laboratoire de physique statistique
laboratoire de physique statistique



P A R M I :

Stokes instability in inhomogeneous membranes: Application to lipoprotein suction of cholesterol-enriched domains - Amar, M. Ben and Allain, J.-M. and Puff, N. and Angelova, M. I.

Abstract : We examine the time-dependent distortion of a nearly circular viscous domain in an infinite viscous sheet when suction occurs. Suction, the driving force of the instability, can occur everywhere in the two phases separated by an interface. The model assumes a two-dimensional Stokes flow; the selection of the wavelength at short times is determined by a variational procedure. Contrary to the viscous fingering instability, undulations of the boundary may be observed for enough pumping, whatever the sign of the viscosity contrast between the two fluids involved. We apply our model to the suction by lipoproteins of cholesterol-enriched domains in giant unilamellar vesicles. Comparison of the number of undulations given by the model and by the experiments gives reasonable values of physical quantities such as the viscosities of the domains.
The leading edge of evaporating droplets - Guena, G. and Poulard, C. and Cazabat, A. M.

Abstract : New experiments on drops evaporating in normal atmosphere from smooth substrates in the situation of complete wetting are reported and compared with the available theoretical model. They are the continuation of previous work with alkane or water sessile drops, which is first briefly summarized. The model accounts very well for the dynamics of the drop radius, but the predictions are only qualitative for the contact angle, especially for small angles. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drops dynamics. Then the main part of the paper concerns new experiments with polydimethylsiloxane oligomers. These silicone oils are similar to alkanes as far as evaporation rate is concerned, but have lower surface tensions, and therefore smaller dynamic contact angles. The purity of the oils appears to be critical for the experiments, and requires a preliminary investigation. Then a systematic study of the drops dynamics is presented, as a basis for forthcoming theoretical work. (c) 2006 Elsevier Inc. All rights reserved.
Gels and glasses in a single system: Evidence for an intricate free-energy landscape of glassy materials - Jabbari-Farouji, Sara and Wegdam, Gerard H. and Bonn, Daniel

Abstract : In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by ``hesitations'' between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated.
Two-photon uncaging with fluorescence reporting: Evaluation of the o-hydroxycinnamic platform - Gagey, Nathalie and Neveu, Pierre and Benbrahim, Chouaha and Goetz, Bernard and Aujard, Isabelle and Baudin, Jean-Bernard and Jullien, Ludovic

Abstract : This paper evaluates the o-hydroxycinnamic platform for designing efficient caging groups with fluorescence reporting upon one- and two-photon excitation. The model cinnamates are easily prepared in one step by coupling commercial or readily available synthons. They exhibit a large one-photon absorption that can be tuned in the near-UV range. Uncaging after one-photon excitation was investigated by H-1 NMR, UV-vis absorption, and steady-state fluorescence emission. In the whole investigated series, the caged substrate is quantitatively released upon photolysis. At the same time, uncaging releases a strongly fluorescent coproduct that can be used as a reporter for quantitative substrate delivery. The quantum yield of double bond photoisomerization leading to uncaging after one-photon absorption mostly lies in the 10\% range. Taking advantage of the favorable photophysical properties of the uncaging coproduct, we use a series of techniques based on fluorescence emission to measure the action uncaging cross sections with two-photon excitation of the present cinnamates. Exhibiting values in the 1-10 GM range at 750 nm, they satisfactorily compare with the most efficient caging groups reported to date. Noticeably, the uncaging behavior with two-photon excitation is retained in vivo as suggested by the results observed in living zebrafish embryos. Reliable structure property relationships were extracted from analysis of the present collected data. In particular, the careful kinetic analysis allows us to discuss the relevance of the o-hydroxycinnamic platform for diverse caging applications with one- and two-photon excitation.
Comment on ``Large melting-point hysteresis of ge nanocrystals embedded in SiO2'' - Caupin, Frederic
Complex dynamics of a unidirectional optical oscillator based on a liquid-crystal gain medium - Montina, A. and Bortolozzo, U. and Residori, S. and Huignard, J. P. and Arecchi, F. T.

