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  • 1
    ISSN: 1292-895X
    Keywords: PACS. 64.60.Cn Order-disorder transformations; statistical mechanics of model systems - 64.70.Ja Liquid-liquid transitions - 68.45.Gd Wetting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We have performed small angle neutron scattering experiments on the binary fluid mixture n-C6H14+n-C8F18 imbibed inside porous Vycor glass in the thermodynamic region corresponding to the bulk critical one. The resulting structure can be represented as the sum of a temperature dependent Lorentzian term and a term describing the interference between the porous matrix, a shell part richer in one component coating the glass surface, and a core part richer in the other component. We observe critical fluctuations extending over distances markedly larger than the mean pore size.
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  • 2
    ISSN: 1292-895X
    Keywords: PACS. 05.90.+m Other topics in statistical physics, thermodynamics, and nonlinear dynamical systems - 68.10.-m Fluid surfaces and fluid-fluid interfaces - 82.65.Dp Thermodynamics of surfaces and interfaces
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We consider the adsorption of an isolated, Gaussian, random, and quenched copolymer chain at an interface. We first propose a simple analytical method to obtain the adsorption/depletion transition, by averaging over the disorder the partition function instead of the free energy. The adsorption thresholds obtained by previous authors at a solid/liquid and at a liquid/liquid interface for multicopolymer chains can be rederived using this method. We also compare the adsorption thresholds obtained for bimodal and for Gaussian disorder; they only agree for small disorder. We focus on the specific case of an ideally flat asymmetric liquid/liquid interface, and consider the situation where the chain is composed of monomers of two different chemical species A and B. The replica method is developed for this case. We show that the Hartree approximation, coupled to a replica symmetry assumption, leads to the same adsorption thresholds as obtained from our general method. In order to describe the properties of the adsorbed (or depleted) chain, we develop a new approximation for long chains, within the framework of the replica theory. In most cases, the behavior of a random copolymer chain can be mapped onto that of a homopolymer chain at an asymmetric attractive interface. The values of the effective adsorption energy are different for a random and a periodic copolymer chain. Finally, we consider the case of uncorrelated annealed disorder. The behavior of an annealed chain can be mapped onto that of a homopolymer chain at an asymmetric non attractive interface; hence, an annealed chain cannot adsorb at an asymmetric interface.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (2000), S. 55-59 
    ISSN: 1292-895X
    Keywords: PACS. 81.05.Rm Porous materials; granular materials - 83.70.Fn Granular solids - 46.40.-f Vibrations and mechanical waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: In horizontally shaken granular material different types of pattern formation have been reported. We want to deal with the convection instability which has been observed in experiments and which recently has been investigated numerically. Using two dimensional molecular dynamics we show that the convection pattern depends crucially on the inelastic properties of the material. The concept of restitution coefficient provides arguments for the change of the behaviour with varying inelasticity.
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  • 4
    ISSN: 1292-895X
    Keywords: PACS. 36.20.Ey Conformation (statistics and dynamics) - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 82.70.Dd Colloids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We present an analytical self-consistent-field (SCF) theory for a neutral polymer brush (a layer of long polymer chains end-grafted to a surface) with annealed excluded volume interactions between the monomer units. This model mimics the reversible adsorption of solute molecules or aggregates, such as small globular proteins or surfactant micelles, on the grafted chains. The equilibrium structural properties of the brush (the brush thickness, the monomer density profile, the distribution of the end segments of the grafted chains) as well as the overall adsorbed amount and the adsorbate density profile are analyzed as a function of the grafting density, the excluded volume parameters and the chemical potential (the concentration) of the adsorbate in the solution. We demonstrate that, when the grafting density is varied, the overall adsorbed amount always exhibits a maximum, whereas the root-mean-square brush thickness either increases monotonically or passes through a (local) minimum. At high grafting densities the chains are loaded by adsorbed aggregates preferentially in the distal region of the brush, whereas in the region proximal to the grafting surface depletion of aggregates occurs and the polymer brush retains an unperturbed structure. Depending on the relative strength of the excluded volume interactions between unloaded and loaded monomers both the degree of loading of the chains and the polymer density profile are either continuous or they exhibit a discontinuity as a function of the distance from the grafting surface. In the latter case intrinsic phase separation occurs in the brush: the dense phase consists of unloaded and weakly extended chains and occupies the region proximal to the surface, whereas a more dilute phase consisting of highly loaded and strongly extended chains forms the periphery of the brush.
