laboratoire de physique statistique
laboratoire de physique statistique




P A R M I :

Designing visually rich, nearly random textures - Ninio, Jacques
SPATIAL VISION 20561+ (2007)

Abstract : Camouflaging textures containing as in real life edges at all orientations, were designed by computer, then manually, for use in stereoscopic vision studies. In the manual procedure, the starting point is either a set of photographs (for instance, of barks) or a manually produced first-generation texture. Then patches are cut zigzagging and assembled into successive generations of textures. The absence of extended edges - straight or curved - and the local heterogeneity of the texture are important camouflaging factors, allowing curved surfaces to be covered with these textures without visible join. Small areas of a texture often suggest a scene, but when the areas are assembled, the suggestive power is lost, and the statistical properties of the texture then dominate. However, when symmetry is introduced (as in the Rorschach test), meaningful scenes emerge again.
Inferring DNA sequences from mechanical unzipping data: the large-bandwidth case - Baldazzi, V. and Bradde, S. and Cocco, S. and Marinari, E. and Monasson, R.

Abstract : The complementary strands of DNA molecules can be separated when stretched apart by a force; the unzipping signal is correlated to the base content of the sequence but is affected by thermal and instrumental noise. We consider here the ideal case where opening events are known to a very good time resolution (very large bandwidth), and study how the sequence can be reconstructed from the unzipping data. Our approach relies on the use of statistical Bayesian inference and of Viterbi decoding algorithm. Performances are studied numerically on Monte Carlo generated data, and analytically. We show how multiple unzippings of the same molecule may be exploited to improve the quality of the prediction, and calculate analytically the number of required unzippings as a function of the bandwidth, the sequence content, and the elasticity parameters of the unzipped strands.
Mental kinetics in visual memory - Ninio, J.
PERCEPTION 3652 (2007)
Cavitation in plants at low temperature: is sap transport limited by the tensile strength of water as expected from Briggs' Z-tube experiment? - Cochard, Herve and Barigah, Tete and Herbert, Eric and Caupin, Frederic
NEW PHYTOLOGIST 173571-575 (2007)

Abstract : Xylem cavitation in plants is thought to be caused by a loss of adhesion at the conduit wall surface because a rupture in the body of the water column was implicitly ruled out by an experiment by Lyman J. Briggs with Z-tube capillaries. However, Briggs reported a drastic increase in cavitation pressure of water below 5 degrees C which, if it were also true in xylem conduits, would suggest that water transport in plants could be limited by water cohesion at low temperature. In this study we have repeated Briggs' experiment using stem segments. Xylem vulnerability curves were obtained with a centrifuge technique at 1, 25 and 50 degrees C on yew (Taxus baccata). Contrary to Briggs' finding, vulnerability to cavitation, measured as per cent loss conductance, did not increase sharply at 1 degrees C and was even less than at 25 degrees C and 50 degrees C. Moreover, the onset of cavitation in yew at 1 degrees C was measured at a much more negative pressure than Briggs' value. This points out an artefact in Brigg's experiment at low temperature possibly related to imperfections in the tube walls which act as cavitation nuclei.
Selective-pivot sampling of radial distribution functions in asymmetric liquid mixtures - Malherbe, J. G. and Krauth, W.
MOLECULAR PHYSICS 1052393-2398 (2007)

Abstract : We present a new method for selectively sampling radial distribution functions and effective interaction potentials in asymmetric liquid mixtures using a Monte Carlo simulation. We demonstrate its efficiency for hard-sphere mixtures, and for model systems with more general interactions, and compare our simulations with several analytical approximations. For interaction potentials containing a hard-sphere contribution, the algorithm yields the contact value of the radial distribution function.
Lagrangian intermittencies in dynamic and static turbulent velocity fields from direct numerical simulations - Leveque, E. and Chevillard, L. and Pinton, J.-F. and Roux, S. and Arneodo, A. and Mordant, N.

Abstract : Three temporal velocity signals are analyzed from direct numerical simulations of the Navier - Stokes ( N - S) equations. The three signals are: (i) the velocity of fluid particles transported by the time-evolving solution ( Eulerian velocity field) of the N - S equations, referred to as the dynamic case; (ii) the velocity of fluid particles transported by a solution of the N - S equations at some fixed time, referred to as the static case; and (iii) the time evolution of the solution of the N - S equations at some fixed positions, referred to as the Eulerian case. The comparison of these three signals aims at elucidating the importance of the overall spacetime evolution of the flow on Lagrangian statistics. It is observed that the static case is, to some extent, similar to the Eulerian case; a feature that can be understood as an ergodicity property of homogeneous and isotropic turbulence and can be related to the process of random sweeping. The dynamic case is clearly different. It bears the signature of the time evolution of the flow. This study emphasizes the importance of the global dynamics of the flow and points out the existence of long-time correlations in the fluid-particle dynamics in the Lagrangian description.
Depinning exponents of the driven long-range elastic string - Duemmer, Olaf and Krauth, Werner

