When fluids are forced to flow, instabilities are commonly observed in simple and complex fluids due to inertia or to elasticity. Shear-thickening, the process by which the viscosity of concentrated particulate dispersions dramatically increases above some critical load, is another widespread phenomenon that can be interpreted in terms of an underlying instability. While recent theoretical work has narrowed down the potential instabilities at play, their experimental demonstration is sorely lacking and mechanisms accurately explaining shear-thickening are still a subject of heated debate.

Our experimental work uncovers for the first time the local origin of the unsteady large-scale dynamics characteristic of shear-thickening in a dense suspension of cornstarch grains. We solicit the suspension under a constant shear stress and image its flow using echography. Introducing and adapting statistical tools from turbulence physics, we pin down the origin of fluctuations: macroscopic regions organize as transient localized bands that travel perpendicular to the flow direction and proliferate as the shear stress is increased. These large-scale “coherent structures” are scattered in a random background reminiscent of turbulence. These key observations urge to introduce stress spatiotemporal correlations in models and to simulate dense three-dimensional systems together with the shearing boundaries

Ce séminaire se situe dans le cadre du projet d'intégration à l'équipe CONNECTOME d'Anne Ducros, professeur et responsable de la recherche clinique sur le thème 'migraines et céphalées' au CHU de Montpellier.
Anne Ducros présentera un état de l'art de la recherche clinique associée aux phénomènes de migraines.
François Molino articulera cette problématique au projet de l'équipe CONNECTOME, tant au niveau du recueil de données d'imagerie que de la modélisation.

Magnetic susceptibility is a physical quantity that plays an important role in magnetic resonance imaging. Often considered to be a nuisance, it offers rather impressive opportunities to tailor image contrast. Starting out with susceptibility weighted imaging, which is exquisitely sensitive to venous blood, hemorrhage and iron storage, the field has more recently moved to quantitative susceptibility mapping (QSM). QSM represents a novel quantitative contrast of an intrinsic physical tissue property that reflects the reaction of tissue to the static main magnetic field. The talk will trace some of the historical developments and will introduce the technical underpinnings of susceptibility-based imagng as well as present some applications.

Gauge-gravity duality allows for the reformulation of certain strongly coupled quantum field theories in terms of higher-dimensional gravity. In these lectures, aimed at the graduate level, we will introduce the general set-up of the correspondence, and describe the dictionary that maps between the observables on the field theory and gravity sides of the duality. We will cover topics such as how, by making use of the gravity dual description, one can compute correlation functions in strongly coupled theories, how to extract the spectrum of composite states, the relation between renormalization group flow and the geometry of the gravity dual, as well as how linear confinement can be modelled on the gravity side. Once the general framework has been introduced, we will discuss some applications to scenarios for strongly coupled physics beyond the Standard Model.

Nano-force spectroscopy: insight into intercellular de-adhesion dynamics

Attila-Gergely Végh PhD

Institute of Biophysics, Biological Research Centre, Temesvári Krt. 62, H-6726 Szeged, Hungary

Proper homeostasis of the central nervous system relies on the structural and functional integrity of the neurovascular unit (NVU). The most important physiological function of the NVU is formation and maintenance of the blood-brain barrier (BBB). Anatomically the cerebral microvascular endothelium together with pericytes, astrocytes, neurons and the extracellular matrix builds up the NVU. From a mechanical point of view, the endothelium is the most exposed to mechanical stress they represent the first defense and signaling line of the BBB. Unfortunately, most of the neoplasms found in the CNS are of metastatic origin. The first and crucial step of brain metastasis formation is the establishment of firm adhesion between the blood travelling tumor cells and the tightly connected layer of the endothelium. Amongst all tumors, the melanoma exhibits elevated frequency to metastasise to the brain.
Intercellular dynamics might present crucial nanomechanical aspects, therefore direct investigation with high accuracy provides important information. Using single-cell force spectroscopy, de-adhesion dynamics of melanoma cells with different level of malignancy (WM35, A2058 and A375) from a confluent layer of brain micro-capillary endothelial cells (hCMEC/D3) was investigated. Based on simple mechanical assumptions, hereby is presented our latest data on comparing the de-adhesion dynamics between the studied cell types. Apparent mechanical properties showed altered characteristics pointing towards cell type dependent aspects. Our results show that nanomechanical properties can be associated to higher metastatic potential and invasive characteristics may rely on stronger adhesive properties mediated by altered tether formation dynamics.

