Showing papers in "Journal De Physique in 1988"

The role of topological constraints in the kinetics of collapse of macromolecules

Abstract: It is shown that the kinetics of collapse of a polymer coil consisting of N segments after an abrupt decrease of temperature is a two-stage process if N >> Ne. The first stage takes a time ∼ N2 and leads to the peculiar state — crumpled, or fractal, globule. Any part of a chain of any scale is itself a globule in this state ; these parts are segregated from each other in space due to the non-phantomness of a chain. The chain fold in the crumpled globule is a fractal line with fractal dimension 3, equal to the space dimension. The second stage is a chain knotting ; it is realized by means of reptation-like mechanism of motion, takes a time ∼ N3 and is accompanied by an increase of globule density.

Gelation of aqueous gelatin solutions. I. Structural investigation

Abstract: We have investigated the structural modifications of aqueous gelatin solutions for various thermal treatments and different concentrations. The structure is explored at different microscopic levels, putting into evidence the conformational coil ~ helix transition of the proteic chains (by optical rotation), the supramolecular structure of the network (by electron microscopy) and the role of solvent (by proton nuclear magnetic resonance). The non-equilibrium nature of the gel phase is demonstrated as well as its disordered character. A phenomenological analysis of the kinetics of helix formation is proposed, followed by an attempt for a microscopic modelling of the mechanisms, based on the data presently known.

Theoretical study of the coherent backscattering of light by disordered media

Abstract: A theoretical study of the coherent backscattering effect of light from disordered semi-inifinite media is presented for various situations including time-dependent effects as well as absorption and amplitude modulation. Particular attention is devoted to the case of anisotropic scattering and to polarization in order to explain quantitatively experimental results. A microscopic derivation of the coherent albedo is given which strongly supports the heuristic formula previously established. In addition the coherent albedo of a fractal system is predicted. The validity of the different approximations used are discussed and some further theoretical developments are presented On inclut les effets dependant du temps, les milieux absorbants et les effets lies a la modulation d'amplitude de la lumiere

Localized structures generated by subcritical instabilities

Abstract: We report the existence of structurally stable pulse-like solutions in the vicinity of an inverted Hopf bifurcation. These localized structures correspond to droplets in first order phase transitions, where they are known to be unstable. We show that the stabilisation mechanism is a non variational effect, i.e. is due to the non existence of a «free-energy» to minimize in the instability problem we consider. We propose this mechanism as an explanation for the existence of localized waves in shear flows or in convection experiments in binary fluid mixtures Etude de l'existence de solutions stables en forme d'impulsion au voisinage d'une bifurcation de Hopf inverse. On montre que le mecanisme de stabilisation est un effet non variationnel. On propose ce mecanisme pour expliquer l'existence de paquets d'ondes localises dans certains ecoulements paralleles ou dans des experiences de convection dans les fluides binaires

Dynamics of living polymers and flexible surfactant micelles : scaling laws for dilution

Abstract: On etend au regime semidilue un modele theorique introduit recemment pour decrire la dynamique de rupture et de recombinaison de chaines polymeriques

Gelation of aqueous gelatin solutions. II. Rheology of the sol-gel transition

Abstract: We have studied the rheological behaviour of aqueous gelatin solutions during the gelation process at several temperatures. The linear viscoelastic regim was investigated by dynamical measurements in the frequency range 0.01 Hz to 1.5 Hz, using a Weissenberg rheogoniometer. The relation between the shear moduli and the helix amount at several temperatures puts into evidence that this structural parameter pilots the gelation process. Analysing the rheological data, we propose a method of deriving the gel point, which could be used for other systems. Finally we show that gelatin gelation can be interpreted within the framework proposed by de Gennes and Stauffer of the percolation theory. The exponents for the static viscosity and the relaxed shear modulus are in agreement with the 3d predictions.

