laboratoire de physique statistique
laboratoire de physique statistique




P A R M I :

Diffusion and Reaction-Diffusion in Steady Flows at Large Peclet Numbers - Pomeau, Y.
in TRANSPORT AND MIXING IN GEOPHYSICAL FLOWS edited by Weiss, JB and Provenzale, A (2008)

Abstract : In many, if not most; geophysical flows molecular diffusion is formally negligible because its relative importance is measured by the inverse of a large Peclet number. This has motivated a number of studies, based in some way or another on various theoretical approaches to turbulence, where the assumed randomness of the turbulent flow plays the center role. If one adds to this complicated situation of the passive scalar the possibility of chemical reactions changing the chemical layout of the fluid; the number of unresolved issues increases dramatically. The chemistry is often such that, without flow, the reaction progresses by a front (the flame front in gaseous combustion) sweeping the system, across which the reaction takes place, from an unburnt (fresh) side to a burnt side. I look at these problems in the case of steady flows made of rolls; as generated by Rayleigh-Benard instability for instance, certainly an idealization of real turbulence, although it is not; clear by how much. The propagation of the chemical reaction in this system depends first on the diffusion of a passive scalar, an interesting question in the limit of a large Peclet number in a flow without open flow lines. The main result there is that the effective diffusion is somewhere in between the molecular diffusion and the ``turbulent'' diffusion. Once chemical reactions are taken into account, that is when one considers the reaction-diffusion case, one finds that the front speed is the laminar propagation velocity (without flow) times the Peclet number to the power 1/4. I refine this last result and give the behavior of the prefactor in the Zel'dovich limit of a narrow reaction zone.
Syndication Strategies in Venture Capital Networks - Mas, David and Vignes, Annick and Weisbuch, Gerard

Abstract : Empirical evidence shows that venture capitalists syndicate to finance start-ups. This paper focuses on the role of the social network generated by these syndication operations. We consider the links developed between venture capitalists, through co-investment rounds, and we study their relationships both through network and econometric analyses. We first demonstrate that the syndication network is not random. Secondly, we show that the different assortativities (degree, spatial, industrial) are positive, suggesting that venture capitalists tend to co-invest with their peers. Thirdly, we measure the influence of different proximities (spatial, national and industrial) on the collaborations between the different players. National and industrial proximity have a strong impact on the determination of links. Finally, we provide evidence that past partners are preferred for future syndication, even if new links do appear regularly.
The science and craft of autostereograms - Ninio, Jacques
SPATIAL VISION 21185-200 (2008)

Abstract : Autostereograms or SIRDS (Single Image Random-Dot Stereograms) are camouflaged stereograms which combine the Julesz random-dot stereogram principle with the wallpaper effect. They can represent any 3D shape on a single image having a quasi-periodic appearance. Rather large SIRDS can be interpreted in depth with unaided eyes. In the hands of computer graphic designers, SIRDS spread all over the world in 1992-1994, and these images, it was claimed, opened a new era of stereoscopic art. Some scientific, algorithmic and artistic aspects of these images are reviewed here. Scientifically, these images provide interesting cues on stereoscopic memory, and on the roles of monocular regions and texture boundaries in stereopsis. Algorithmically, problems arising with early SIRDS, such as internal texture repeats or ghost images are evoked. Algorithmic recommendations are made for gaining a better control on the construction of SIRDS. Problems of graphic quality (smoothness of the represented surfaces, or elimination of internal texture repeats) are discussed and possible solutions are proposed. Artistically, it is proposed that SIRDS should become less anecdotal, and more oriented towards simple geometric effects, which could be implemented on large panels in natural surrounds.
Symmetry influences colour perception: the transparent sheet model - Ninio, J.
PERCEPTION 37147 (2008)
Depth perception following monocular fusion of two images with a squinting eye - Rychkova, S. I. and Ninio, J.
PERCEPTION 3715-16 (2008)
Dedication to Pierre Lallemand on the occasion of his retirement - d'Humieres, Dominique and Krafczyk, Manfred and Luo, Li-Shi and Rubinstein, Robert
Laser Doppler measurement of inertial particle and bubble accelerations in turbulence - Volk, R. and Mordant, N. and Verhille, G. and Pinton, J. -F.
EPL 81 (2008)

