Biologists have recently come to appreciate that eukaryotic cells are home to a multiplicity of non-membrane bound compartments, many of which form and dissolve as needed for the cell to function. These dynamical “liquid droplets” enable many central cellular functions – from ribosome assembly, to RNA regulation and storage, to signaling and metabolism. While it is clear that these compartments represent a type of separated phase, what controls their formation, how specific biological components are included or excluded, and how these structures influence physiological and biochemical processes remain largely mysterious. I will discuss recent experiments on phase separated droplets both in vitro and in vivo, and will present theoretical results that highlight a novel “magic number” effect relevant to the formation and control of two-component phase separated liquid droplets.

In this talk I will discuss the hierarchy problem from a known, but less mainstream direction. I will argue that the hints we have observed for high energy physics may in fact be a smokescreen, with the new physics responsible for these hints lying much closer to accessible energies. A new mechanism, the clockwork, will be proposed as an underlying explanation and connections to extra-dimensional scenarios will be sketched, alongside LHC phenomenology.

An analogy is traced between behavior of relativistic electrons in vacuum and electrons in narrow-gap semiconductors described by a two-band k.p model. Energy dispersion in the forbidden gap, statistical properties of electron gas, interband tunneling in electric field and energy dependence of the spin magnetic moment are used to illustrate the analogy. It is emphasized that it is much easier to observe semirelativistic effects in semiconductors than relativistic effects in vacuum.

Résumé : Au cours de ce travail, nous avons proposé une caractérisation "multi-échelle" des matériaux mous à travers
des montages originaux permettant la mesure simultanée des quantités macroscopiques et microscopiques. Dans
une première partie de nos travaux, nous avons associé une méthode optique introduite récemment, la diffusion
multiple de la lumière résolue spatialement (PCI-DWS), avec une sollicitation thermique, afin de suivre l’évolution
de la microstructure d’échantillons à base de corps gras lors d’une rampe de température. Ces expériences ont permis
d’identifier des transitions de phase par PCI-DWS et de les localiser spatialement dans des échantillons hétérogènes.
Dans une deuxième série d’expériences, la même méthode optique a été couplée à des essais mécaniques réalisés
avec une machine de traction commerciale. Pour des éprouvettes de polymère semi-cristallin, nous avons mesuré la
déformation de l’éprouvette par PCI-DWS, sans avoir recours à des marqueurs de surface, contrairement aux méthodes
d’imagerie couramment utilisées. Pour le même polymère, nous avons suivi la dynamique microscopique lors d’essais
de traction dans les deux régimes, élastique et plastique. Nous avons montré que la dynamique microscopique et la
relaxation de la contrainte lors de tests de traction à déformation imposée sont liées par une relation étonnement
simple, que nous avons pu modéliser. Dans une dernière partie de la thèse, nous avons conçu et réalisé un prototype
d’instrument permettant de mesurer simultanément la dynamique microscopique, la force et le déplacement lors
d’essais de traction sur des élastomères. Grâce à cet instrument, nous avons pu mettre en évidence l’existence de
précurseurs dynamiques microscopiques qui précèdent de milliers de secondes l’apparition de signes macroscopiques
de la défaillance du matériau.



Abstract : In this thesis, we introduce a "multi-scale" characterization of soft matter based on novel setups that couple
macroscopic measurements (temperature, force, deformation) to measurements of the microscopic dynamics. In a
first series of experiments, we use a recently introduced optical method, space-resolved diffusion wave spectroscopy
(PCI-DWS), to follow the microscopic dynamics of fat materials during a temperature ramp. This allows us to detect
phase transitions and to localize them in space for heterogeneous samples. In a second series of experiments, the
same optical method is coupled to mechanical measurements performed on a commercial universal traction machine.
For a semi-crystalline polymer, we measure by PCI-DWS the mesoscopic deformation field under tension, with no
need to pre-treat the sample surface as in conventional imaging methods. For the same polymer, we measure the
microscopic dynamics during tensile stress relaxation tests, both in the linear and non-linear regime. We find a
remarkably simple relationship between the microscopic dynamics and the macroscopic stress relaxation, and propose
a simple model to rationalize it. In the last part of the thesis, we have designed and implemented a setup prototype
to measure simultaneously the microscopic dynamics (by PCI-DWS), the force and the deformation during tensile
tests on elastomers. Thanks to this apparatus, we unveil dynamic precursors preceding by thousands of seconds any
microscopic sign of the incipient material failure.

