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School of Civil and Environmental Engineering and Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, U.S.A.
This talk will present results from numerical simulations of a stratified turbulent wake with non-zero net momentum over a broad range of body-based Reynolds number and Froude numbers. The focus will be on the wake-radiated internal gravity wave (IGW) field. At the higher Reynolds numbers values considered, which are one order of magnitude higher than those attained in the laboratory, secondary Kelvin-Helmholtz instabilities and turbulence in the wake core drive persistent IGW radiation up to much later times than commonly expected. Through two-dimensional wavelet analysis, the wavelengths, group/phase velocities and orientation angle of the wake-emitted IGW field are estimated. At the higher Reynolds number value, IGW beams/packets propagate at an angle closer to the horizontal plane suggesting that the wave-generation mechanism is no longer viscously controlled but serves to efficiently extract momentum from the mean wake flow. Finally, the steepness of the near-field waves increases with Reynolds and Froude number, indicating potential for enhanced remote wave breaking in geophysical/naval environments.
National Center for Atmospheric Research, Boulder
Dans de nombreux écoulements turbulents (par exemple en deux dimensions, ou en présence de certaines symétries), les interactions à longue portée conduisent à une organisation en structures cohérentes à grande échelle. La mécanique statistique permet d'obtenir un certain nombre de ces structures comme des équilibres statistiques et de construire les diagrammes de phase dans les différents ensembles statistiques. Dans cet expose, j'examinerai le rôle de la géométrie du domaine, en me concentrant sur le cas d'une sphère en rotation, pour lequel les propriétés dynamiques et thermodynamiques sont assez originales.
The mathematics and physics of knots has a fascinating history,
starting from a model of atom suggested by W.Thompson (Lord Kelvin).
Knots are now implicated in many places, from hydrodynamics to field
theory. But in polymers they are the most obvious. Knots in DNA are
known and important. Recently, the survey of the protein data bank
found that evolution for some as yet unknown reason preferred
unknotted proteins, although a few beautiful counterexamples were
found, including Gordian knot in one particular protein. Having no or
very few knots is also a non-trivial constraint, which appears to be
relevant to understand how DNA is packed in our cells. In theoretical
aspect, the field was long dominated by either highly abstract
mathematics or computer simulations. Recently, some progress was made
in the direction of physical understanding of knots. In the talk, all
these various aspects will be reviewed in some mixture.
David Wilkowski, Claus Dieter Ohl
1) General presentation of the research activities in Physic at NTU, Singapore by David Wilkowski
During the talk the different research activities will be presented; ranging from material science and nanotechnologies to complex system through cold atoms and ultrafast phenomena.
2) Cavitation in Confined Volumes by Claus Dieter Ohl
Cavitation phenomena in the real world are typically confined by one or more boundaries. Confining cavitation in small channels allows to study their interaction with cells, the formation of emulsions, and even sonochemical reactions in far greater detail as it would be possible in the bulk. However, it was expected that boundary layers will hinder bubble collapse more and more as the structure sizes are reduced. In this presentation the channel size is reduced even further, thus from microfluidic to nanofluidic channels. In microfluidic channels cavitation bubbles are generated with focused laser pulses and with acoustic waves. Acoustic cavitation in micrometer sized allows the formation of homogeneous emulsions, rapid rupture of cells (yeast and bacteria), and the dispersion of nanoparticles. While laser induced cavitation bubbles allow the study of homogeneous nucleation of cavitation and bubble dynamics in nanofluidic channels.
Understanding the physical mechanism behind glass formation is a lasting problem in statistical mechanics and condensed matter physics. Interestingly from a theoretical point of view, there are reasons to think that the viscous slowing down of super-cooled liquids' dynamics, preluding glass formation, is due to a new kind of thermodynamic transition, the ideal glass transition. This is conjectured to be an entropy vanishing critical point with many peculiarities, notably an exponential growth of correlation time in the critical region.
Assessing the mere existence of the ideal glass transition in real systems is particularly difficult. I will propose a new promising route to make substantial improvements in the understanding and in the description of this elusive transition. The idea is to induce a glass transition by pinning particles from an equilibrium configuration. I will characterize the particular features of the induced glass transition, called Random Pinning Glass Transition (RPGT) and discuss the advantages of studying it. I will also focus on other pining particles procedures to contrast the different outcomes and unveil the peculiar nature of the RPGT.