Abstract : We use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time resolution, 30 microseconds, resolves the fastest scales of the fluid motion. Particles are tracked for mean durations of the order of 10 Kolmogorov time scales and their accelerations are measured. For neutrally buoyant particles (fluid tracers), this technique matches the performance of the silicon strip detector technique introduced at Cornell University (Voth G. A. et al., J. Fluid Mech., 469 (2002) 121). This reference dynamics is then compared to that of slightly heavier solid particles (density 1.4) and to air bubbles. We observe that the dynamics of the particles strongly depends on their density. Bubbles have a much faster dynamics and experience much higher accelerations than fluid tracers. Although the particles dynamics are different, we find that the probability distribution functions of accelerations normalized to the variance always remain very close to the one for the fluid tracers. Copyright (C) EPLA, 2008.
Generic behaviours in impact fragmentation - Sator, N. and Mechkov, S. and Sausset, F.
EPL 81 (2008)

Abstract : We present a simple numerical model for investigating the general properties of fragmentation. By use of molecular dynamics simulations, we study the impact fragmentation of a solid disk of interacting particles with a wall. Regardless of the particular form of the interaction potential, the fragment size distribution exhibits a power law behaviour with an exponent that increases logarithmically with the energy deposited in the system, in agreement with experiments. We expect this behaviour to be generic in fragmentation phenomena. Copyright (C) EPLA, 2008.
Spontaneous generation of spiral waves by a hydrodynamic instability - Habibi, M. and Moller, P. C. F. and Ribe, N. M. and Bonn, D.
EPL 81 (2008)

Abstract : The coiling of a thin filament of viscous fluid falling onto a surface is a common and easily reproducible hydrodynamic instability. Here we report for the first time that this instability can generate regular spiral patterns, in which air bubbles are trapped in the coil and then advected horizontally by the fluid spreading on the surface. We present a simple model that explains how these beautiful patterns are formed, and how the number of spiral branches and their curvature depends on the coiling frequency, the frequency of rotation of the coiling center, the total flow rate, and the thickness of the spreading fluid film. Copyright (C) EPLA, 2008
Reconstructing a random potential from its random walks - Cocco, S. and Monasson, R.
EPL 81 (2008)

Abstract : The problem of how many trajectories of a random walker in a potential are needed to reconstruct the values of this potential is studied. We show that this problem can be solved by calculating the probability of survival of an abstract random walker in a partially absorbing potential. The approach is illustrated on the discrete Sinai (random-force) model with a drift. We determine the parameter (temperature, duration of each trajectory,...) values making reconstruction as fast as possible. Copyright (C) EPLA, 2008
Experimental evidence of non-Amontons behaviour at a multi-contact interface - Scheibert, J. and Prevost, A. and Frelat, J. and Rey, P. and Debregeas, G.
EPL 83 (2008)

Abstract : We report on normal stress field measurements at the multicontact interface between a rough elastomeric film and a smooth glass sphere under normal load, using an original MEMS-based stress-sensing device. These measurements are compared to Finite-Elements Method (FEM) calculations with boundary conditions obeying locally Amontons' rigid-plastic-like friction law with a uniform friction coefficient. In dry contact conditions, significant deviations are observed which decrease with increasing load. In lubricated conditions, the measured pro. le recovers almost perfectly the predicted pro. le. These results are interpreted as a consequence of the finite compliance of the multicontact interface, a mechanism which is not taken into account in Amontons' law. Copyright (C) EPLA, 2008.
The spectrum of the fractional Laplacian and First-Passage-Time statistics - Katzav, E. and Adda-Bedia, M.
EPL 83 (2008)

Abstract : We present exact results for the spectrum of the fractional Laplacian in a bounded domain and apply them to First-Passage-Time (FPT) statistics of Levy flights. We specifically show that the average is insufficient to describe the distribution of the FPT, although it is the only quantity available in the existing literature. In particular, we show that the FPT distribution is not peaked around the average, and that knowledge of the whole distribution is necessary to describe this phenomenon. For this purpose, we provide an efficient method to calculate higher-order cumulants and the whole distribution. Copyright (C) EPLA, 2008.
Some applications of magnetic resonance imaging in fluid mechanics: Complex flows and complex fluids - Bonn, Daniel and Rodts, Stephane and Groenink, Maarten and Rafai, Salima and Shahidzadeh-Bonn, Noushine and Coussot, Philippe

