laboratoire de physique statistique
laboratoire de physique statistique



P A R M I :

``Antiferromagnetism'' in social relations and Bonabeau model - Weisbuch, Gerard and Stauffer, Dietrich

Abstract : We here present a fixed agents version of an original model of the emergence of hierarchies among social agents first introduced by Bonabeau et al. Having interactions occurring on a social network rather than among ``walkers'' does not drastically alter the dynamics. But it makes social structures more stable and give a clearer picture of the social organisation in a ``mixed'' regime, where finite ordered domains appear. (C) 2007 Elsevier B.V. All rights reserved.
Superfluid equation of state of dilute composite bosons - Leyronas, X. and Combescot, R.

Abstract : We present an exact theory of the BEC-BCS crossover in the Bose-Einstein-condensate (BEC) regime, which treats explicitly dimers as made of two fermions. We apply our framework, at zero temperature, to the calculation of the equation of state. We find that, when expanding the chemical potential in powers of the density n up to the Lee-Huang-Yang order, proportional to n(3/2), the result is identical to the one of elementary bosons in terms of the dimer-dimer scattering length a(M), the composite nature of the dimers appearing only in the next order term proportional to n(2).
Bose-Einstein condensation in semiconductors: The key role of dark excitons - Combescot, Monique and Betbeder-Matibet, Odile and Combescot, Roland

Abstract : Bose-Einstein condensation in semiconductors is controlled by the nonelementary-boson nature of excitons. Pauli exclusion between the fermionic components of composite excitons produces dramatic exchange couplings between bright and dark states. In microcavities, where bright excitons and photons form polaritons, they force the condensate to be linearly polarized, as observed. In bulk, they also force linear polarization, but of dark states, due to interband Coulomb scatterings. To evidence this dark condensate, indirect processes are thus needed.
Dynamics of non-newtonian droplets - Bartolo, Denis and Boudaoud, Arezki and Narcy, Gregoire and Bonn, Daniel

Abstract : We study the impact and subsequent retraction dynamics of aqueous liquid droplets upon high-speed impact on hydrophobic surfaces. Often a spectacular ``rebound'' of the droplet can be observed: after the impact and expansion, the drop retracts rapidly, leading to ejection of part of the material from the surface. We show how non-Newtonian flow properties can be used to slow down the retraction sufficiently to completely inhibit rebound. The slowing down is due to non-Newtonian normal stresses generated near the moving contact line of the droplet. We provide a quantitative theory for the slowing down, and show that the non- Newtonian effects profoundly change the contact line dynamics.
Effective exponents in the long-range critical wetting of alkanes on aqueous substrates - Weiss, Volker C. and Bertrand, Emanuel and Rafai, Salima and Indekeu, Joseph O. and Bonn, Daniel

Abstract : Alkanes on water show a two-stage wetting transition. Upon raising the temperature, a first-order transition from a molecularly thin to a mesoscopically thick liquid film is followed by a continuous divergence of the film thickness. This second transition is brought about by long-range interactions between adsorbate and substrate and is, therefore, referred to as long-range critical wetting. The divergence of the film thickness is theoretically expected to occur according to the asymptotic power law l similar to(T(w,c)-T)(beta)(s), with beta(s)=-1. This value has indeed been found for pentane on pure water; however, for hexane on salt solutions of different concentrations, beta(s)=-0.73 was found for a 1.5M solution of NaCl and beta(s)=-0.57 for a 2.5M salt solution. In addition, for hexane on a 2.5M solution of NaCl, an exponent of alpha(s)=0.1 was found from contact-angle measurements, differing greatly from the theoretically expected value of alpha(s)=-1. Using Dzyaloshinskii-Lifshitz-Pitaevskii theory, we calculate effective local exponents in order to explain the experimental findings. Taking into account the uncertainty of the exponents derived from experiments as well as the temperature range in which the measurements were carried out, a reasonable agreement between theory and experiment is found, thereby providing a consistent approach to resolving the apparently anomalous behavior of hexane on brine. The experimentally observed exponents beta(s)=-0.57 and alpha(s)=0.1 are also compatible with a long-range tricritical wetting transition, which is characterized by beta(s)=-1/2 and alpha(s)=0; this alternative explanation of the experimental findings is neither supported nor completely ruled out by our calculations.
Roughness of moving elastic lines: Crack and wetting fronts - Katzav, E. and Adda-Bedia, M. and Ben Amar, M. and Boudaoud, A.