Gauge-gravity duality allows for the reformulation of certain strongly coupled quantum field theories in terms of higher-dimensional gravity. In these lectures, aimed at the graduate level, we will introduce the general set-up of the correspondence, and describe the dictionary that maps between the observables on the field theory and gravity sides of the duality. We will cover topics such as how, by making use of the gravity dual description, one can compute correlation functions in strongly coupled theories, how to extract the spectrum of composite states, the relation between renormalization group flow and the geometry of the gravity dual, as well as how linear confinement can be modelled on the gravity side. Once the general framework has been introduced, we will discuss some applications to scenarios for strongly coupled physics beyond the Standard Model.

Heavy neutrinos that couple appreciably to Standard Model particles are a common prediction of low-scale neutrino mass models. If kinematically accessible, such hypothetical particles can be produced copiously at the Large Hadron Collider, and its potential successors, and shed light on the mysterious origin of tiny neutrino masses [1]. In this seminar, the results of a newly proposed search methodology for heavy neutrinos at hadron colliders are presented. The analysis proposal focuses on an unusual (but "safe" [2]) implementation of jet vetoes to separate signal from background. Nontrivial increases in sensitivity at the High-Luminosity LHC and potential successor hadron colliders are discussed.

[1] https://arxiv.org/abs/1711.02180
[2] https://arxiv.org/abs/1805.09335

Glasses and supercooled liquids are examples of frustrated systems: their characteristically slow dynamics and high viscosities are often thought as being linked to the impossibility for the constituents to satisfy local geometrical constraints at a global scale. Starting from experimental evidence in colloidal glasses, I will show that an analysis of local geometrical motifs in real space allows us to identify signatures of an emergent non-crystalline local order in an exceptionally deeply supercooled regime [1]. To understand how such emerging features affect the frustration of supercooled liquids, I will rationalise the experimental evidence with numerical models studied within the framework of large deviations of structural-dynamical observables [2]. First, I will show that an accurate measure of the fluctuations contains deep dynamical and thermodynamical information on very low temperature equilibrium states. Then, I will report the results for three distinct models of glass-formers analysed within this framework [2,3,4], for which slow dynamics corresponds to phase coexistence in an augmented dynamical space, with a non-trivial temperature dependence that allows us to reinterpret a variety of scenarios for the low-temperature behaviour of supercooled liquids in terms of amorphous-to-amorphous transitions.

[1] J. E. Hallett, F. Turci, C. P. Royall, Local structure in deeply supercooled liquids exhibits growing lengthscales and dynamical correlations. To appear in Nature Communications (2018).
[2] F. Turci, C. P. Royall, T. Speck, Non-equilibrium phase transition in an atomistic glassformer: The connection to thermodynamics, Phys. Rev. X 7, 031028 (2017)
[3] F. Turci and coworkers, Experimental evidence for a structural-dynamical transition in trajectory space, Phys. Rev. Lett. 119, 028004, (2017)
[4] F. Turci, T. Speck and C. P. Royall, Structural-dynamical transition in the Wahnström mixture. EPJ E 41: 54 (2018).

Modélisation ab-initio des interactions électron-phonon dans les matériaux 2D. (candidature CNRS)
Les mécanismes impliquant les interactions électron-phonon sont au coeur des propriétés électroniques et vibratoires des matériaux. Leur compréhension et notre capacité à les simuler est donc essentielle à l'élaboration de nouvelles technologies exploitant les matériaux 2D.
Tout comme les techniques expérimentales, les méthodes ab-initio ont dû évoluer pour traiter
correctement cette nouvelle catégorie de matériaux.

Je présente ici le développement et l'application de la théorie des perturbations de la fonctionnelle densité pour simuler les matériaux 2D, isolés ou chargés par effet de champ.
Cet outil a notamment permis la modélisation détaillée du couplage electron-phonon et du transport électronique dans le graphène; du comportement des phonons optiques polaires (interaction de Fröhlich et séparation des modes LO et TO); ainsi que la dépendance en dopage du spectre Raman des dichalcogénures de métaux de transitions.

Ce développement ab-initio est également l'un des outils principaux dans un effort de découverte et caractérisation de nouveaux matériaux 2D par calculs numériques à haute densité. Notre base de donnée en recense près de 2000. En m’appuyant sur la quantité et diversité des informations à disposition, j’ai pu dresser un portrait général des interactions electron-phonon dans les matériaux 2D.