of epithelial tissues. Nat. Phys. 15: 79-88 (2019)

Relations between quantum field theories and vertex operator algebras (VOAs) have proven ubiquitous. I will discuss one such instance, where VOAs arise on the boundary of topologically twisted 3d supersymmetric quantum field theories. These VOAs are defined from twisted non-Abelian quiver gauge theories by restricting to the boundary sector and performing a BRST reduction. The quiver description plays a key role, with parallels between the geometry of the associated quiver variety and structures of the corresponding VOA. There are two interconnected perspectives here: i) physically, BRST closed operators allow to construct an explicit homomorphism from affine W-algebras into the H-twist VOAs of particular quiver gauge theories, while ii) mathematically, the VOAs are defined as a chiralization of an extended quiver variety. The latter point of view is particularly powerful as it allows to implement a reduction procedure for the quiver diagrams, which translates to free-field realisations when lifted to the VOAs.

Potentiel candidat CNRS de l'équipe NMAT

Berglund Hubsch are Calabi-Yau of chain and loop types. We investigate the case when they are defined in the same weighted projective space. We show that they are deformation of each other. We also show that their mirrors are the same manifold for the 79 “Good” cases but not for the 77 “Bad” cases.

The rheology of concentrated non-Brownian suspensions has undergone a small revolution in the last 15 years when the importance of the role played by solid contacts between particles was realized. Considering these contacts has allowed to explain the continuous or discontinuous shear-thickening in dense suspensions [1] and, more recently, the shear-thinning observed beyond the shear-thickening transition [2], i.e. in frictional non-Brownian suspensions [3]. I will start by presenting a number of questions about suspension dynamics that would remain unanswered if we didn't take particle-to-particle contact into account. I will then focus on shear thinning to show that the decrease in viscosity recorded as shear stress increases can be explained by the variable friction between particles [4]. To this aim, I will first show how the study of the microstructure of suspensions under shear has revealed the existence of solid contacts between particles and that they are made possible by the presence of roughness on the particle surface. I will then present some numerical results that show how solid friction between particles increases the viscosity of suspensions, and how modelling inter-particle contact by an elasto-plastic mono-contact gives a very good account of the shear-thinning observed in concentrated non-Brownian suspensions. I will conclude by presenting measurements of the friction coefficient between two particles using an AFM [4], which clearly validate this scenario and highlight the close links between the microscopic friction properties of the particles and the macroscopic rheological behaviour of suspensions. [1] R. Mari, R. Seto, J.F. Morris and M.M. Denn, J. Rheol., 2014, 58(6), 1693-1724. [2] G. Chatté, J. Comtet, A. Niguès, L. Bocquet, A. Siria, G. Ducouret, F. Lequeux, N. Lenoir, G. Ovarlez and A. Colin, Soft Matter, 2018, 14(6), 879-893. [3] L. Lobry, E. Lemaire, F. Blanc, S. Gallier and F. Peters, J. Fluid Mech., 2019, 860, 682-7LF. [4] M. Arshad, A. Maali, C. Claudet, L. Lobry, F. Peters and E. Lemaire, Soft Matter, 2021, 17(25), 6088-6097.

During my PhD at MSC (Paris), I investigated capillary flows of suspensions — systems where micrometric particles interact with dynamic free interfaces, such as in droplet spreading or thin film destabilization under their own weight. The suspension obtained upon the addition of the particles can in some cases follow the laws of simple fluids. In other cases, these laws require adjustments or, sometimes, entirely new dynamics emerge, where dissipation, capillarity, and extreme confinement play a key role. Next, I will present two projects from my postdoctoral work at the James Franck Institute (Chicago). The first explores the addition of solid particles to an acoustically levitated droplet, where the shape and stability result from the competition between acoustic pressure, gravity, capillarity, and particle confinement. The second project delves into the phenomenon of shear thickening in colloidal suspensions and investigates how the addition of granular particles affects the suspension’s rheology. Finally, I will present several project ideas for a potential application to the CNRS at the Laboratoire Charles Coulomb. My research would focus on two main axes. The first would investigate marine microplastics, aiming to refine the understanding and modeling of ballast effects caused by biofouling and clay aggregation on their surfaces. The second would focus on soil contamination by microplastics and nanoplastics, with particular attention to their diffusion, fragmentation, and impact on the mechanical and biological properties of soils.