Abstract : The review deals with applications of magnetic resonance imaging (MRI) techniques to study flow. We first briefly discuss the principles of flow measurement by MRI and give examples of some applications, such as multiphase flows, the MRI rheology of complex fluid flows, and blood flows in the human body.
A caged retinoic acid for one- and two-photon excitation in zebrafish embryos - Neveu, Pierre and Aujard, Isabelle and Benbrahim, Chouaha and Le Saux, Thomas and Allemand, Jean-Francois and Vriz, Sophie and Bensimon, David and Jullien, Ludovic
Physics - The force of fluctuations - Balibar, Sebastien
NATURE 451136-137 (2008)
Shear thickening of cornstarch suspensions as a reentrant jamming transition - Fall, Abdoulaye and Huang, N. and Bertrand, F. and Ovarlez, G. and Bonn, Daniel

Abstract : We study the rheology of cornstarch suspensions, a non-Brownian particle system that exhibits shear thickening. From magnetic resonance imaging velocimetry and classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid, and then solid again when it shear thickens. For the onset of thickening we find that the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.
The spectrum of large powers of the Laplacian in bounded domains - Katzav, E. and Adda-Bedia, M.

Abstract : We present exact results for the spectrum of the Nth power of the Laplacian in a bounded domain. We begin with the one-dimensional case and show that the whole spectrum can be obtained in the limit of large N. We also show that it is a useful numerical approach valid for any N. Finally, we discuss implications of this work and present its possible extensions for non-integer N and for 3D Laplacian problems.
Current noise through a kondo quantum dot in a SU(N) fermi liquid state - Mora, Christophe and Leyronas, Xavier and Regnault, Nicolas

Abstract : The current noise through a mesoscopic quantum dot is calculated and analyzed in the Fermi liquid regime of the SU(N) Kondo model. The results connect the Johnson-Nyquist noise to the shot noise for an arbitrary ratio of voltage and temperature, and show that temperature corrections are sizable in the usual experiments. For the experimentally relevant SU(4) case, quasiparticle interactions are shown to increase the shot noise.
The traveling-wave approach to asexual evolution: Muller's ratchet and speed of adaptation - Rouzine, Igor M. and Brunet, Eric and Wilke, Claus O.

Abstract : We use traveling-wave theory to derive expressions for the rate of accumulation of deleterious mutations under Muller's ratchet and the speed of adaptation under positive selection in asexual populations. Traveling-wave theory is a semi-deterministic description of an evolving population, where the bulk of the population is modeled using deterministic equations, but the class of the highest-fitness genotypes, whose evolution over time determines loss or gain of fitness in the population, is given proper stochastic treatment. We derive improved methods to model the highest-fitness class (the stochastic edge) for both Muller's ratchet and adaptive evolution, and calculate analytic correction terms that compensate for inaccuracies which arise when treating discrete fitness classes as a continuum. We show that traveling-wave theory makes excellent predictions for the rate of mutation accumulation in the case of Muller's ratchet, and makes good predictions for the speed of adaptation in a very broad parameter range. We predict the adaptation rate to grow logarithmically in the population size until the population size is extremely large. (C) 2007 Elsevier Inc. All rights reserved.
Analyzing single-bond experiments: Influence of the shape of the energy landscape and universal law between the width, depth, and force spectrum of the bond - Husson, Julien and Pincet, Frederic

Abstract : Experimentalists who measure the rupture force of a single molecular bond usually pull on that bond at a constant speed, keeping the loading rate r=df/dt constant. The challenge is to extract the energy landscape of the interaction between the two molecules involved from the experimental rupture force distribution under several loading rates. This analysis requires the use of a model for the shape of this energy landscape. Several barriers can compose the landscape, though molecular bonds with a single barrier are often observed. The Bell model is commonly used for the analysis of rupture force measurements with bonds displaying a single barrier. It provides an analytical expression of the most likely rupture force which makes it very simple to use. However, in principle, it can only be applied to landscapes with extrema whose positions do not vary under force. Here, we evaluate the general relevance of the Bell model by comparing it with another analytical model for which the landscape is harmonic in the vicinity of its extrema. Similar shapes of force distributions are obtained with both models, making it difficult to confirm the validity of the Bell model for a given set of experimental data. Nevertheless, we show that the analysis of rupture force experiments on such harmonic landscapes with the Bell model provides excellent results in most cases. However, numerical computation of the distributions of the rupture forces on piecewise-linear energy landscapes indicates that the blind use of any model such as the Bell model may be risky, since there often exist several landscapes compatible with a given set of experimental data. Finally, we derive a universal relation between the range and energy of the bond and the force spectrum. This relation does not depend on the shape of the energy landscape and can thus be used to characterize unambiguously any one-barrier landscape from experiments. All the results are illustrated with the streptavidin-biotin bond.