Charged platelet suspensions, such as swelling clays, disc-like mineral crystallites or exfoliated nanosheets are ubiquitous in nature. Their puzzling phase behaviors are nevertheless still poorly understood: while Laponite and Bentonite clay suspensions form arrested states at low densities, others, like Beidellite and Gibbsite exhibit an equilibrium isotropic-nematic
transition at moderate densities. These observations raise fundamental questions about the influence of electrostatic interactions on the isotropic-nematic transition and more generally on the organization of charged platelets. We investigate the competition between anisotropic excluded-volume and
electrostatic interactions in suspensions of thin charged disks, by means of Monte-Carlo simulations. We show that the original intrinsic anisotropy of the electrostatic potential between charged platelets, obtained within the non-linear Poisson-Boltzmann formalism, not only rationalizes the generic features of the complex phase diagram of charged colloidal platelets, but also predicts the existence of novel structures and arrested states upon varying density and ionic strength.
In this talk I will describe the motion of a massive intruder in a granular gas, namely a gas of macroscopic particles that collide inelastically.
In a dilute regime, the presence of dissipative forces is not detected by any violation of the fluctuation-dissipation relation, which is always satisfied. On the contrary, in a denser regime, autocorrelation and response are not proportional one to the other, as confirmed by numerical simulations. By combining generalized response theory and fluctuation relations, it is possible to identify a local velocity field, generated by the particles surrounding the intruder, necessary to have a good description of the dynamics.
In the second part, I will describe an experiment on a quasi-2D granular fluid which allows one to define and measure a non-equilibrium coherence length, confirming the presence of the correlation in the velocity field.
The statics and dynamics of thin strings are a classical subject in mechanics, and form the core of many open problems in geometrically nonlinear elasticity. I will begin by discussing what catenaries and lariats have in common, then present new results on rotating strings. With simple examples, I will illustrate a framework for the dynamics of curves which centers on the role of the stress, a multiplier field enforcing the metrical constraint of inextensibility. Finally, I will show how a rapidly unfurling string harbors a surprising instability.
Chemistry of life is totally controlled by a collection of enzymes which make highly efficient
at low (room) temperature only selected biochemical reactions within an organized scheme (metabolism).
Chemical reactions which require reorganization of chemical structures, necessarily involve
electron transfers. Though the standard Marcus theory describes quite well most non-biochemical
electron transfer reaction obeying Arrhenius laws with large activation energies, we argue
that it must fail when this activation energy becomes too small (roughly smaller than Debye energy)
as it is generally the case for biochemistry which operates at low temperature.
We show that in the cases of low energy barriers (as it is in biochemistry), the intrinsic (coherent) dynamics
of the electrons must be involved. We derive an improved theory where electronic dynamics remains coupled
to the nuclei but not slaved (unlike in the standard Born-Oppenheimer theory which
is generally used). Our theory becomes equivalent to the standard theory when the activation
energies is large, but is different at low energy.
Our improved formalism allows one to build simple toy models for ultrafast electron
transfer in the photosynthetic reaction center, for biomotors, signal transmission...
When parameters are finely tuned, our models can exhibit spectacular phenomena
mimicing those observed in biophysics. It might show a new direction for
solving many puzzling questions in biophysics still without convincing answer up to now.
Transcriptional regulation is the complex system of interactions used to produce precise gene expression at specific times and locations. Encapsulating our understanding of these interactions in computational models allows the exploration and prediction of the cellular response to changing environments, which can provide invaluable support and guidance to experimental research. Cells use several different transcription regulation mechanisms,including: transcription factor interactions, nucleosome positioning, chromatin remodeling, and transcriptional interference. These mechanisms are used individually or in combination to create a complex regulation environment. In this work we automatically create stochastic models that encompass all these regulatory methods into a single model for any given DNA sequence. The results from simulations of the generated model can be visualized to understand the interactions between components that regulate transcription.
I will discuss the thermodynamic efficiency at maximum power of a molecular machine described as a Brownian particle diffusing in a tilted periodic potential. I will consider both 1-D and 2-D models and show that loosely coupled machines operate with a smaller efficiency at maximum power than their strongly coupled counterparts.