Abstract : A unidirectional optical oscillator consists of an optical amplifier whose outgoing field is reinjected into the incoming one by means of a ring cavity. We used recently a liquid crystal light valve as optical amplifier and reported experimental evidence of cavity field oscillations. The light-valve provides a high gain and a wide transverse size, activating a large number of cavity modes, both transverse and longitudinal. A mean-field approximation along the cavity axis is not suitable for this system, thus we introduce a model which accounts for the activation of different transverse and longitudinal modes and shows numerically that their interaction generates three-dimensional spatiotemporal pulses localized along the cavity axis. The generation of these pulses is experimentally verified.
A novel type of optical oscillator based on a liquid crystal gain medium - Residori, Stefania and Bortolozzo, Umberto and Montina, Alberto and Arecchi, Fortunato Tito and Huignard, Jean-Pierre

Abstract : A nonlinear optical medium results by the collective orientation of liquid crystal molecules tightly coupled to a transparent photoconductive layer made of a thin photorefractive BSO crystal. For this new kind of liquid crystal light-valve, we have recently demonstrated signal beam amplification and self- pumped phase conjugation. After giving a description of the device and its properties, we show here that the liquid crystal light-valve, when inserted in a ring optical cavity, has a gain large enough to overcome the losses, thus resulting in unidirectional field oscillation. The wide transverse size and the high nonlinearity of the liquid crystal medium allow us to explore dynamical regimes where a huge number of longitudinal and transverse modes are simultaneously amplified in the cavity. Up to date, such regimes have remained inaccessible for all the known types of optical cavities. In particular, we report the observation of high amplitude spatiotemporal pulses appearing in random space points and confined along the three space directions. Then, we present a theoretical model that takes into account the longitudinal dependence of the field in the liquid crystal optical oscillator, and show that spatiotemporal pulses confined in 3D space directions arise from the random superposition of many longitudinal and transverse modes oscillating at different frequencies.
Doubts about quantal analysis - Ninio, Jacques
Freezing of helium-4: Comparison of different density functional approaches - Caupin, Frederic and Ancilotto, Francesco and Barranco, Manuel and Mayol, Ricardo and Pi, Marti

Abstract : Solidification of superfluid helium-4 has been addressed within the framework of density functional theory. Early studies used a variational approach, approximating the density distribution in the solid phase by a sum of Gaussians on each lattice site. Recently, we performed an unconstrained minimization of the functional describing the helium system as reported by Ancilotto et al. (Phys. Rev. B, 72, 214522, 2005). At sufficiently high density, we find stable solid like solutions, which exhibit an anisotropic density profile around each lattice site. We compare these results to the previous variational approach, and attempt to improve the family of trial functions by adding a variational parameter to account for anisotropy.
Supersolidity and superfluidity of grain boundaries - Sasaki, S. and Ishiguro, R. and Caupin, F. and Maris, H.-J. and Balibar, S.

Abstract : We have looked for dc mass transport through solid He-4 in a simple experiment with two communicating vessels filled with solid He-4 in equilibrium with liquid He-4. Through good quality crystals, we have observed no mass transport, in contradiction with the hypothesis of a Bose-Einstein condensation of vacancies. Through crystals containing grain boundaries, we have found superfluid flow along these grain boundaries. We discuss these results in the context of other experiments on supersolidity.
Homogeneous nucleation of He-4 crystals by acoustic waves - Ishiguro, R. and Caupin, F. and Balibar, S.

Abstract : We have recently found experimental evidence for the homogeneous nucleation of crystals in metastable liquid He-4 at high pressure. For this we combined the focusing of a high intensity spherical acoustic wave with a simple light scattering technique. We discuss the analysis used to distinguish between nucleation of bubbles in the negative pressure swings of the wave from nucleation of crystals in the positive swings. We also discuss the interest of our results and future developments of our experiment in the general context of the study of nucleation and instability limits in phase diagrams.
Optical solitons as quantum objects - Pomeau, Yves and Le Berre, Martine
CHAOS 17 (2007)