Abstract : We investigate propagating fronts in disordered media that belong to the universality class of wetting contact lines and planar tensile crack fronts. We derive from first principles their nonlinear equations of motion, using the generalized Griffith criterion for crack fronts and three standard mobility laws for contact lines. Then we study their roughness using the self-consistent expansion. When neglecting the irreversibility of fracture and wetting processes, we find a possible dynamic rough phase with a roughness exponent of zeta=1/2 and a dynamic exponent of z=2. When including the irreversibility, we conclude that the front propagation can become history dependent, and thus we consider the value zeta=1/2 as a lower bound for the roughness exponent. Interestingly, for propagating contact line in wetting, where irreversibility is weaker than in fracture, the experimental results are close to 0.5, while for fracture the reported values of 0.55-0.65 are higher.
Off-diagonal long-range order, cycle probabilities, and condensate fraction in the ideal Bose gas - Chevallier, Maguelonne and Krauth, Werner

Abstract : We discuss the relationship between the cycle probabilities in the path-integral representation of the ideal Bose gas, off-diagonal long-range order, and Bose-Einstein condensation. Starting from the Landsberg recursion relation for the canonic partition function, we use elementary considerations to show that in a box of size L(3) the sum of the cycle probabilities of length k > L(2) equals the off-diagonal long-range order parameter in the thermodynamic limit. For arbitrary systems of ideal bosons, the integer derivative of the cycle probabilities is related to the probability of condensing k bosons. We use this relation to derive the precise form of the pi(k) in the thermodynamic limit. We also determine the function pi(k) for arbitrary systems. Furthermore, we use the cycle probabilities to compute the probability distribution of the maximum-length cycles both at T=0, where the ideal Bose gas reduces to the study of random permutations, and at finite temperature. We close with comments on the cycle probabilities in interacting Bose gases.
From production networks to geographical economics - Weisbuch, Gerard and Battiston, Stefano

Abstract : Although standard economics textbooks are seldom interested in production networks, modern economies are more and more based upon supplier/customer interactions. One can consider entire sectors of the economy as generalised supply chains. We will take this view in the present paper and study under which conditions local failures to produce or simply to deliver can result in avalanches of shortage and bankruptcies and in localisation of the economic activity. We will show that a large class of models exhibit scale free distributions of production and wealth among firms and that regions of high production are localised. (c) 2007 Elsevier B.V. All rights reserved.
Life and death of a fakir droplet: Impalement transitions on superhydrophobic surfaces - Moulinet, S. and Bartolo, D.

Abstract : We show that the equilibrium state of a water drop deposited on a superhydrophobic surface cannot be solely determined by its macroscopic contact angle but also depends on the drop size. Following the evolution of the interface of evaporating droplets, we demonstrate that the liquid can explore a succession of equilibrium conformations which are neither of the usual fakir nor Wenzel types. A comprehensive description of the transition between these wetting states is provided. To do so, we have taken advantage of microfabrication techniques and interference microscopy which allows for the ``3D'' imaging of the liquid interface. In addition, we propose a simple theoretical description of the interface geometry which goes beyond the standard two-state picture for superhydrophobicity. This model accounts correctly for all our experimental observations. Finally, guided by potential microfluidic applications we propose an efficient design strategy to build robust liquid repellant surfaces.
Lattice Boltzmann and analytical modeling of flow processes in anisotropic and heterogeneous stratified aquifers - Ginzburg, Irina and d'Humieres, Dominique

Abstract : We present analytical and Lattice Boltzmann (LB) solutions for steady-state saturated flows in 2D and 3D anisotropic heterogeneous aquifers. The analytical solution is easy to use and extends the known ones for ground-water whirls to more general combinations of the anisotropic properties of two-layered systems. The Bakker and Hernker's ``multi-layered'' semi-analytical solution and the LB results are compared to the analytical solution for a broad range of anisotropic heterogeneous diffusion tensors. The main components of the LB scheme, the eigenvalues of the linear collision operator and/or the equilibrium functions, become discontinuous when the anisotropy changes between the layers. It is shown that the evolution equation of the LB method needs to be modified at the interfaces in order to satisfy the continuity conditions for the diffusion function and/or its tangential derivatives. The existing LB schemes for anisotropic advection-dispersion equations are formulated in a more general framework in which the leading-order interface corrections are constructed and analyzed for linear and highly nonlinear exact solutions. We also present some stability aspects of these schemes, introduce specified normal gradient boundary conditions and discuss the computation of total and local fluxes. The interface analysis developed here applies to generic LB schemes with discontinuous collision operators. (C) 2007 Elsevier Ltd. All rights reserved.
Preparation and characterization of particles with small differences in polydispersity - Schoepe, H. J. and Marnette, O. and van Megen, W. and Bryant, G.
LANGMUIR 2311534-11539 (2007)

