Internships 2008-2009

J’étudie un système appellé capillary pumped loop(CPL) ou boucle de refroidissement à pompage capillaire. L’objectif du système est de refroidir des éléments d’électronique de puissance d’une chaine de traction ferroviaire. La CPL est constituée d’un évaporateur (où sont posés les éléments à refroidir) et où le fluide s’évapore et d’un condenseur. Le mouvement du fluide n’est pas assuré par une pompe comme dans les machines thermiques traditionnelles mais par un saut de pression capillaire.
En effet, l’évaporateur est constitué dune mèche poreuse(responsable du saut de pression capillaire) : le liquide arrive par le bas de cette mèche, monte par capillarité et se vaporise. La vapeur ainsi produite est évacuée vers une conduite vapeur puis vers le condenseur où elle se condense. Le liquide ainsi produit retourne à l’évaporateur et le cycle recommence.
Cette technologie de refroidissement est utilisée dans le spatial sur les satellites depuis de nombreuses années. ALSTOM veut l’appliquer dans le ferroviaire mais cela pose des problèmes. En effet, les flux de chaleur engendrés par l’électronique de puissance sont tels qu’il arrive qu’une poche de vapeur envahisse le poreux et l’assèche ce qui provoque l’arrêt du pompage capillaire et le désamorçage de la boucle toute entière.
Mon objectif est donc de mettre au point un modèle physique permettant de rendre compte des écoulements(de liquide et de vapeur) qui ont lieu au sein de la mèche poreuse. Il s’agit concrètement d’étudier les échanges couplés de masse et d’énergie en milieu poreux avec un changement de phase. Ce modèle aurait pour but de prévoir le désamorçage du système.
Je dispose chez ALSTOM d’une CPL instrumenté sur laquelle je ne peux mesurer que des températures en divers points de la boucle et quelques pressions. Je n’ai pas accès à la mèche poreuse et ne dispose pas de l’instrumentation nécessaire afin de voir ce qui s’y passe. je peux simplement constater si le système désamorce ou pas (arrêt du pompage capillaire)...

<span style="font-weight: bold;">Batot, G.
We commonly know that the cell is the unit of living beings, but we have often got the biased image of a non-understandable biologic object  : a big amino acids and proteins mixture. How does that organization persist ? We don’t know enough, we just have a great catalog of chemical reactions which take place inside the cell. However, since few years, it is now possible to make real mechanical experiments which results are meaningful and hopeful. In fact, recently a lot of elements indicate that the rigidity of cell’s living environment is important in it’s development [6,8]. The object of this report is to study the influence of the substrat’s rigidity on the cell. Firstly we detailed the existed knowledge of cellular contraction. Secondly we observed precisely the rigidity-sensing phenomen through different experiments where we caught a single cell between two lamellas. Finally we analyzed how to simulate and find numerically the result of our previous experiences.

Bimbard, E.
This report presents a way of preparing and characterizing electromagnetic in an arbitrary coherent superposition of Fock states up to the 2 photon level. We used the process of spontaneous parametric down-conversion, in which a pump photon is non deterministically split in two entangled lower frequency photons. One of the outputs is used as trigger, in which we detect incoming photons to herald the presence of photons in the other arm, called signal. We can then perform measurements on the remotely prepared light in the signal channel, via the technique of time domain homodyne tomography to characterize the produced states. This experiment was used to produce single-photon Fock states until now, via a simple detection of trigger photons. We developed it to a more complex level, where making use of weak coherent states mixed with the trigger beam, we can remotely prepare the expected states in the signal channel and analyse them.

Blein, A.
The discovery of the cosmic microwave background (CMB) in 1965 provided an observational basis for cosmology which was crucial in determining our standard cosmological model, that of a spatially flat Universe consisting mainly of dark matter. The Q/U Imaging ExperimenT (QUIET) is designed to make very sensitive measurements of the polarization of the CMB, which contains valuable cosmological information. We present a brief overview of the theoretical framework for the study of CMB anisotropies, as well as the basis for the operation of the QUIET polarization modules. We then describe the data analysis which has been done thus far to perform data cuts and enable us to produce anisotropy maps and power spectrums. We have created a 50 Hz phase switch flag which will be discussed, and an extensive study of the 50 Hz imbalance has been made.