A synchrotron X-ray study of competing undulation and electrostatic interlayer interactions in fluid multimembrane lyotropic phases

Abstract: Recent X-ray work [1] in a quaternary lyotropic lamellar phase demonstrated that entropically induced undulation forces dominated the free energy of interaction between fluid membranes swollen by dodecane. Here we report on a comprehensive X-ray study of competing electrostatic and undulation forces in two multimembrane systems in the lamellar Lα phase as a function of the intermembrane distance. In each case, the negatively charged membrane is composed of a mixture of Sodium Dodecyl Sulfate (SDS) and pentanol, while the solvent separating the membranes is either pure water or brine (~ 0.5 mole. 1-1 of NaCl). A specialized high resolution X-ray spectrometer enables us to measure, as a function of the intermembrane distance (d ), the exponent η (d ) which characterizes the algebraic decay of layer correlations. In turn, η (d ) is directly related to the intermembrane interactions. The results clearly show that when diluting with pure water the interactions are dominated by long range electrostatic forces. In the brine dilution system, the addition of free ions (NaCl) to the solvent yields a small Debye length (λD/d<

Optical coherent backscattering by random media : an experimental study

Abstract: A detailed experimental study of coherent backscattering of light from aqueous suspensions of polystyrene microspheres is presented. Emphasis is on the effects of particle size, of absorption due to added dye and of light polarization on the shape and height of the backscattering cone. For parallel polarization of incident and scattered beams, the scalar diffusion theory, parametrized by the transport mean free path l*, agrees well with our data up to surprizingly large scattering angles (ql * ~ 1) and quantitatively accounts for the rounding of the cones due to absorption. No deviations from the usual Gaussian statistics of scattered fields is observed up to λ/l* ∼ 0.1.

On electrically driven pattern-forming instabilities in planar nematics

Abstract: The theoretical description of the threshold and near-threshold behavior of electrohydrodynamic convection in nematic liquid crystals in the (low-frequency) conduction regime is reconsidered. We present essentially the full three-dimensional linear stability analysis of the basic state and a major part of the weakly-dependence are treated rigorously but the flexoelectric effect is neglected. Related transitions like the periodic splay-twist instability, which is relevant in polymer materials with positive dielectric anisotropy, are also considered. We give criteria for finding the oblique-roll state and for the competition between different instabilities. Comparison with experiments is made wherever possible. Good qualitative and sometimes quantitative agreement is found Analyse lineaire tridimensionnelle complete de l'etat de base et theorie faiblement non lineaire de l'etat convectif pour decrire le comportement au seuil, et pres du seuil, de la convection electrohydrodynamique de cristaux liquides nematiques dans le regime de conduction (basse frequence). Traitement rigoureux des conditions aux limites aux plaques superieure et inferieure et de la variation temporelle. Extension de l'analyse a diverses transitions, comme l'instabilite de «splay-twist» periodique observee dans des polymeres a anisotropie dielectrique positive. Criteres pour l'observation de l'etat des rouleaux obliques et la competition entre differentes instabilites. Comparaison aux donnees experimentales

Shape transformations of the aggregates in dilute surfactant solutions : a small-angle neutron scattering study

Abstract: The brine rich corner of the phase diagrams of quasitemary (surfactant/alcohol/brine) systems shows the domains of stability of three different phases : the classical micellar phase L1, the highly swollen lamellar phase L α and an isotropic phase with quite special physical properties herein denoted L2*. The successive phase transformations are triggered by increasing values of the alcohol/surfactant ratio. We here report an extensive small angle neutron scattering investigation of the morphological transformations of the aggregates all along the phase sequence. The following shape sequence is unambiguously evidenced : globules, rods and finally wide bilayers. The primary structures have the same morphology at a local scale (bilayers) in both the Lα and L2* phases. Only the large scale arrangement of this elementary object changes from L α to L2*. The very low q scattering patterns of L 2* suggest that the structure of this phase has a bicontinuous topology where the infinite bilayer separates two infinite interwoven equivalent self-connected brine domains.