Our group has been working for several years on fracture, or rather on crack propagation in soft materials. We developed several original methods and tools to characterize the damage taking place at the tip of a crack. In this seminar I will introduce the topic of fracture of soft materials and its important unsolved scientific questions, then present our picture of the structure of a typical crack from a materials point of view and from available characterization techniques such as optical microscopy and SEM, infrared microscopy, Small and Wide angle X-ray scattering and last but not least the incorporation of mechanoluminescent and fluorescent molecules in the materials.

We investigate the behavior of transient self-assembled and permanently crosslinked gels subjected to extreme mechanical stresses. On the one hand, we study the dynamics of freely expanding sheets produced by impacting a (Newtonian or viscoelastic) liquid drop or a bead of polymeric gel on solid surfaces in minimal dissipation conditions, achieved using either a small solid target or a repellent surface. Upon impact, the drop or bead is transformed into a thin sheet that expands and retracts due to elasticity. For viscoelastic fluids with a relaxation time smaller than the typical lifetime of the sheet, the dynamics of the viscoelastic sheet is similar to that of Newtonian liquids with equal viscosity. The maximal expansion of the sheet decreases with the viscosity and is quantitatively modeled using an energy balance between inertia, surface tension and viscous shear dissipation on the solid target that can be accounted by measuring an effective velocity of the expanding sheet at short time scale. We further show that the shear dissipation can be substantially eliminated by performing impact experiments on a solid surface based on an inverse Leidenfrost effect. Experiments performed using elastic beads of various elastic moduli, viscoelastic or liquid drops of various surface tensions reveal a universal scaling behavior of the maximum expansion with the impact velocity, with a dynamics that mimics that of a conventional spring-mass system. We furthermore show that, for drops impacting a solid target, a similar scaling holds once the viscous dissipation is accounted by replacing the impact velocity with the effective velocity. Another fascinating behavior of viscoelastic fluids is a heterogeneous expansion of the sheet with the occurrence of cracks, revealing the elastic nature of the viscoelastic fluid. On the other hand, we study the uniaxial deformation and the fracture of reversible double transient networks by coupling extensional rheology to fast imaging. We provide a state diagram that delineates the regime of fracture without necking of the filament, when it is stretched at a rate larger than the inverse of the slowest relaxation time of the networks. We quantitatively demonstrate that dissipation processes are not relevant in our experimental conditions and that, depending on the density of nodes in the networks, fracture occurs in the linear elastic regime, or in a non-linear elastic regime preceded by a considerable strain hardening. In addition, analysis of the crack opening profiles indicates, for weakly connected networks, deviations from a parabolic shape close to the crack tip, which is expected for the linear elasticity of a brittle fracture. We demonstrate a direct correlation between the amplitude of the deviation from the parabolic shape and the amount of non-linear elasticity.

Granular materials such as sand, powders, foams etc. are ubiquitous in our daily life, as well as in industrial and geotechnical applications. Although these disordered systems form stable structures if unperturbed, in practice they do relax because of the presence of unavoidable external influences such as tapping or shear. Often it is tacitly assumed that for granular systems this relaxation dynamics is similar to the one of thermal glass-formers, but in fact experimental difficulties have so far prevented to determine the dynamic properties of three dimensional granular systems on the particle level. This lack of experimental data, combined with the fact that in these systems the motion of the particles involves friction, makes it very challenging to come up with an accurate description of their relaxation dynamics. In this talk I will present results from X-ray tomography that allowed to determine the microscopic relaxation dynamics of hard granular ellipsoids that are subject to an oscillatory shear. This dynamics shows that on the particle level the dynamical behavior of granular systems are qualitatively different from the one of thermal glass-formers and instead more similar to the one of complex fluids.

Porous materials are at the heart of numerous important applications: adsorption (gas sensing, pollutants entrapment, drug delivery, chromatography), energy (porous materials are used for hydrogen storage, fuel cells and batteries), environment (phase separation, water treatment, nuclear waste storage), Earth science (the soil porosity ensures exchange between the soil and the atmosphere), etc. In this colloquium, I will present the state-of-the-art on adsorption, phase transition and transport in multiscale porous materials which possess pore sizes spanning several orders of magnitude (from the nm scale to the macroscale). I will first discuss the different adsorption regimes encountered when a fluid is set in contact with a porous material: from irreversible adsorption/capillary condensation for the largest pores to reversible/continuous filling for the smallest pores. We will see that simple thermodynamic modeling allows rationalizing these different regimes provided that a description of the shift of the critical point of the confined fluid is taken into account. Then, I will show how transport in such media can be described without having to rely on macroscopic concepts such as hydrodynamics (which is known to breakdown at the nm scale). Using parameters and coefficients available to experiments, we will see how transport coefficients can be rigorously obtained from simple models in the framework of statistical mechanics. While these results mostly pertain to basic science, they will be discussed in connection with important examples such as hydrocarbon recovery from shale gas plants.