Bonart, J.
Les équations fondamentales de la magnétohydrodynamique - à savoir l’équation de Navier-Stokes généralisée par un terme décrivant la force de Lorentz et par l’équation d’induction - mènent à des instabilités linéaires intéressantes, dès qu’elles sont placées dans un référentiel tournant. Au début nous nous sommes intéressés au profil de rotation du soleil : Nous partions du modèle proposé dans [1] ("A simple model for solar isorotation contours", S. Balbus 2009, MNRAS) qui tente de reproduire le profil de rotation du soleil et en améliorant ce modèle nous obtenions des résultats remarquables. Ensuite nous avons généralisé les relations de dispersions des ondes magnétohydrodynamique dans un référentiel tournant en considérant un milieu d’une résistance non négligeable. Cette recherche a mené à une découverte d’un nouveau type d’instabilité très intéressant que nous avons pu expliquer avec un modèle de double-diffusion.

Boulier, T.
In 1960 Dzyaloshinskii, Lifshitz and Piatevskii showed that under some circumstances the Casimir force between 2 macroscopic bodies could be repulsive[4, 3]. This was measured last year by J. Munday[10], from the Capasso Group (Harvard School of Engineering and Applied Sciences). This internship was focused on designing an experiment attempting to use this force to counterbalance gravity and make micrometer-sized spherical particles levitate above a plate. This work was conducted under the direction of F. Capasso, Professor at the Harvard School of Engineering and Applied Science. The system used is the same as in the previous experiment which measured the force (Gold - Liquid Bromobenzene - Silica). The distance between the plate and the sphere is measured indirectly, by observing with a holographic-microscopic setup the modifications
in the Brownian motion of the sphere (a hydrodynamic effect due to the very close presence of the plate, from which we can derive the distance[8, 7]). After thinking and putting into place the setup some levitation was qualitatively observed, but its origin has not yet been proved to be a repulsive Casimir effect

Calandre, T.
The purpose of this internship was to characterize a two-dimensional system composed of hard particles, which have a ‘snowman’ shape (two sticked-together- disks, not necessarily of same diameter). Ideal system (without any interactions, except for hard core interactions) was modelized by computer simulation, while I tried different ’real’ systems, with different kind of interactions between particles. To do that, I have used different interface’s properties, such as an air water interface. The aim of this work was to characterize these unstressed systems, in order, then, to study their behavior under stress.

Dechant, B.
In the framework of this “stage” High Harmonic Generation has been studied both on a theoretical-conceptual and on an experimental level. After putting the current developments into the historical context and giving some motivations for the study of high harmonic generation, the basics of this phenomenon are briefly reviewed. Some emphasis is put on the crucial points of phase-matching as well as the spectral properties of harmonic spectra and ways how to satisfy the phase-matching condition and modify the spectra are presented. Also, the interesting implications of high harmonic generation in the time domain - namely the generation of sub-femtosecond pulses - are outlined. Next, the important step from single-coulour high harmonic generation towards two-colour high harmonic generation is made. The availability of certain relative parameters between the fundamental and second harmonic fields is pointed out and ways to use these new degrees of freedom to control the properties (spectral, temporal, spatial) of high harmonics are described. In the second part of this report, the experimental results are presented and discussed both for the one-colour high harmonic generation experiment with a 30fs pulse and some of the two-colour experiments.

Filippone, M.
Le sujet de mon stage est l’étude du rapport entre l’intrication quantique et les transitions de phase dans des systèmes de spin total S à la limite thermo-
dynamique (S → ∞) et à température nulle. Ces systèmes étant gouvernés par des hamiltoniens de la forme :
H = −A S^m_x − B S^n_z
m et n entiers positifs et S_i opérateurs de spin. J’ai déterminé les ordres et les points des transitions et j’ai étudié comment certaines mesures d’emmêlement sur le fondamental du système, comme la concurrence et l’entropie d’intrication, nous renseignent sur la présence et la nature de ces transitions. Cela est intéressant vu la difficulté qu’il y a à definir des paramètres d’ordre dans les systèmes sujets à transition de phase.
A cause des ambiguités des comportements de ces mesures, nous avons décidé d’étudier aussi le comportement aux transitions de la susceptibilité de la fidélité et, maintenant, je suis en train d’étudier la présence d’emmêlement à des températures non nulles.

Freulon, V.
In this experimental work, different parts of the setup required to measure the current-phase relation in a ring containing a function made of ferromagnetic barrier are studied. Hall crosses are first produced in a clean room using Gallium Arsenide substrate and connected. The measurement setup is tehn improved so as to be able to detect jumps with a lower amplitude than 1 microT in the magnetic field. A method based on amplitude modulation is also used. The corsses are also used to test the magnetic field produced by a superconducting electromagnet. It is finally shown that the considered junctions behave just like Josephson junctions.