Compressed sensing is triggering a major evolution in signal
acquisition that changes completely the way we think about
experiments and measurements. The idea is that most data, signals and
images, that are usually compressible and have redundancy, can be
reconstructed from much fewer measurements than what was usually
considered necessary, resulting in a drastic gain of time, cost, and
Compressed sensing consists in sampling a sparse signal using some random projections, and later using computational power for its exact reconstruction, so that only the necessary information is measured. This has been applied to many situations, from medical imagery and one-pixel-camera to confocal microscopy, acoustic holography or DNA micro-array analysis in biology.
In this talk, I will start by a general instruction to compressed sensing for physicists and discuss the state of the art reconstruction algorithms. Currently used reconstruction techniques are however limited to acquisition rates still higher than the true density of the signal. By using a mapping to a statistical physics problem, and motivated by the theory of crystal nucleation, I will introduce a new algorithm, and new measurement protocols, that achieves exact reconstruction of the signal even at measurement rates very close to the lowest possible ones.
In this talk I will review the research activities of our team in the field
of physics and biophysics of soft matter, especially concerning the
electrically charged macromolecules (DNA, polypeptide, synthetic polymers).
The major issue in this field is to understand the physics of biopolymers,
such as conformational and structural changes of these macromolecules under
the action of an external force. I will discuss the challenges we have
overcome to obtain and manipulate DNA and homopolypeptides, in order to
determine their structure using atomic force microscopy. I'll show how
these simple experiments illustrate a new highly sensitive approach to
biological problems, in which physicists have combined their expertise to
better understand them.
The delicate nanostructure of soft matter, such as polymers, emulsions, and colloidal suspensions, can be self assembled or controlled for a variety of fascinating applications familiar in everyday life; this allows us to also study a variety of fundamental ideas about phase transitions, statistical mechanics, and non-equilibrium physics. Precisely because of the soft nanostructures, these materials are strongly susceptible to mechanical stresses and flow fieilds. I will discuss two examples of soft matter and our attempts to understand some of the interesting roles that mechanical forces play: (i) biomembranes, which are ubiquitous motifs whose important dynamical function is increasingly being recognized, and (ii) hydrodynamic instabilities and transitions in soft matter in flow, whose understanding is crucial to many common products, from plastics to shampoos to pastes.
Hollow vortices are vortices whose interior is at rest. They posses
vortex sheets on their boundaries and can be viewed as a
desingularization of point vortices. We give a brief history of point
vortices. We then obtain exact solutions for hollow vortices in
linear and nonlinear strain and examine the properties of streets of
hollow vortices. The former can be viewed as a canonical example of a
hollow vortex in an arbitrary flow, and its stability properties
depend. In the latter case, we reexamine the hollow vortex street of
Baker, Saffman and Sheffield and examine its stability to arbitrary
disturbances, and then investigate the double hollow vortex street.
Implications and extensions of this work are discussed.
Small RNA (sRNA) play critical regulatory roles in organisms across all kingdoms of life: from the control of stress response in bacteria to the regulation of development in animals. sRNAs act at the post-transcriptional level via base-pairing with the targeted messenger RNAs (mRNA), leading to suppression of translation and/or to promotion of degradation of the mRNA. In this talk, I will present my work on the modeling of the sRNA regulation. In a first part, I will discuss the Langevin-like model we use to describe the stochastic dynamic of the coupled system composed by the sRNA and its targets. In particular, our formalism accounts for the stochastic nature of the underlying biochemical reactions, including the effect of transport by diffusion of the interacting molecules. I will also present the general physical properties of this type of regulation. In particular, we will discuss the fluctuations of the system that we estimated using the fluctuation-dissipation theorem derived from the linear-noise approximation. In a second part, I will apply this formalism to concrete biological situations. In particular: 1) Our model suggest that the presence of weak targets (which make transient complexes with the sRNAs) helps maintaining the intrinsic fluctuations of the main/strong targets at a low level, without significantly affecting their mean responses. 2) In the context of bacterial regulation, a detailed modeling of the translation process coupled to our stochastic description of sRNA regulation, suggest that ribosomes recruit sRNA molecules and that an increase of the affinity between the ribosome and the mRNA leads to a more efficient but more noisy regulation of the target.