Abstract : We perform a high-precision calculation of the critical exponents for the long-range elastic string driven through quenched disorder at the depinning transition, at zero temperature. Large-scale simulations avoid finite-size effects and improve accuracy. We explicitly demonstrate the equivalence of fixed-velocity and fixed-driving-force simulations. The roughness, growth, and velocity exponents are calculated independently, and the dynamic and correlation length exponents are derived. The critical exponents satisfy known scaling relations and agree well with analytical predictions.
3D aggregation of wet fibers - Py, C. and Bastien, R. and Bico, J. and Roman, B. and Boudaoud, A.
EPL 77 (2007)

Abstract : Wet fibrous structures such as nanotube carpets or macroscopic brushes tend to self-assemble into bundles when the liquid evaporates. The aggregation process relies on a balance between capillary attraction provided by liquid bridges and restoring torque due to structure stiffness. The final self-organized structure is found to result from a cascade of pairing of smaller bundles into bigger ones. We first describe, both experimentally at a macroscopic scale and theoretically, the case of a single pair of fibers and then generalize this description to more complex 3D assemblies. We finally show the relevance of our results to micro-scale experiments from the literature. Copyright (C) EPLA, 2007.
Periodic lipidic membrane tubes - Campelo, F. and Allain, J. -M. and Ben Amar, M.
EPL 77 (2007)

Abstract : We investigate the formation of two-phase lipidic tubes of membrane in the framework of the Canham-Helfrich model. The two phases have different elastic moduli (bending and Gaussian rigidity), different tensions and a line tension prevents the mixing. For a set of parameters close to experimental values, periodic patterns with arbitrary wavelength can be found numerically. A wavelength selection is detected via the existence of an energy minimum. When the chemical composition induces an important enough size disequilibrium between both phases, a segregation into two half infinite tubes is preferred to a periodic structure. Copyright (C) EPLA, 2007.
Magnetic field reversals in an experimental turbulent dynamo - Berhanu, M. and Monchaux, R. and Fauve, S. and Mordant, N. and Petrelis, F. and Chiffaudel, A. and Daviaud, F. and Dubrulle, B. and Marie, L. and Ravelet, F. and Bourgoin, M. and Odier, Ph. and Pinton, J.-F. and Volk, R.
EPL 77 (2007)

Abstract : We report the first experimental observation of reversals of a dynamo field generated in a laboratory experiment based on a turbulent flow of liquid sodium. The magnetic field randomly switches between two symmetric solutions B and -B. We observe a hierarchy of time scales similar to the Earth's magnetic field: the duration of the steady phases is widely distributed, but is always much longer than the time needed to switch polarity. In addition to reversals we report excursions. Both coincide with minima of the mechanical power driving the flow. Small changes in the flow driving parameters also reveal a large variety of dynamo regimes.
Fracture surfaces of heterogeneous materials: A 2D solvable model - Katzav, E. and Adda-Bedia, M. and Derrida, B.
EPL 78 (2007)

Abstract : Using an elastostatic description of crack growth based on the Griffith criterion and the principle of local symmetry, we present a stochastic model describing the propagation of a crack tip in a 2D heterogeneous brittle material. The model ensures the stability of straight cracks and allows for the study of the roughening of fracture surfaces. When neglecting the effect of the nonsingular stress, the problem becomes exactly solvable and yields analytic predictions for the power spectrum of the paths. This result suggests an alternative to the conventional power law analysis often used in the analysis of experimental data. Copyright (C) EPLA, 2007.
Escape from a potential well, stochastic resonance and zero frequency component of the noise - Petrelis, F. and Aumaitre, S. and Mallick, K.
EPL 79 (2007)

Abstract : For an overdamped particle evolving in a potential and submitted to colored noise, we study numerically the problems of first passage time at the top of the potential and of stochastic resonance. In the limit where the correlation time of the noise is small compared to the other characteristic times of the problem we show that properties of the solution related to long time behavior are controlled by the zero-frequency component of the noise. Copyright (C) EPLA, 2007.
1-D random landscapes and non-random data series - Fink, T. M. A. and Willbrand, K. and Brown, F. C. S.
EPL 79 (2007)

Abstract : We study the simplest random landscape, the curve formed by joining consecutive data points f1,..., f(N+1) with line segments, where the f(i) are i.i.d. random numbers and fi = fj. We label each segment increasing (+) or decreasing (-) and call this string of +'s and -'s the up-down signature sigma. We calculate the probability P(sigma(f)) for a random curve and use it to bound the algorithmic information content of f. We show that f can be compressed by k = log(2)1/P(sigma) - N bits, where k is a universal currency for comparing the amount of pattern in different curves. By applying our results to microarray time series data, we blindly identify regulatory genes.
The shear modulus of wet granular matter - Moller, P. C. F. and Bonn, D.
EPL 80 (2007)