Guénot, D.
We study the effect of the motion of cold ion Coulomb crystals in a linear Paul trap on coherent coupling with an optical field in a cavity at the single photon level. After studying the different modes of vibration of a Coulomb crystal and the ions dynamic, we show how to use the coupling between the ions and the cavity field in order to characterize these modes and extract some thermodynamical properties and structural information of the plasma.

Hesse, J
L’intêret de mon project est d’apprendre plus sur l’optimisation des activités dans le cerveau. Andrea Hasenstaub qui mesure la consumation d’énérgie des "fast-spiking PV+ interneurons" dans son labo. Elle a déjà fait un "single compartment model" de ce neuron, mais elle m’a demandé de faire un modèle plus réaliste. Son idée est que un "fast-spiking" neurone doit en même temps être rapide et avoir un bilan d’énergie optimale. Elle peut déjà montrer que cela marche pas mal, mais son graphe pour l’énergie pour un seul spike a la mauvaise courbure. Je fais le modèle avec NEURON. Une autre question que je traîte est si les junctions gap dans un reseau d’interneurone augmentent ou baissent l’activité, spécialement dans le reseau des amacrine cells dans le rétine. Alors mon projet consiste en modélisation sur le niveau de neurons.

Jian, P.
The field of quantum optics investigates the properties of light at a quantum level. It has successfully been used for tests of the quantum mechanics, and for the development of quantum information science. In particular, the field of multimode quantum imaging explores how the spatial properties of optical images can be used for quantum optics experiments. The goal is to reproduce in a single laser beam with multiple spatial modes the same results as in multiple non-copropagating beams. We report here two techniques we have developed for the manipulation and the detection of a multimode beam. We present a novel method for the manipulation of beams : we show that any linear operation can be done on a multimode beam using a Spatial Light Modulator and present an algorithm for the design of any mode conversion. We also report how a scheme of multipixel homodyne detection can be used for the detection of the properties of a multimode beam.

Kral, Q.
Tout le travail est basé sur des observations d’un nuage moléculaire en effondrement. Les premières phases de la formation d’étoiles de grosses masses sont encore assez mal comprise. On essaye de simuler l’effondrement d’un nuage de 1000 masses solaires et de regarder dans quelles conditions la simulation se rapproche le plus de se que l’on observe. L’energie thermique du nuage est assez bien connue, on essaye par contre de voir l’impact de la turbulence interstellaire (typiquement, l’explosion de supernovae ou des vents provenants du milieu environnant) et du champ magnétique sur l’effondrement de ce nuage. On peut ainsi espèrer comprendre un peu plus en détails l’impact de ces deux phénomènes qui d’habitude sont traités indépendamment, et "trancher" suivant les deux scénarios qui ont été proposés pour la formation d’étoile de grosses masses. Pour les simulations numériques, ramses3d est utilisé.

Kurkjian, H.
In order to achieve quantum computation on neutral atoms, for example in an optical lattice, it is necessary to control each atomic q-bit independently, this could never be performed to this day. During this internship, we aimed to achieve single-atom addressing of Ytterbium atoms in optical lattice using very strong magnetic field gradients to spatially resolve their levels. To this day, we have shown successful optical tweezer atom moving as well as cooling down to a temperature below 1microK in a crossed dipole trap, and have successfully observed global fluorescence and could even go down to observing only one hundred atoms’ fluorescence. Checking our Signal to Noise ratio, we estimated that with a few improvements, we are very close to single-atom detection. Single-atom detection is naturally the first step towards single-atom addressing. For fluorescence observing, we notably invested time building up a new 399 nm frequency locked laser system : some of the light obtained from an External Cavity Laser Diode was locked to the resonance of a high finesse Ultra-Low Expansion cavity after acousto-optic modulation, while another part was amplified and send to the experiment for Magneto-Optical Trapping (MOT) and fluorescence detection. Finally, I would like to highlight the importance I attached, during this internship, in acquiring experimental knowledge of atomic Physics basic technique, that which my previous studies couldn’t bring me.