Abstract : The intensity of classical bright solitons propagating in linearly coupled identical fibers can be distributed either in a stable symmetric state at strong coupling or in a stable asymmetric state if the coupling is small enough. In the first case, if the initial state is not the equilibrium state, the intensity may switch periodically from fiber to fiber, while in the second case the asymmetrical state remains forever, with most of its energy in either fiber. The latter situation makes a state of propagation with two exactly reciprocal realizations. In the quantum case, such a situation does not exist as an eigenstate because of the quantum tunneling between the two fibers. Such a tunneling is a purely quantum phenomenon without counterpart in the classical theory. We estimate the rate of tunneling by quantizing a simplified dynamics derived from the original Lagrangian equations with test functions. This tunneling could be within reach of the experiments, particularly if the quantum coherence of the soliton can be maintained over a sufficient amount of time. (c) 2007 American Institute of Physics.
On the definition and modeling of incremental, cumulative, and continuous growth laws in morphoelasticity - Goriely, Alain and Ben Amar, Martine

Abstract : In the theory of elastic growth, a growth process is modeled by a sequence of growth itself followed by an elastic relaxation ensuring integrity and compatibility of the body. The description of this process is local in time and only corresponds to an incremental step in the total growth process. As time evolves, these incremental growth steps are compounded and a natural question is the description of the overall cumulative growth and whether a continuous description of this process is possible. These ideas are discussed and further studied in the case of incompressible shells.
Quasi-static rheology of foams. Part 2. Continuous shear flow - Kabla, Alexandre and Scheibert, Julien and Debregeas, Georges

Abstract : The evolution of a bidimensional foam submitted to continuous quasi-static shearing is investigated both experimentally and numerically. We extract, from the images of the sheared foam, the plastic flow profiles as well as the local statistical properties of the stress field. When the imposed strain becomes larger than the yield strain, the plastic events develop large spatial and temporal correlations, and the plastic flow becomes confined to a narrow shear band. This transition and the steady-state regime of flow are investigated by first focusing on the elastic deformation produced by an elementary plastic event. This allows us to understand (i) the appearance of long-lived spatial heterogeneities of the stress field, which we believe are at the origin of the shear-banding transition, and (ii) the statistics of the dynamic fluctuations of the stress field induced by plastic rearrangements in the steady-state regime. Movies are available with the online version of the paper.
Quasi-static rheology of foams. Part 1. Oscillating strain - Kabla, Alexandre and Debregeas, Georges

Abstract : A quasi-static simulation is used to study the mechanical response of a disordered bidimensional aqueous foam submitted to an oscillating shear strain. The application of shear progressively extends the elastic domain, i.e. the strain range within which no plastic process occurs. It is associated with the development of an irreversible normal stress difference, and a decrease in the shear modulus, which are both signatures of the appearance of anisotropy in the film network. Beyond this mechanical measurement, the evolution of the structural properties of the foam is investigated. We focus in particular on the energy E-0 defined as the minimum line-length energy under zero shear stress. For strain amplitude less than similar to 0.5, this quantity is found to decay with the number of applied cycles as a result of the curing of topological defects. However, for higher strain amplitude, plastic events appear to increase the structural disorder and tend to gather near the shearing walls. This process is a precursor of the shear-banding transition observed in fully developed flows, which will be studied in the companion paper. Movies are available with the online version of the paper.
Effect of selection on ancestry: An exactly soluble case and its phenomenological generalization - Brunet, E. and Derrida, B. and Mueller, A. H. and Munier, S.

Abstract : We consider a family of models describing the evolution under selection of a population whose dynamics can be related to the propagation of noisy traveling waves. For one particular model that we shall call the exponential model, the properties of the traveling wave front can be calculated exactly, as well as the statistics of the genealogy of the population. One striking result is that, for this particular model, the genealogical trees have the same statistics as the trees of replicas in the Parisi mean-field theory of spin glasses. We also find that in the exponential model, the coalescence times along these trees grow like the logarithm of the population size. A phenomenological picture of the propagation of wave fronts that we introduced in a previous work, as well as our numerical data, suggest that these statistics remain valid for a larger class of models, while the coalescence times grow like the cube of the logarithm of the population size.
Energetics and dynamics of SNAREpin folding across lipid bilayers - Feng Li and Pincet, Frederic and Perez, Eric and Eng, William S. and Melia, Thomas J. and Rothman, James E. and Tareste, David