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  • 5
    ISSN: 1292-895X
    Keywords: PACS. 42.25.Fx Diffraction and scattering - 43.35.+d Ultrasonics, quantum acoustics, and physical effects of sound - 43.80.+p Bioacoustics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: Shear break-up of reversible fractal clusters is investigated by ultrasound and multiple light scattering in the low shear regime. We consider a dense suspension of Rayleigh scatterers (particles or clusters) with acoustic properties close to those of the surrounding liquid so that the attenuation of the ultrasonic coherent field is weak and multiple scattering is negligible. The concept of variance in local particle volume fraction is used to derive an original expression of the ultrasound scattering cross-section per unit volume for Rayleigh fractal clusters. On the basis of a scaling law for the shear break-up of aggregates, then we derive the shear stress dependence of the ultrasound scattered intensity from a suspension of reversible fractal clusters. In a second part, we present rheo-acoustical experiments to study the shear break-up of hardened red cell aggregates in plane-plane flow geometry and we examine both the self consistent field approximation and the scaling laws used in microrheological models. We further compare the ability of acoustical backscattering and optical reflectometry techniques to estimate the critical disaggregation shear stress and the particle surface adhesive energy. Finally, the microrheological model from Snabre and Mills [#!ref5!#] based on a fractal approach is shown to describe the non Newtonian behavior of a dense distribution of hardened red cell aggregates.
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  • 6
    ISSN: 1292-895X
    Keywords: PACS. 61.30.-v Liquid crystals - 61.30.Cz Theory and models of liquid crystal structure - 61.30.Jf Defects in liquid crystals
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We investigated SmC* films sandwiched between silane coated glass plates and observed formation of textures exhibiting a uniform tilt of the smectic layers with respect to the boundary plates. The layer tilt angle increases from zero to as the sample is cooled from the smectic A phase to room temperature. These films show linear electro-optical effects because the permanent polarization can be aligned so that it has a component parallel to the applied field without changing the layer structure. Our analysis indicates that mainly two effects determine the layer tilt. On the one hand, the surface tension tends to minimise the layer tilt. On the other hand, the surface energy promotes the director to be normal to the boundary plates.
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  • 7
    ISSN: 1292-895X
    Keywords: PACS. 05.70.Jk Critical point phenomena - 74.80.Bj Granular, melt-textured, and amorphous superconductors; powders
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: The dynamics of intermittent granular flow through an orifice at the bottom of a granular bin and the associated clogging due to formation of arches blocking the outlet, is studied numerically in two dimensions. When the hole size is less than the grain diameter, only a single grain is removed from the system so that the system self-organizes to a steady state and the distribution of the grain displacements decays as power laws. On the other hand, when hole sizes are within few times of the grain diameter, the outflow distributions are also observed to follow a power law.
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  • 8
    ISSN: 1292-895X
    Keywords: PACS. 87.68.+z Biomaterials and biological interfaces - 87.16.Dg Membranes bilayers and vesicles - 68.10.-m Fluid surfaces and fluid-fluid interfaces
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: Using both theoretical modeling and computer simulations we study a model system for DNA interactions in the vicinity of charged membranes. We focus on the polarization of the mobile charges in the membranes due to the nearby charged rods (DNA) and the resulting screening of their fields and inter-rod interactions. We find, both within a Debye-Hückel model and in Brownian dynamics simulations, that the confinement of the mobile charges to the surface leads to a qualitative reduction in their ability to screen the charged rods to the degree that the fields and resulting interactions are not finite-ranged as in systems including a bulk salt concentration, but rather decay algebraically and the screening effect is more like an effective increase in the multipole moment of the charged rod.
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  • 9
    ISSN: 1292-895X
    Keywords: PACS 83.10.Hh Flow of solids – 83.10.Lk Multiphase flows – 83.20.Hn Structural and phase changes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We investigate the rheological properties of a cubic fcc phase of micelles obtained by aggregation of a triblock copolymer (PEO)127(PPO)48(PEO)127 in water as selective solvent. The resulting soft solid is submitted to a range of stresses varying from 20 to 800Pa in Couette geometry. Creep and flow behaviour can be distinguished and interpreted in terms of structural changes previously observed by SAXS under flow. Contrasting with other systems, no discontinuity in the flow behaviour is associated with the structural changes. The strong shear thinning is interpreted from the scattering data, as resulting from the nucleation of a new structure of hexagonal compact planes parallel to the Couette walls. This creates a lubricating domain in the gap, whose size grows with the applied shear rate. We argue moreover that the very existence of flow (as a steady state opposed to creep) is associated with this so-called layer-sliding structure in a fraction, however small, of the sample.
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  • 10
    ISSN: 1292-895X
    Keywords: PACS 61.30.Jf Defects in liquid crystals – 61.72.Lk Linear defects: dislocation disclinations – 61.72.Mm Grain and twin boundaries
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: Focal Conic Domains (FCDs) in smectic phases often assemble according to a particular rule, experimentally discovered by G. Friedel, the law of corresponding cones (l.c.c.). This paper reports various results relating to this type of association. First we show that a l.c.c. contact between 2 focal conic domains has a vanishing energy, yielding metastable local equilibrium. Then we use some projective properties of conic sections to extend the celebrated Apollonian tiling, which describes a tilt grain boundary (TiGB) of vanishing disorientation made of toric focal conic domains, to any TiGB. Finally we present a realistic model of the energy of the TiGB, which we compare to the energy of a TiGB split into dislocations, and to the energy of a curvature wall. This model explains why FCD tilings show macroscopic zones not filled with FCDs.
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