Lallemand, I.
Réalisé sous la direction d’Emmanuelle Gouillar t au sein du laboratoire Surface du Verre et Interfaces (Saint-Gobain/CNRS), ce stage visait à aborder les mécanismes du mélange d’un fluide à seuil (carbopol) d’un point de vue expérimental. L’utilisation de fluides possédant cette propriété rhéologique au sein de nombreux mélangeurs industriels motivait cette étude, pour laquelle un protocole de mélange chaotique avait été préalablement retenu mais dont de nombreux aspects restaient à définir. La première par tie du stage a été consacrée à la conception et à l’assemblage du dispositif expérimental, dans lequel une petite tache opaque de fluide à seuil est mélangée à un fluide transparent de même rhéologie selon une géométrie 2D au sein d’un réser voir en rotation. La mesure de l’intensité lumineuse transmise à l’aide d’une caméra permet ainsi d’obtenir l’évolution temporelle du champ de concentration de la tache, dont sont extraites des grandeurs statistiques permettant de caractériser l’efficacité du mélange (moyenne et écar t standard). Au cours de la deuxième par tie du stage, les motifs de mélange obtenus pour différentes configurations des vitesses de rotation des mélangeurs et du réser voir ont d’abord été étudiés sur les images fournies par la caméra. Dans cer tains cas, nous avons pu obser ver la formation de lobes dont nous avons notamment relié le nombre au rappor t de la période de rotation du réser voir à celle des mélangeurs. Par la suite, en dépit de la découver te tardive d’un défaut de la table lumineuse, le ralentissement de la décroissance de l’écar t standard du champ de concentration a pu être observé sur plusieurs expériences, et relié à la présence de lobes.

Lanéry, S.
My subject was about the theoretical study of temperature measurement on very small systems (far beyond what is at the moment feasible experimentally). Fur this purpose I modeled such a measurement process : the "thermometer" is made of a small spins set (up to about a hundred) and brought in contact with a bath at some temperature (in particular I interested myself to temperatures extremely low compared to the energy splitting).The measured value of the temperature is deduced from a measurement of the total magnetization and the central point is to study the fluctuations of this measured temperature, with analytical and numerical aspects.

Le Brun, A.
This report carried out after a long internship done during the second half of the first year of my Master within the cosmology group of the Max Planck Institut for Astrophysics (MPA) in Garching, Germany is about the growth of dark matter haloes in the Millennium-II simulation. The purpose is to study some of the numerous aspects of this growth in the framework of the standard cosmological model or ΛCDM model by making use of the Millennium-II simulation which have been recently completed at MPA. This growth is not only interesting by itself but also because it can be related to the growth of the galaxies that inhabit those haloes. My report will contain firstly a short introduction to the cosmological standard model, some information about the Millennium-II simulation and about galaxies, then a brief description of the work conducted during this internship and lastly some of the results obtained concerning the growth of the dark matter haloes and of the galaxies they host. I was thus interested in trying to explain why some types of galaxies were much denser in the past than they are at the present time.

Lehe, R.
I investigate the heating of charged particles (electrons, protons, ions) by magnetohydrodynamic turbulence, using numerical simulations. The purpose of this work is to contribute to a research effort that aims at understanding heating processes in astrophysical plasmas (e.g., in the solar wind or solar corona, in accretion disks, etc...). The simulations evolve the turbulent magnetohydrodynamic fields using the Athena code, and compute the tra jectories of individual test particles in those fields. A major part of my work consisted in developing the particle integrator, and I describe it here in some detail. The results of the simulation highlight different heating regimes depending on, for instance, the particles’ charge-to-mass ratio, their energy, or the resistivity of the fluid. An interpretation in terms of cyclotron resonance and Landau resonance is discussed.

Louis, T.
This thesis concludes the internship I did at the LNB Lab under the supervision of Anze Slozar and David Schlegel and deals with the detection of the physical phenomenon called Baryon acoustic oscillations. This work is part of a more general project that aims to constraint the dynamics of the Universe to ultimately better understand the dark energy. This field is right on the boundaries between theory and observation, for previous observation projects have led to the acquiring of a great quantity of theoretical data. This thesis focuses on finding better ways to treat those data and to extract the most relevant ideas.