A domain theory for linear and nonlinear aging effects in spin glasses

Abstract: We present a theory of aging in spin glasses subjected to a time-dependent temperature and external magnetic field. An arbitrary nonequilibrium spin glass state is imagined to be decomposable into a collection of «(T 1 ,H 1 )-domains» (for any pair (T 1 ,H 1 )) through a comparison of this state to an equilibrium state at a temperature T 1 and in a field H 1 . The theory postulates a time evolution for the domains (comprising both growth and breakup), as well as an equation for the magnetic relaxation within a domain. Of crucial importance is the interplay between two characteristics lengths: i) the time-dependent linear size of a domain, and ii) an overlap length l (ΔT,ΔH); the latter indicates up until which length scale two thermodynamic equilibrium states differing by ΔT and ΔH are indistinguishable. We show that the theory explains a variety of experimental aging effects as have been observed in particular by Refregier et al. and by Lundgren et al. Theorie du vieillissement dans les verres de spin soumis a une temperature et un champ magnetique externe dependant du temps, basee sur l'hypothese qu'un etat de verre de spin arbitraire hors equilibre peut etre decompose en une collection de «domaines (T 1 ,H 1 )» par comparaison a un etat d'equilibre a une temperature T 1 dans un champ H 1 . Equations d'evolution decrivant la croissance et la fracturation des domaines, equation de relaxation magnetique a l'interieur d'un domaine. Role crucial de l'interaction de deux longueurs caracteristiques definissant la dimension lineaire d'un domaine et l'echelle d'indiscernabilite de deux etats d'equilibre thermodynamique differant par ΔT et ΔH. Interpretation satisfaisante d'une serie de resultats experimentaux

Phase diagram of polyelectrolyte solutions

Abstract: Diagramme de phase de polystyrenesulfonate en fonction du degre de polymerisation et de la concentration

Bending moduli of polymeric surfactant interfaces

Abstract: Extension de la theorie des «brosses» polymeriques greffees en bout de chaine a des polymeres attaches sur des interfaces courbees

The calculated electronic and magnetic structures of Fe4N and Mn 4N

Abstract: Using the augmented spherical wave method (A. S. W.) we calculate the electronic and magnetic structures of Fe 4 N and Mn 4 N. Both nitrides are found to order magnetically exhibiting ferro- and ferrimagnetic spin alignment respectively. The magnetic moments found are in good agreement with neutron data. Both systems can be described via covalent magnetism. Comparison with Goodenough's phenomenological model as well as with Heusler alloys is given Calcul des structures electroniques et magnetiques en utilisant la methode de «l'onde spherique augmentee». Les deux composes s'ordonnent magnetiquement avec un alignement ferro- et ferrimagnetique. Les valeurs des moments magnetiques sont en bon accord avec celles de la litterature. Comparaison avec le modele phenomenologique de Goodenough et avec les alliages de Heusler

Periodic systems of frustrated fluid films and « micellar » cubic structures in liquid crystals

Abstract: Etude des configurations ordonnees possible permettant de resoudre la frustration d'un systeme periodique de films fluides frustres, tel que ceux construits par des molecules amphiphiles en presence d'eau

Domains of attraction in neural networks

Abstract: On calcule le domaine d'attraction du point fixe d'une memoire a reseau de neurones en fonction des champs magnetiques locaux. Ces resultats, combines a des algorithmes d'apprentissage standard rendent possible la construction de memoires associatives saturees

Rod like micellar structures in perfluorinated ionomer solutions

Abstract: Une structure en bâtonnets arranges sur un reseau hexagonal est proposee dans des solutions et gels d'ionomeres perfluores. Deux differentes approches ont permis l'obtention du diametre des bâtonnets

Flattening of grooves on a crystal surface: a method of investigation of surface roughness