Abstract : The strength of different wet granular materials is investigated as a function of the liquid volume fraction by measuring the elastic shear modulus, G'. We show that the optimum strength is achieved at a very low liquid volume fraction of 1-3\%. Surprisingly we find that the macroscopic strength of different wet granular materials depends with a power of 2/3 on the microscopic elastic modulus of the individual grains, with a power of -1/3 on the radius, and with a power of 1/3 on the surface tension. This can be explained by assuming that the attractive capillary force between two grains deforms the grains elastically, yielding a ``spring constant'' for further deformation. Averaging over many grain- grain orientations allows us to predict the macroscopic shear modulus in excellent agreement with our experiments. Copyright (C) EPLA, 2007
Contact line stability of ridges and drops - Mechkov, S. and Oshanin, G. and Rauscher, M. and Brinkmann, M. and Cazabat, A. M. and Dietrich, S.
EPL 80 (2007)

Abstract : Within the framework of a semi-microscopic interface displacement model in the small slope approximation we analyze the linear stability of sessile ridges and drops of a non-volatile liquid on a homogeneous, partially wet substrate, for both signs and arbitrary amplitudes of the three-phase contact line tension. Focusing on perturbations which correspond to deformations of the three-phase contact line, we find that drops are generally stable while ridges are subject only to the long-wavelength Rayleigh-Plateau instability leading to a breakup into droplets, in contrast to the predictions of capillary models which take line tension into account. We argue that the short-wavelength instabilities predicted within the framework of the latter macroscopic capillary theory occur outside its range of validity and thus are spurious. Copyright (c) EPLA, 2007.
Folding of flexible rods confined in 2D space - Boue, Laurent and Katzav, Eytan
EPL 80 (2007)

Abstract : We propose a statistical field theory to study the properties of elastic rods confined in 2D containers. Using a mean-field evaluation of the path integral, we show how a self-reorganization of the folding pattern between disordered and ordered configurations above a critical density leads to a more efficient packing. In addition, we predict the existence of a jamming transition for higher densities. The nature of this jamming transition is compared with similar observations in experiments on packing of flexible structures. The advantage of this approach is that it puts on an equal footing the geometrical features (such as self-avoidance) and the mechanical response to confinement. Copyright (C) EPLA, 2007.
Supersolidity and superfluidity - Balibar, Sebastien

Abstract : Is it possible that a solid flows like a liquid, or even without friction like a superfluid? At first sight, the crystalline order in real space looks contradictory with the coherent motion which is responsible for superfluidity. However, several authors considered this possibility in 1969 - 1970, and experiments done by Kim and Chan in 2004 suggest that solid 4 He is indeed a superfluid solid - a `supersolid' - below about 100 mK. This article summarizes my present understanding of this paradoxical issue.
Scaling laws of turbulent dynamos - Fauve, Stephan and Petrelis, Francois

Abstract : We consider magnetic fields generated by homogeneous isotropic and parity invariant turbulent flows. We show that simple scaling laws for the dynamo threshold, magnetic energy and Ohmic dissipation can be obtained depending on the value of the magnetic Prandtl number.
Errors and alternatives in prebiotic replication and catalysis - Ninio, Jacques

Abstract : The work on nonenzymatic nucleic acid replication performed by Leslie Orgel and co-workers over the last four decades, now extended by work on artificial selection of RNA aptamers and ribozymes, is generating some pessimism concerning the `naked gene' theories of the origin of life. It is suggested here that the low probability of finding RNA aptamers and ribozymes within pools of random sequences is not as disquieting as the poor gain in efficiency obtained with increases in information content. As acknowledged by Orget and many other authors, primitive RNA replication and catalysis must have occurred within already complex and dynamic environments. I, thus, propose to pay attention to a number of possibilities that bridge the gap between `naked gene' theories, on one side, and metabolic theories in which complex systems self-propagate by growth and fragmentation, on the other side. For instance, one can de-emphasize nucleotide-by-nucleotide replication leading to long informational polymers, and view instead long random polymers as storage devices, from which shorter oligomers are excised. Catalytic tasks would be mainly performed by complexes associating two or more oligomers belonging to the same or to different chemical families. It is proposed that the problems of stability, binding affinity, reactivity, and specificity could be easier to handle by heterogeneous complexes of short oligomers than by long, single-stranded polymers. Finally, I point out that replication errors in a primitive replication context should include incorporations of alternative nucleotides with interesting, chemically reactive groups. In this way, an RNA sequence could be at the same time an inert sequence when copied without error, and a ribozyme, when a chemically reactive nucleotide is inadvertently introduced during replication.
Single-molecule micromanipulation techniques - Neuman, K. C. and Lionnet, T. and Allemand, J.-F

Abstract : Single-molecule micrornanipulation techniques traditionally have been developed for biophysical applications, but they are being increasingly employed in materials science applications such as theology and polymer dynamics. Continuing developments and improvements in single-molecule manipulation techniques afford new opportunities in a broad range of fields. In this review we present an overview of current single-molecule manipulation techniques, with an emphasis on optical and magnetic tweezers, followed by a description of the elastic properties of single biopolymers. We then review die use of micrornanipulation techniques to locally probe material properties. To provide some insight into biophysical questions addressed by these techniques, we describe two applications, which further serve to illustrate the power and versatility of single-molecule micromanipulation techniques. We conclude with a brief discussion of emerging applications and techniques.