Nicolas, T.
In this report, I present recent advances in the building of a new cooling device for atoms or molecules, based on Helium buffer-gas cooling. We aim at temperatures ranging from 4K to 15K, and an interseting feature of our device is its size : about 8” wide for 16” high, small compared to other experiments on cold atoms/molecules, which typical size is usually around 1m. The species to be cooled are first heated until their vapor pressure is high enough, and then guided to a cold Copper cell using a continuous Helium flow : this is the buffer gas. In the short term, we aimed at obtaining a continuous, cold, optically coherent, high optical density (around 100) and clean (compared to laser ablation sources) flow of Rubidium. The optical coherence of the sample was to be demonstrated using EIT (Electromagnetically Induced Transparency). When the device is ready and when its versatility is proven, it is going to be shared with other research groups to test various ideas that are now theoretically studied. These ideas range from quantum information and cold chemistry to production of lasers with interesting frequencies, like 123 nm, corresponding to the Lyman α line in Hydrogen. After 6 months, we had built and assembled all the various parts of the cryostat, and cooled the first samples of Rubidium. The lowest temperature we measured was 21K, but with a very low optical density of 0.4. We could increase the optical density to 2.9 by increasing the Helium flow, but this was to the expense of temperature, which reached 400K. This is because the Helium flow was too high to allow enough time to thermalize with the walls and then cool the atomes of Rubidium. However one of our objectives has been achieved since the source is effectively clean, stable and continuous : one absorption signal was measured stable for about 20 minutes. New improvements are now being done on the device to be able to control the density of the buffer gas in the cell (very important parameter) without changing the flow (since too high flows don’t allow enough time for the buffer gas to thermalize).

<span style="font-weight: bold;">Oger, G.
I present a numerical model first introduced by Vicsek et al. [3] for the collective motion of self-propelled particles. In this model a group of point-wise particles move at each time step by taking the average direction of their neighboring particles, with a random perturbation (noise). It can be seen as a non-equilibrium equivalent of models like the Ising model for spins. First I investigate the dynamic behavior and phase transition of the system (from random motion to orientationnaly ordered motion) to find the results of Vicsek, with a lot of similarities to equilibrium systems. Then I introduce a new parameter in the system : hydrodynamic interactions to take into account the fluid particles are moving in, and I try to characterize their action on the order of the system.

Paillat, S.
The experiments conducted during this training course constitute a first investigation of a phenomenon which can appear in certain volcanic eruptions : eruptions of gas without magma. The forecast of these types of dangerous eruptions is a challenge for volcanologists. Experiments using materials analogous to the crust (gelatine) are set up. The aim is to measure the material surface deformation caused by the movement of gas through this gelatine. Air is simply injected though the gelatine with a pressure difference between a tank containing the gelatine and the laboratory and the acceleration of the gelatine near its surface is measured.
Whereas a continuous air flow is injected, the flow through the gelatine is discontinuous. Slugs of air are observed reaching periodically the surface of the gelatine. The movement of these slugs and their release at the gelatine surface are directly correlated with the period measured for the surface acceleration. Other types of flow are observed but not analysed in this report. The experimental observations show that the period of the surface oscillation varies systematically with the rigidity of the gelatine (characterised by the Young’s modulus) and the volumetric flowrate of the gas through the crack formed. In addition, the horizontal dimensions of the crack (width and thickness) increase systematically with gas volumetric flowrate, whereas the vertical dimension of the crack is related to the gelatine depth and rigidity, and the initial conditions of gas injection. Dimensional considerations are used to identify an approximate relationship that explains the systematic experimental observations. The ratio of the volume of gas in each gas slug and the period of oscillation is simply proportional to the initial continuous volumetric gas flux, as expected. A range of possible pressure scales exist for the flow and elastic deformation, and it is found that the ratio of elastic pressure of the gelatine and the pressure required to open the crack by the flow control the flowrate and oscillation period. The longer horizontal dimension of the crack emerges as the length scale that controls the flow, and two dimensionless groups formed from these parameters satisfactorily collapse the experimental data. Further quantitative analysis requires more experimental observations, which will also help to constrain the processes that open the crack and partition the initially continuous flow into discrete gas slugs. 

Parrain, D.
Quantum optics investigates the behaviour of light at a quantum level. Laser is an essential tool in this field because it delivers spatially and temporaly coherent light. Optical cavities or resonators are used in order to control modes delivered by the laser. A cavity is said locked when it delivers continuously a given mode. This is realised by a feedback control on the lenght of the cavity. Usually, this feedback control is done with analog instrument. This report aims at showing a manner of digitally locking an optical cavity. the digital program we developped also permits to analyse the quality of the lock in order to improve it.

Piatecki, S.
I devoted this internship to studying how the ground state of a lattice of classical antiferromagnetically coupled spins can be affected by the presence of electrons. The model I used let electrons hop from site to site with a tight-binding hamiltonian ; electrons interact with the local moments according to Hund’s coupling, which tends to align the local spins parallel to their spins. I first concentrated on the interplay of electrons with four spins coupled two by two, then with spins on a checkerboard lattice. Eventually, I developed a description of the ground-state in terms of loops.