The Kardar-Parisi-Zhang (KPZ) equation describes the stochastic evolution of a growing surface. Fluctuations of the interface around its mean value are characterized by universal scaling exponents and scaling functions, which also describe fluctuations of the free energy for a directed polymer in a random medium, and fluctuations of the current of particles in lattice gases. In one dimension, some of these scaling functions are related to distributions of largest eigenvalues for large random matrices. They have been measured in experiments.
Hydrodynamics plays a crucial role in many cellular processes. One example is the locomotion of cells such as bacteria, spermatozoa, and essentially half of the microorganisms on earth. These organisms typically possess flagella, slender whiplike appendages which are actuated in a periodic fashion in a fluid environment, thereby giving rise to propulsion. The basic physics of fluid-based cell locomotion was laid out in the 1950-70s by applied mathematicians. A recent resurgence in the field driven by new experimental data has allowed the community to ask a new series of questions on the nonlinear and nonlocal hydrodynamics of swimming cells. After a short introduction to the field, we present our recent work on the nonlocal and nonlinear hydrodynamics of swimming cells. We first model the observed synchronization of flagella between cells swimming in close proximity. We then determine the diffusive behavior of cells in noisy environments. We finally address the locomotion of cells in complex (polymeric) fluids. We conclude by presenting an overview of our current modeling work in small-scale mechanics
There is considerable industrial interest in novel flexible, transparent electrodes for electro-optical applications, also because of dwindling natural reserves of indium, a component of transparent electrodes used, e.g., in LCD display technology. For this purpose frantic research is currently being conducted worldwide into polymeric composites containing electrically conducting inorganic and metallic nanowires, carbon nanotubes, grafite flakes, graphene and so on. One of the objectives of this work is to get as high as possible a conduction for as low as possible a nanoparticle loading but progress is slow. Unclear is why, e.g., carbon nanotubes dispersed in plastic matrix materials can have such widely diverging electrical percolation thresholds even when their mean physical dimensions and other characteristics seem very similar. In an effort to shed light on this, we applied continuous space connectedness percolation theory to collections of anisometric particle
s with arbitrary polydispersity in length, width and levels of conduction between them. We find that the percolation threshold is extremely sensitive to even quite modest degrees of polydispersity and of alignment incurred in the processing of the fluid composites before they set and become the final solid product. We also find that the way polydispersity influences the percolation threshold depends on whether or not the length and width distributions are dependent on each other. Finally, we make connection between percolation of rods in continuous space and that on a Bethe lattice.
DNA denaturation is a physical process in the course of which the double strand can open locally thanks to thermal fluctuations. Within a denaturation bubble the two fluctuating single strands have a bending rigidity 50 times weaker than that of the unopened helix, which increase its conformational entropy. The DNA conformation will in turn influence the bubble creation process. This mutual influence naturally leads to a theoretical model coupling the local internal DNA states (open or closed base pairs) and the local DNA elasticity.
This internal-external coupling allows us to address and answer still open questions, such as how conformational fluctuations modify its thermal denaturation, and why DNA chains are kinked when observed on surfaces by AFM.
Finally, this elastic-base pairing coupling holds the key to studying the closure dynamics of a pre-equilibrated denaturation bubble and yields insight into the long closure times observed experimentally.
L'impact d'une goutte sur une surface conduit à de belles formes dynamiques qui résultent d'une
interaction subtile entre les effets d'inertie, les propriétés du fluide et des caractéristiques du substrat. Lors de ce séminaire, je présenterai une expérience où les impacts successifs de gouttes conduisent à d’étonnantes structures élancées que nous avons appelées tours granulaires. Elles sont créées par un goutte à goutte d’une suspension granulaire dense sur une surface absorbante comme, par exemple, du papier buvard ou une couche de granulaire sec. Ces tours résultant d’une solidification rapide de la goutte lors de l’impact sont analogues à de nombreuses structures rencontrées dans la nature, comme les coulées de lave solidifiées, les stalagmites de glace ou de calcaire. La hauteur peut être déterminée par un équilibre entre le flux de liquide en excès et le drainage à travers la tour granulaire. La vitesse d'impact, le temps de chute libre et la densité de la suspension contrôlent le diamètre de la tour et le détail de sa morphologie. Je montrerai que ces paramètres peuvent être manipulés pour obtenir divers structures lisse ou ondulée, axisymétrique, en zigzag ou parfois même chirale.