Abstract: The decay of a profile artificially produced on a crystal surface below its roughening transition is studied by means of a qualitative theory. In the case of a bidirectional modulation the height h(t) is a linear function of time t and the lifetime τ is proportional to λ 3 h(0), where λ is the wavelength. As suggested by Bonzel et al. the formation of facets is a sensitive information to determine whether a surface is below its roughening temperature. Near the flat part the profile has a (x-x 0 ) 3/2 singularity as in a crystal at equilibrium. The case of a unidirectionnel modulation is still open although some heuristic give a qualitative agreement with experimental results Etude theorique et qualitative de l'evolution de sillons creuses a la surface d'un monocristal en dessous de sa temperature de transition rugueuse

Neutron resonance spin echo, bootstrap method for increasing the effective magnetic field

Abstract: We show that it is possible to use a bootstrap technique to increase the precession angle, and hence the instrument resolution for a given maximum magnetic field intensity in Neutron Resonance Spin Echo (NRSE) spectrometry. The limits, theoretical as well as technical, to this effective field amplification are discussed Il est possible d'utiliser la technique du bootstrap pour accroitre l'angle de precision et ainsi la resolution pour un champ magnetique maximal donne en spectrometrie d'echos de spin de neutrons polarises. Discussion des limites theoriques et techniques a l'amplification du champ effectif

Elastic and plastic deformations of charge density waves

Abstract: An elastic theory of CDW deformations under electric fields is presented, on the basis of a simple phase Hamiltonian. The emphasis is put on the internal rigidity of the modulation which indeed resembles an «electronic solid», and on the anisotropy of its elastic constants. CDW's continous deformations are described by a strain tensor given by the relative variations of the wavevector of the modulation. Stresses occur due to the combined effect of external electric field and pinning points. Under low stresses the globally pinned condensate retains phase continuity: the structure has no defects. Under larger stresses dislocation loops are nucleated involving local amplitude variations of the CDW. We describe the nucleation process for elementary loops under an external field and compare as a consequence thresholds for impurity depinning and boundary depinning (surfaces or interfaces). A characteristic sample length is found under which contact pinning prevails and a coherent deformation (polarization) is built Theorie de la deformation sous champ electrique utilisant un simple hamiltonien de phase, basee sur la rigidite interne de la modulation et sur l'anisotropie des constantes elastiques de l'ODC. Sous faibles contraintes le condensat, globalement ancre, conserve la continuite de phase et la structure est sans defauts; sous de plus fortes contraintes on observe la nucleation de boucles de dislocation. Analyse du processus de nucleation des boucles elementaires et comparaison des seuils de desancrage des impuretes et des bords; longueur caracteristique au-dessous de laquelle l'ancrage aux contacts domine et une deformation coherente (polarisation) apparait

A 6‐D Structural Model for the Icosahedral (Al,Si)‐Mn Quasicrystal

Abstract: A 6-dimensional (6-D) periodic model is proposed for the Al-Mn-Si icosahedral quasiperiodic crystal. The model results from an embedding of the periodic cubic α structure in 6-D. In the Janner-Janssen-Bak description, it consists of three concentric spherical shells of respectively Mn, Al and Al aligned in perpendicular space around the lattice nodes and two additional shells of Al around the body centers. This model is shown to match the X-ray powder diffraction data with a satisfactory residual R-factor of 0.128 Ce modele est construit a partir de la representation a 6 dimensions de la phase cristalline cubique approximante α. Dans le formalisme de Janner-Janssen-Bak, il consiste en trois couronnes spheriques concentriques, l'une de manganese et les deux autres d'aluminium centrees autour des nœuds du reseau hypercubique a 6 dimensions, et deux couronnes additionnelles d'aluminium centrees au milieu des diagonales principales de l'hypercube. Le modele verifie les donnees des diagrammes de diffraction RX

Collapse of grafted chains in poor solvents

Abstract: 2014 A mean field theory of the collapse of grafted chains (consisting of N monomers) in a poor solvent is presented. The collapse behaviour of nonoverlapping grafted chains is identical to that of free coils but with no phase separation. This « strong » collapse, in which the coil radius R decreases continuously from R ~ N3/5 in a good solvent to R ~ N1/3 in a poor solvent, is replaced by « weak » collapse for densely grafted layers. Such a layer when undergoing « weak » collapse becomes thinner, yet the chains remain stretched even in poor solvents, and the layer thickness is linear in N past the collapse. For low densities the « weak » collapse is associated with a first order phase transition. An increase in grafting density takes the system through a critical point into a gradual collapse regime. J. Phys. France 49 (1988) 547-550 MARS 1988, Classification Physics Abstracts 36.20E 61.25H 68.35R 86.25D