Abstract : Membrane fusion occurs when SNAREpins fold up between lipid bilayers. How much energy is generated during SNAREpin folding and how this energy is coupled to the fusion of apposing membranes is unknown. We have used a surface forces apparatus to determine the energetics and dynamics of SNAREpin formation and characterize the different intermediate structures sampled by cognate SNAREs in the course of their assembly. The interaction energy-versus-distance profiles of assembling SNAREpins reveal that SNARE motifs begin to interact when the membranes are 8 nm apart. Even after very close approach of the bilayers (similar to 2-4 nm), the SNAREpins remain partly unstructured in their membrane-proximal region. The energy stabilizing a single SNAREpin in this configuration (35 k(B)T) corresponds closely with the energy needed to fuse outer but not inner leaflets (hemifusion) of pure lipid bilayers (40-50 k(B)T).
Statics and dynamics of adhesion between two soap bubbles - Besson, S. and Debregeas, G.

Abstract : An original set-up is used to study the adhesive properties of two hemispherical soap bubbles put into contact. The contact angle at the line connecting the three films is extracted by image analysis of the bubbles profiles. After the initial contact, the angle rapidly reaches a static value slightly larger than the standard 120 degrees angle expected from Plateau rule. This deviation is consistent with previous experimental and theoretical studies: it can be quantitatively predicted by taking into account the finite size of the Plateau border (the liquid volume trapped at the vertex) in the free energy minimization. The visco-elastic adhesion properties of the bubbles are further explored by measuring the deviation Delta theta(d)(t) of the contact angle from the static value as the distance between the two bubbles supports is sinusoidally modulated. It is found to linearly increase with Delta r(c)/r(c) , where r(c) is the radius of the central film and Delta r(c) the amplitude of modulation of this length induced by the displacement of the supports. The in-phase and out-of-phase components of Delta theta(d)(t) with the imposed modulation frequency are systematically probed, which reveals a transition from a viscous to an elastic response of the system with a crossover pulsation of the order 1 rad.s(-1). Independent interfacial rheological measurements, obtained from an oscillating bubble experiment, allow us to develop a model of dynamic adhesion which is confronted to our experimental results. The relevance of such adhesive dynamic properties to the rheology of foams is briefly discussed using a perturbative approach to the Princen 2D model of foams.
Membrane Recruitment of Scaffold Proteins Drives Specific Signaling - Pincet, Frederic
PLOS ONE 2 (2007)

Abstract : Cells must give the right response to each stimulus they receive. Scaffolding, a signaling process mediated by scaffold proteins, participates in the decoding of the cues by specifically directing signal transduction. The aim of this paper is to describe the molecular mechanisms of scaffolding, i.e. the principles by which scaffold proteins drive a specific response of the cell. Since similar scaffold proteins are found in many species, they evolved according to the purpose of each organism. This means they require adaptability. In the usual description of the mechanisms of scaffolding, scaffold proteins are considered as reactors where molecules involved in a cascade of reactions are simultaneously bound with the right orientation to meet and interact. This description is not realistic: (i) it is not verified by experiments and (ii) timing and orientation constraints make it complex which seems to contradict the required adaptability. A scaffold protein, Ste5, is used in the MAPK pathway of Saccharomyces Cerevisiae for the cell to provide a specific response to stimuli. The massive amount of data available for this pathway makes it ideal to investigate the actual mechanisms of scaffolding. Here, a complete treatment of the chemical reactions allows the computation of the distributions of all the proteins involved in the MAPK pathway when the cell receives various cues. These distributions are compared to several experimental results. It turns out that the molecular mechanisms of scaffolding are much simpler and more adaptable than previously thought in the reactor model. Scaffold proteins bind only one molecule at a time. Then, their membrane recruitment automatically drives specific, amplified and localized signal transductions. The mechanisms presented here, which explain how the membrane recruitment of a protein can produce a drastic change in the activity of cells, are generic and may be commonly used in many biological processes.
Coiling of elastic ropes - Habibi, M. and Ribe, N. M. and Bonn, Daniel

Abstract : A rope falling onto a solid surface typically forms a series of regular coils. Here, we study this phenomenon using laboratory experiments (with cotton threads and softened spaghetti) and an asymptotic ``slender-rope'' numerical model. The excellent agreement between the two with no adjustable parameters allows us to determine a complete phase diagram for elastic coiling comprising three basic regimes involving different force balances (elastic, gravitational, and inertial) together with resonant ``whirling string'' and ``whirling shaft'' eigenmodes in the inertial regime.