[J. Chopin et A. Kudrolli, Phys. Rev Lett. 107, 208304 (2011)].
Dans ce séminaire, je présenterai deux études qui mettent en évidence l'importance des conditions aux limites et des interfaces sur les écoulements de fluides.
Le première étude théorique porte sur le glissement de fluides simples au voisinage de surfaces dites super-hydrophobes, connues pour leurs propriétés super-lubrifiantes.
Je présenterai à la fois un calcul semi-analytique ainsi que des lois d'échelle permettant de prédire le glissement en fonction des caractéristiques des surfaces. Je m'intéresserai en particulier
à des surfaces super-hydrophobes fractales pour lesquelles il n'existait encore aucune prédiction.
La deuxième étude expérimentale porte sur l'ascension capillaire de fluides complexes.
Un résultat surprenant au premier abord est l'absence de dépendance entre la hauteur finale et la taille du capillaire.
A l'aide d'un modèle basé sur la rhéologie des fluides à seuil, je décrirai à la fois la hauteur
finale d'ascension ainsi que la dynamique de l'ascension capillaire. Je conclurai ce séminaire par quelques perspectives.
During the recent eruption of Shinmoe-dake volcano, southwester Japan, an interesting sequence of harmonic tremor was observed. The process is reproduced by a laboratory experiment using a children's beeping toy made of a straw and a balloon, and a non-newtonian hair gel.
When a cell replicates its DNA, each base must be copied once and only once per cell cycle. A failure to complete replication normally can lead to cell death, or worse. In this talk, I will discuss how ideas from statistical physics can help understand how replication is organized and controlled. We focus on two successful applications. In the first, we show how the replication of budding yeast, long thought to have deterministic timing control, is controlled by stochastic mechanisms. In the second, we show that the DNA replication program in B cells is altered during development, as the cell commits to producing a particular antibody. The overall theme is that passive stochastic control of the replication program plays a more important role than had been believed until recently.
We present a numerical study of the magnetic field generated by the Taylor-Green vortex. We show that periodic boundary conditions can be used to mimic realistic boundary conditions by prescribing the symmetries of the velocity and magnetic fields. This gives insight in some problems of central interest for dynamos: the possible effect of velocity fluctuations on the dynamo threshold, the role of boundary conditions on the threshold and on the geometry of the magnetic field generated by dynamo action. In particular, we show that an axial dipolar dynamo similar to the one observed in a recent experiment can be obtained with an appropriate choice of the symmetries of the magnetic field.
The nonlinear saturation is studied and a simple model explaining the magnetic Prandtl number dependence of the super/sub critical nature of the dynamo transition is given.
Dans ce séminaire je présenterai deux problèmes très différents de cinétique dans des systèmes complexes.
Dans le premier problème, nous examinons les comportements complexes de moteurs moléculaires connectés par un élément rigide. Un exemple de tel comportement est la génération d'oscillations spontanées, qui pourraient jouer une rôle dans les oscillations de muscles cardiaques ou le battement de flagelles. De telles oscillations ont récemment été observées dans des conditions in vitro, et nous les comparons à une théorie. Nous décrivons ensuite une autre instabilité jouant un rôle dans le transport intracellulaire, le mouvement bidirectionnel, au cours duquel on assiste à des inversions de vitesse d'une assemblée de moteurs. Nous calculons un potentiel effectif hors équilibre dans ce système actif et dérivons une formule analytique pour le temps de renversement de la vitesse, révélant ainsi l'existence d'un nombre minimal de moteurs requis pour observer le mouvement bidirectionnel.