Statistics of a polymer in a random potential, with implications for a nonlinear interfacial growth model

Abstract: We examine and extend recent results for the statistics of a gaussian polymer chain of length t in a quenched random potential μ (r). This problem can be mapped onto one involving the nonlinear evolution equation h (r, t) = ∇2h + (∇h)2 - μ(r) for a growing interface in the presence of a time-independent, but spatially varying, random flux - μ (r). [The corresponding problem for a directed polymer, equivalent to a flux random in both time and space, was studied by M. Kardar, G. Parisi and Y.-C. Zhang, Phys. Rev. Lett. 56 (1986) 889.] A free chain in a true white-noise potential, in space dimensions d = 2, 3, is predicted to collapse to a linear size R ˜ v1 /(d - 4)[ln V] 1/(d - 4) where v is the mean-square potential fluctuation (presumed small) and V the volume of the system. This agrees with a recent replica calculation of S. F. Edwards and M. Muthukumar (J. Chem. Phys., 89 (1988) 2435), but with an extra logarithmic dependence on V. The difference between this result and that for an annealed potential is commented upon, with reference to the phenomenon of localization as it applies to polymers in a quenched environment ; the effect of a saturating potential is discussed. In contrast, a chain with one end fixed at a particular place (say at r) is typically arranged as a tadpole consisting of a collapsed head region (occupying a deep minimum of μ) connected to r by an extended tail. The free energy surface of such a chain F (r, t ) consists asymptotically of a series of conical valleys, separated by sharp ridges. Several results for the statistics of this surface are obtained. The evolved profile in the equivalent interfacial growth model is obtained from h (r, t ) = - F (r, t ), and hence corresponds to a series of conical mountains.

Theory of spin-anisotropic electron-electron interactions in quasi-one-dimensional metals

Abstract: We construct a theoretical model for one-dimensional conductors taking into account the effects of spin-dependent interactions due to spin-orbit or electronic dipole-dipole couplings. We show that a new type of interaction which does not conserve the z-component of the spin is generated, and that in the presence of time-reversal and inversion symmetry four different coupling constants are necessary to describe the most general case. The spin-dependence of the interactions is computed for spin-orbit and dipolar coupling. The use of bosonization and a renormalization group calculation allow us to obtain the correlation functions and the zero-temperature phase diagram of a strictly one-dimensional system. There is a gap in the spin excitations in the whole phase diagram which leads to fully anisotropic spin-density wave phases. The behaviour under a magnetic field is studied and we find a spin-flop transition. In order to describe quasi-one-dimensional systems we introduce an effective interchain coupling and treat it in the mean-field approximation. The phase diagram is qualitatively different in the case Construction d'un modele qui prend en compte les interactions dues au couplage spin-orbite et les interactions dipole-dipole entre electrons, ce qui induit un nouveau type d'interaction qui ne conserve pas la composante z du spin et qui necessite quatre constantes differentes dans le cas le plus general s'il existe une symetrie par renversement du temps et une symetrie d'espace. Emploi de la bosonisation et d'un calcul de renormalisation pour obtenir les fonctions de correlation et le diagramme de phase du systeme a temperature nulle. Observation de phases ondes de densite de spin totalement anisotropes, d'une transition de type spin-flop sous champ magnetique. Introduction d'un couplage interchaines, traite a l'approximation du champ moyen. Obtention de diagrammes de phases tres differents suivant que les anisotropies sont faibles ou fortes