Abstract : Colloidal particles are widely used both in fundamental research and in materials science. One important parameter influencing the physical properties of colloidal materials is the particle size distribution (polydispersity) of the colloidal particles. Recent work on colloidal crystallization has demonstrated that even subtle changes in polydispersity can have significant effects. In this study we present centrifugation techniques for subtly manipulating the width and the shape of the particle size distribution, for polydispersities less than 10\%. We use scanning electron microscopy as well as dynamic and static light scattering to characterize the particle size distributions. We compare the results and highlight the difficulties associated with the determination of accurate particle size distributions.
Viscosity of a dense suspension in Couette flow - Huang, Nicolas and Bonn, Daniel

Abstract : We study the rheology of a granular paste, i.e. a dense suspension of non-Brownian particles, quantitatively at steady state, in a cylindrical Couette cell. Previous studies have shown a discrepancy between local and global measurements of the viscosity for these materials, making it impossible to predict their resistance to flow. Using both MRI investigation techniques and classical rheology studies, we show that agreement between local and global measurements can be obtained, provided the migration of particles inside the gap is taken into account. As found by Leighton \& Acrivos (J. Fluid Mech. vol. 181, 1987, p. 415), the migration leads to a particle density gradient in the flow, the highly sheared regions being less dense in particles. Here, by comparing the local viscosity and particle density measurements from MRI with the macroscopic relation between viscosity and the volume fraction, it is shown that global and local measurements agree with each other. This consequently allows us to define a viscosity for dense suspensions.
Wetting properties of grain boundaries in solid He-4 - Sasaki, Satoshi and Caupin, Frederic and Balibar, Sebastien

Abstract : We have observed boundaries between hcp He-4 crystal grains in equilibrium with liquid He-4. We have found that, when emerging at the liquid-solid interface, a grain boundary makes a groove whose dihedral angle 2 theta is nonzero. This measurement shows that grain boundaries are not completely wet by the liquid phase, in agreement with recent Monte Carlo simulations. Depending on the value of theta, the contact line of a grain boundary with a solid wall may be wet by the liquid. In this case, the line is a thin channel with a curved triangular cross section, whose measured width agrees with predictions from a simple model. We discuss these measurements in the context of grain boundary premelting and for a future understanding of the possible supersolidity of solid He-4.
Creation of intercellular bonds by anchoring protein ligands to membranes using the diphtheria toxin T domain - Perier, Aurelie and Gourier, Christine and Pichard, Sylvain and Husson, Julien and Lajeunesse, Evelyne and Babon, Aurelie and Menez, Andre and Gillet, Daniel
FEBS LETTERS 5815480-5484 (2007)

Abstract : We describe the creation of cell adhesion mediated by cell surface engineering. The Flt3-ligand was fused to a membrane anchor made of the diphtheria toxin translocation domain. The fusion protein was attached to the surface of a cell by an acid pulse. Contact with another cell expressing the receptor Flt3 lead to its activation. This activity involved direct cell-cell contact. A mean force of 20 nN was needed to separate functionalized cells after 5 min of contact. Overall, we showed that it is possible to promote specific cell-cell adhesion by attaching protein ligands at the surface of cells. (c) 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.
What can we learn from synaptic weight distributions? - Barbour, Boris and Brunel, Nicolas and Hakim, Vincent and Nadal, Jean-Pierre

Abstract : Much research effort into synaptic plasticity has been motivated by the idea that modifications of synaptic weights (or strengths or efficacies) underlie learning and memory. Here, we examine the possibility of exploiting the statistics of experimentally measured synaptic weights to deduce information about the learning process. Analysing distributions of synaptic weights requires a theoretical framework to interpret the experimental measurements, but the results can be unexpectedly powerful, yielding strong constraints on possible learning theories as well as information that is difficult to obtain by other means, such as the information storage capacity of a cell. We review the available experimental and theoretical techniques as well as important open issues.
Elastocapillary coalescence: Aggregation and fragmentation with a maximal size - Boudaoud, Arezki and Bico, Jose and Roman, Benoit