Dans un deuxième temps j'aborderai des questions de cinétique de réaction entre molécules attachées à des polymères. Des exemples de telles réactions sont la formation de boucle dans un polymère (réaction de «cyclisation»), ou la réaction d'un monomère donné avec une cible fixe en espace confiné. Ces deux problèmes sont inspirés de interviennent potentiellement dans des situations biologiques (formation de boucles dans l’ADN ou lors du repliement d’une protéine, «recherche» du noyau d’une cellule par un génome viral...). Ce sont des problèmes non-markoviens car ils comportent des variables cachées (les positions de tous les monomères), à ce jour il n’existe aucune théorie pouvant reproduire les résultats de simulations numériques. La plupart des théories existantes tentent de remplacer le problème non-markovien par un problème markovien effectif. Nous proposons une théorie permettant de garder le caractère non-markovien du problème et qui permet de calculer de manière approchée la distribution de probabilité des variables cachées au moment de la réaction. Nous obtenons des résultats qui correspondent très bien aux simulations numériques.
La théorie des grandes déviations s'intéresse aux événements rares de processus stochastiques ayant une probabilité exponentiellement faible de se réaliser. La théorie tire ses racines de la physique statistique d'équilibre, et est de plus en plus utilisée ces jours-ci par les physiciens travaillant sur les systèmes hors d'équilibre. Cet exposé tentera de faire un survol de cette théorie en se concentrant sur deux applications particulières, soit, le problème des entropies non-concaves des systèmes avec interactions à longue portée, et les fluctuations de systèmes stationnaires hors d'équilibre.
We report on experimental studies of spatio-temporally
heterogeneous stick-slip motions in the sliding friction
between a hard polymethyl methacrylate (PMMA, plexiglass)
block and a soft poly-dimethyl siloxane (PDMS, silicone)
gel plate. We observe large and rapid slip events preceded
by an alternation of active and less active periods.
The probability distributions of the force drop, a quantity
analogous to seismic moment, obey a power law
similar to Gutenberg-Richter's empirical law for the
frequency-size statistics of earthquakes, and the exponents
of the power law vary with the plate velocity. We propose
a simple model to explain this velocity dependence. Finally,
we introduce our preliminary results on the visualization
of shear stress fields during stick-slip cycles. We observe
a clear change in spatial patterns of the stress field
towards a large slip event.
Orbital dynamics that lead to longitudinal libration of celestial bodies also result in an elliptically deformed equatorial core-mantle boundary. Such a non-axisymmetric boundary allows for the existence of topographic coupling between the assumed rigid mantle and the underlying low viscosity fluid.
Here we use a coupled numerical-experimental approach to investigate the effect of topographic coupling on the flow in the fluid contained within librating bodies. We report the first evidence that longitudinal libration can drive intermittent, turbulence in particular bands of libration frequency. A local theoretical analysis suggests that our results can be interpreted as the cyclical breakdown of an elliptical instability that grows on the base flow produced by the libration of the container. When the background flow remains laminar, we observe a zonal flow that is independent of the ellipticity and results from non-linear interactions in the Ekman boundary layer. Our experiments suggest that intermittent, space-filling turbulence can exist in the cores and sub-surface oceans of librating planets due to elliptical instability.
Flow problems involving mixing process are met in a wide variety of natural systems and industrial activities. Mixing span an enormous range of length an time scales and encompass Reynolds numbers varying over forty order of magnitude. We will present two examples of mixing in fluid dynamics, both were studied with an experimental approach. First the buoyancy driven mixing of two fluids of different densities in the confined geometry of a tilted tube. The fluids are initially separated in an unstable configuration (the heavier fluid is above the lighter fluid) and the tilt angle is fixed for each experiment.Velocity and concentration measurements are performed in a vertical diametral plan. The flow displays a large variety of behaviors depending on the the tilt angle and the density contrast (which are the two control parameters), from a laminar counter-flow to a turbulent mixing thought a laminar flow with turbulent bursts. A secondary flow is identified and plays an important role for the momentum transport. The second study is focused on periodically driven laminar flows (the Reynolds number is in order of 1e -3). Time-periodic viscous flow in a square cylindrical domain is considered. The flow is generated via translation of the top or bottom wall of the cylinder under specific forcing protocols. Each forcing protocol has a particular configuration consisting of n steps. The n-step flow (n = 3) is approximated as a piece-wise linear combination of n separate steady flows. Using 3D particle tracking velocimetry (3D PTV) we are interested in the formation and interaction of coherent structures due to fluid inertia, which play an important role in 3D mixing by geometrically determining the tracer transport. The disintegration of these structures by fluid inertia reflects an essentially 3D route to chaos.