Ramananarivo, S.
A landslide happens when the soil resistance along a failure surface isn’t sufficient to counteract the moving forces. Previous studies showed that the presence of roots, even dead ones, increases the soil shearing resistance. The goal of this project is to study the reinforcement effect of roots with regard to natural time evolution of the growing root system. The root architecture and several mechanical properties related to soil stability will be measured over a growing period, in conjunction with the evolution of soil shear resistance.

Raynaud, R.
The study of transient detectability both requires the simulation of a light curve according to a model of transients, and the definition of an observing strategy according to LOFAR technical specifications. Then, the efficiency of a strategy is quantified via the determination of a probability of detection. The analyse of the level curves of the probability as function of the flux scale and the time scale of the source enables to find the best strategy for different transients. The results show a correlation between the probabilities of detection and characterisation : for fast transients, the former becomes constant while the latter falls near zero. Moreover, if the probability of detection just mainly depends on the sensitivity of the telescope—which varies with the bandwidth, the frequencies of observation and the integration time—, the probability of characterisation is in addition sensitive both to the whole duration of observation and to the time elapsed between each data point.

Reys, V.
The following report describes how the Schwinger-Dyson formalism may be used to understand the dynamics of the ghosts and gluons in the Gribov-Zwanziger action. An overview of this theory will be presented, and then we will establish the Schwinger-Dyson equations of these fields. These equations will be used to obtain the values of the critical infrared exponents of the various propagators.

Solard, G.
We study conjugate operators to the energy-momentum operators in standard free massless quantum field theory. They are conjugate in the sense that they verify non-trivial commutation relations with the energy-momentum operators. Thus one can say that they are coordinate operators. We first show a way to build such operators starting from the generators of conformal transformations which give them a natural role in the conformal algebra. But since some difficulties arise from this definition, we will settle for simpler and less constrained operators. We then use these newly defined operators to device an S-matrix in curved space. Finally, we try to construct fields using these new coordinates operators.

Stril, A.
The Joint Dark Energy Mission (JDEM) will probe the expansion rate of the universe through di_erent techniques (graviational weak lensing, supernovae dimming, and baryon acoustic oscillations). The measurements should allow to rule out or con_firm some dark energy models (cosmological constant, quintessence, etc). We use the Fisher matrix formalism to quantify the merit of this experiences and assess their combined power in constraining the dark energy equation of state and the gravitational growth of structure.
I am also working on topics related to the Sunyaev-Zel’dovich (SZ) effect such as probing the Lemaître-Tolman-Bondi model and trying to establish a thermal SZ signal - halo mass relation of a galaxy cluster catalog.

Tourkine, P.
The work presented here is split into two parts. The first part concerns Stephan adhesion for Newtonian and yield stress fluids. This Stephan adhesion is the reason why two rigid plates confining some fluid are hard to pull apart. This part deals with the adhesive strength of those material from the theoretical and experimental point of view. It also presents the instabilities associated with these adhesive pull of tests, including cavitation and the Saffman-Taylor instability, known as fingering. The second and main part presents the work done with magnetorheological fluids. We discovered a new kind of instability, that arises in the same pull of adhesive tests just like the fingering, which we call flowering. We analyse this instability mostly from the phenomenological point of view, in a simpler wall like geometry.

<span style="font-weight: bold;">Valon, L.
Up until now, the time dependent properties of the cell have been described using a viscoelastic formulation. During my internship, I validated a new description of cell mechanics called poroelasticity. In contrast to the single phase viscoelastic description of the cell, poroelasticity models the cell as a biphasic system constituted of a solid porous meshwork bathed in a liquid phase. In my experiments, I used an atomic force microscope to apply a localized external mechanical stress onto the cell and measure ensuing cellular relaxation. I showed that the poroelastic formulation fits the experimental data significantly better than the viscoelastic model at both short and long time-scales. In addition, the poroelastic formulation can also accurately model major changes in the cell physiology. I showed that the cellular fluid fraction played a major role in the time-dependent properties of the cell and that actin filaments were more important than microtubules in modulating cellular poroelasticity. This poroelastic description should permit a better understanding of the role of the cytoskeleton, the fluid phase and macromolecular crowding in cell mechanics. All of these elements are involved in internal diffusivity and therefore may play an important role for the cell motility and cellular shape changes during the cell cycle and especially metaphase.