Abstract : Aggregation processes generally lead to broad distributions of sizes involving exponential tails. Here, experiments on the capillary-driven coalescence of regularly spaced flexible structures yields a self-similar distribution of sizes with no tail. At a given step, the physical process imposes a maximal size for the aggregates, which appears as the relevant scale for the distribution. A simple toy model involving the aggregation of nearest neighbors exhibits the same statistics. A mean-field theory accounting for a maximal size is in agreement with both experiments and numerics. This approach is extended to iterative fragmentation processes where the largest object is broken at each step.
Fluctuations of the number of particles within a given volume in cold quantum gases - Astrakharchik, G. E. and Combescot, R. and Pitaevskii, L. P.

Abstract : In ultracold gases many experiments use atom imaging as a basic observable. The resulting image is averaged over a number of realizations and mostly only this average is used. Only recently the noise has been measured to extract physical information. In the present paper we investigate the quantum noise arising in these gases at zero temperature. We restrict ourselves to the homogeneous situation and study the fluctuations in particle number found within a given volume in the gas, and more specifically inside a sphere of radius R. We show that zero-temperature fluctuations are not extensive and the leading term scales with sphere radius R as R-2 ln R (or ln R) in three- (or one-) dimensional systems. We calculate systematically the next term beyond this leading order. We consider first the generic case of a compressible superfluid. Then we investigate the whole Bose-Einstein-condensation (BEC) -BCS crossover, and in particular the limiting cases of the weakly interacting Bose gas and of the free Fermi gas.
Repulsive and attractive critical Casimir forces - Rafai, Salima and Bonn, Daniel and Meunier, Jacques

Abstract : When confining vacuum fluctuations between two identical walls, the Casimir force manifests itself as a mutual attraction of the walls. When confining concentration fluctuations of a binary liquid mixture, an analogous force should exist near the critical temperature T-C; it is called the critical Casimir force. Here we show experimentally that this purely entropic force can be either attractive or repulsive, depending on the boundary conditions for the fluctuations. For symmetrical boundary conditions an attractive force is found while asymmetrical ones lead to a repulsive force. This is observed directly by confining the fluctuations in a thin (approximate to 100 angstrom) wetting film. Depending on the boundary conditions either a thinning or a thickening of the film is observed when T -> Tc. (c) 2007 Elsevier B.V. All rights reserved.
Deriving structure from evolution: metazoan segmentation - Francois, Paul and Hakim, Vincent and Siggia, Eric D.

Abstract : Segmentation is a common feature of disparate clades of metazoans, and its evolution is a central problem of evolutionary developmental biology. We evolved in silico regulatory networks by a mutation/selection process that just rewards the number of segment boundaries. For segmentation controlled by a static gradient, as in long-germ band insects, a cascade of adjacent repressors reminiscent of gap genes evolves. For sequential segmentation controlled by a moving gradient, similar to vertebrate somitogenesis, we invariably observe a very constrained evolutionary path or funnel. The evolved state is a cell autonomous `clock and wavefront' model, with the new attribute of a separate bistable system driven by an autonomous clock. Early stages in the evolution of both modes of segmentation are functionally similar, and simulations suggest a possible path for their interconversion. Our computation illustrates how complex traits can evolve by the incremental addition of new functions on top of pre-existing traits.
Confinement free energy of surfaces bearing end-grafted, polymers in the mushroom regime and local measurement of the polymer density - Li, Feng and Pincet, Frederic
LANGMUIR 2312541-12548 (2007)

Abstract : End-tethered polymer chains usually adopt mushroomlike structures on the surface when their density is low. The behaviors of these surface-attached hemicoils are described by existing polymer theory. Dolan and Edwards derived the free energy of a single polymer chain confined between two planar surfaces. Their theory was used to approximate the steric interaction free energy, E, of two identical surfaces bearing polymers in the mushroom regime and to compare with experimental data obtained from surface force measurements. However, because of a mislabeled plot in the original paper, experimental force profiles did not seem to fit the free energy approximation satisfactorily. We have correctly relabeled the involved plot and derived a new simple expression for E. In order to verify this expression, we have performed experiments on PEG(45) polymers incorporated in lipid bilayers using a surface force apparatus. The measured force profiles are in perfect agreement with the prediction. We show that such measurements can be used to determine the local density of grafted polymer with good precision.