Residual stresses and plasticity are well-known companions of all glassy materials. In the years, it was understood that macroscopic stress fields can be tuned to severely affect some physical properties of the material, like for example the strength in tempered glasses. Despite this, their nature at the microscopic length scale is still unclear: their behaviour is tightly linked to plasticity, i.e. local non-affine deformations that in glasses behave as stress sources coupled to the elasticity of the material. In this talk I will discuss recent X-Ray Photon Correlation Spectroscopy (XPCS) results exploring the interplay between plasticity, stress fields and the microscopic dynamics in very different systems i.e., colloidal and atomic glasses. In the colloidal system, I will show how the dynamics is dominated by residual stresses trapped-in during the preparation procedure, which give rise to a very rich phenomenology in the observed dynamical properties, as the presence of compressed relaxation functions, ballistic dynamics and dynamical heterogeneities [1]. In the second part of the talk, I will focus on oxide glasses illuminated with hard X-rays. I will show that the absorbed photons create point defects which behave as plastic regions, and their number is directly controlled by the delivered dose [2]. Following the atomic dynamics on different length-scales, I will show how the glass can behave both elastically and plastically, eventually reaching a state where the microscopic dynamical properties resemble that of a flowing system [2]. Finally, I will present a CNRS project aiming at pushing our understanding of plasticity at the microscopic length-scale. I will propose to leverage the properties of new thermo-sensitive colloidal glasses to investigate plastic activity for different level of glass equilibration (i.e., residual stresses) and external applied perturbations, combining cutting-edge experiments involving rheology, time- and space- resolved visible scattering and synchrotron-based X-ray photon correlation. [1] F. Dallari, A. Martinelli, et al., Sci. Adv. 6(12), eaaz2982 (2020). [2] A. Martinelli et al., Phys. Rev. X CR, 04LF31 (2023).

Living organisms use dynbamic, isothermal processes like self-organization to function and evolve. Imitating these processes with synthetic soft matter can lead to innovative and bio-inspired materials. DNA nanotechnology offers programmability and versatility for creating desired 2D and 3D nanostructures. Although reconfigurability of DNA nanostructures is possible, building growing and evolving structures is still challenging. Our recent work in Damien Baigl’s team demonstrated that using an adjusted salted buffer allows DNA strands to self-assemble into programmed and reconfigurable morphologies at room temperature [1]. In this seminar, I will discuss how we can go further self-assemble DNA nanostructures at 37°C in bio-relevant buffers, and notably in the presence of cells. I will also show that the method enabled us to induce the living growth of DNA nanotubes, either in bulk solution or inside biomimetic micro-compartments. Finally, I will present the self-organization of nanotubes with electrostatic interactions, resulting into large DNA superstructures that exhibit reversible dynamics triggered by external stimuli. All these results open up perspectives for the development of dynamic and evolving architectures inside living and/or temperature-sensitive environments. 1. Rossi-Gendron, C.; El Fakih, F.; Bourdon, L.; Nakazawa, K.; Finkel, J.; Triomphe, N.; Chocron, L.; Endo, M.; Sugiyama, H.; Bellot, G.; et al. Isothermal Self-Assembly of Multicomponent and Evolutive DNA Nanostructures. Nat. Nanotechnol. 2023, doi:LF.LF38/s41565-023-01468-2.

In theories with compact dimensions, Bubbles of Nothing (BoN) are vacuum decays in which the compactification manifold C shrinks to a point, with the instability expanding at the speed of light and leaving nothing (not even spacetime) behind. The Swampland Cobordism Conjecture predicts spacetime-ending configurations (known as bordisms to nothing) such as these being commonplace in a consistent theory of Quantum Gravity. Most constructions of BoN or more general bordisms to nothing found in the literature feature simple instances of C or singular bordisms, which cannot be approached from the effective field theory. Assuming the solution mediating such decay to nothing is homeomorphic to a smooth description, one can go a step further, and obtain topological properties from those of C. Through the use of Morse-Bott theory, these can be translated into information on the number and types of topology changes the compact manifold experiences as we move towards the tip of the bordism, as well as the location of possible cobordism defects. I will illustrate these results with different examples coming from String Theory and, time permitting, study more complicated arrangements such as BoN collisions. The talk is based on the paper https://arxiv.org/abs/24LF.2CR72.