The technological revolution we witnessed over the last decade has resulted in social and technological networks that have detailed structured in multiple and inter operating scales.Examples are provided by the Internet, the social Web, the new WiFi communication technologies and transportation and mobility infrastructures. Understanding the interactions between the technological constraints and the social interactions that mutually compete to guide the development of the network is one of the challenges that we must face. We review the application of statistical analysis of these networks to the modeling of Human Behavior and Epidemic Spreading. As an application to human behavior we use present how the use of proxy information from the analysis of online traces is able to characterize human behavior on and off line. Using the empirical traffic patterns generated by a thousand users, we develop a model of how individuals navigate online that is able to characterize several properties of Web traffic that cannot be reproduced by traditional Markovian models. Finally, we present the Global Epidemic and Mobility (GLEaM) model that integrates sociodemographic and population mobility data in a spatially structured stochastic disease approach to simulate the spread of epidemics at the worldwide scale. We discuss the ßexible structure of the model that is open to the inclusion of different disease structures and local intervention policies
Quantum turbulence (QT) is turbulence in a superfluid. A pure superfluid lacks any viscosity, enabling the energy cascade to be extended to much smaller scales than seen in classical fluids. Due to the irrotational aspect of the flow, QT is characterized by polarized bundles of quantized vortex lines. At the inter-vortex scale, the quasi-Kolmogorov 3D energy cascade in the flow gets transferred onto these vortex lines as propagating Kelvin waves. It is these Kelvin waves that transfer energy to finer scales, until the energy can be dissipated into heat. It was originally believed that by using wave turbulence theory to describe these Kelvin waves, the energy transfer between them was a local process - resulting in the Kozik-Svistunov (KS) energy spectrum. I will present some calculation that show that the Kelvin wave interaction are non-local, thereby invalidating the KS spectrum and present a recently proposed non-local theory for Kelvin waves.
Foamy virus is an atypical retrovirus which shares similarities with HIV and hepatitis B viruses. Some parts of his replication cycle remain unclear, especially its infection process. We followed the entry of dual color fluorescent virus particles in living cells by high resolution 3D imaging, namely using a spinning disk microscope. We developed several analysis tools allowing to achieve 3D single virus tracking and 3D dynamic colocalization calculation. This colocalization analysis is used to detect when the two differently labeled proteins spatially separate. Thus we show that prototype foamy virus can enter the cell by two different pathways: fusion and endocytosis.
I will also briefly introduce an innovative 3D tracking microscope enabling real-time tracking (feedback approach) of single fluorescent particles with nanometer accuracy and 32ms time resolution.
Computational Epidemiology Lab, ISI Foundation, Turin - IT
The 2009 H1N1 influenza pandemic is just the latest example of how human mobility helps drive infectious diseases. Travel has grown explosively in the last decades, contributing to an emerging complex pattern of traffic flows that unfolds at different scales, shaping the spread of epidemics. Restrictions on people's mobility are thus investigated to design possible containment measures. By considering a theoretical framework in terms of reaction-diffusion processes, it is possible to study the invasion dynamics of epidemics in a meta-population system with heterogeneous mobility patterns. The system is found to exhibit a global invasion threshold that sets the critical mobility rate below which the epidemic is contained. The results provide a general framework for the understanding of the numerical evidence from detailed data-driven simulations that show the limited benefit provided by travel flows reduction in slowing down or containing an emerging epidemic.
On general grounds, a non-equilibrium temperature can be consistently
defined from generalized fluctuation-dissipation relations only if it is
independent of the observable considered. In this talk I will argue that
the dependence on the choice of observable generically occurs when the
phase-space probability distribution is nonuniform on constant energy
shells. I will relate quantitatively this observable dependence to a
fundamental characteristics of non-equilibrium systems, namely, the
Shannon entropy difference with respect to the equilibrium state with
the same energy. The results will be illustrated on various mean-field
We study the dynamics of the shear-banding flow of wormlike micelles in Taylor-Couette cell. We show that, at vanishing Reynolds, the flow exhibit Taylor-like vortices that eventually leads to turbulent flow. We propose a scenario that could explain the observed behavior by an analogy with the elastic instabilities known for polymer solutions.