Hedgehog structure in nematic and magnetic systems

Abstract: The core structure of hedgehog point disclinations in magnetic and nematic liquid crystal systems is investigated within Landau theory. Exact spherically symmetric solutions are found, with isotropic cores. In the nematic hedgehog the solution is uniaxial everywhere. Nematic hedgehogs are very much larger than magnetic hedgehogs when measured with respect to the order parameter healing length Etude de la structure des points singuliers «herissons» dans les systemes nematiques et magnetiques dans le cadre d'une theorie de Landau et mise en evidence de solutions exactes pour la symetrie spherique et des cœurs isotropes: solution partout uniaxiale pour les herissons nematiques, beaucoup plus grands que les herissons magnetiques, avec des longueurs mesurees par rapport a la longueur de relaxation du parametre d'ordre

The Euclidean matching problem

Abstract: We study the matching problem in finite dimensions. The Euclidean correlations of the distances can be taken into account in a systematic way. With respect to the case of independent random distances which we have studied before, the adjonction of Euclidean triangular correlations gives rise to corrections which vanish when the dimension of space goes to infinity, and remain relatively small in any dimensions Nous etudions le probleme du couplage («matching») en dimension finie. Les correlations euclidiennes entre les distances peuvent etre prises en compte de maniere systematique. Par rapport au cas des distances aleatoires independantes que nous avions etudiees precedemment, les correlations triangulaires euclidiennes engendrent des corrections qui s'annulent dans la limite ou la dimension de l'espace tend vers l'infini, et restent relativement petites a toute dimension

Solutions of polydiacetylenes in good and poor solvents : a light and neutron scattering study

Abstract: The properties of the P-3BCMU and P-4BCMU solutions in good and poor solvents have been investigated, by use of light and neutron scattering techniques. In good solvents, P-3BCMU and P-4BCMU dissolve with a yellow color and form semi-rigid coils in a trans isomeric state. Their internal structure is better described by a continuous model of a wormlike ribbon than by a model of discrete defects separating misoriented rigid platelets of variable lengths. At room temperature and for both polydiacetylenes, the longitudinal persistence length is found to be close to 160 A and the mean square radial radius of gyration, which is related to the side extension of the macromolecules, is of the order of 60 A2. The conjugated chain stiffness does not seem to depend on the presence or absence of Hydrogen bonds between neighboring side groups, as long as a small enough proportion of such bonds are formed. In poor solvents, specifically for red solutions of P-4BCMU in toluene, the chains form complex aggregated structures of large molecular weight. No evidence for red solutions of isolated rod molecule has been found, even for concentrations as low as 0.5 x 10-3 g 1-1 with high molecular weight P-4BCMU (Mw =1.67 x 106 g mole-1) or 0.5 gl-1 with low molecular weight P-4BCMU (Mw ~ 4.32 x 104 g mole-1). This strongly suggests that all experimental

Pair wave functions for strongly correlated fermions and their determinantal representation

Abstract: We show that a (Jastrow projected or not) BCS wave function with any fixed number of fermions can be represented as a determinant. This is important for their numerical investigation, with applications to liquid He 3 and Hubbard's model. We also propose a new type of wave function, describing a mixed state composed of paired and unpaired fermions. The physical properties of such a state are tentatively discussed On montre que l'amplitude de toute fonction BCS (de Jastrow projetee ou non) a nombre de particules fixe est un determinant, ce qui est important pour leur etude numerique et qui permet une approche variationnelle de ces fonctions, appliquee a 3 He liquide et au modele de Hubbard. Proposition d'un nouveau type de fonction d'onde decrivant un etat mixte compose de fermions apparies et celibataires. Discussion des proprietes physiques de cet etat

Associating polymers and shear thickening

Abstract: The shear-thickening property observed in ionomer solutions is discussed. Scattering and dynamic modulus data suggest that the ionic groups cluster into multiplets and that these control the relaxation of stress. A simple mechanism for shear thickening is described ; it leads to the expectation that these clusters must be bound with a certain minimal strength. Associations with the strength required are argued to pose serious problems for solubility. These problems worsen with increasing number of ions in a multiplet. The competing interactions which determine this number are described.