Biomedical subjects

Stéphane Roux

Publications and source records attributed to Stéphane Roux.

10 recordsLinked to original sources

PubMed2005

Synthesis, characterization of dihydrolipoic acid capped gold nanoparticles, and functionalization by the electroluminescent luminol.

The use of gold nanoparticles as biological probes requires the improvement of colloidal stability. Dihydrolipoic acid (DHLA), a dithiol obtained by the reduction of thioctic acid, appears therefore very attractive for the stabilization and the further functionalization of gold nanoparticles because DHLA is characterized by a carboxylic acid group and two thiol functions. The ionizable carboxylic acid groups ensure, for pH > or = 8, the water solubility of DHLA-capped gold (Au@DHLA) nanoparticles, prepared by the Brust protocol, and the stability of the resulting colloid by electrostatic repulsions. Moreover almost all DHLA, adsorbed onto gold, adopts a conformation allowing their immobilization by both sulfur ends. It is proved by sulfur K-edge X-ray absorption near edge structure spectroscopy, which appears as an appropriate tool for determining the chemical form of sulfur atoms present in the organic monolayer. Such a grafting renders the DHLA monolayers more resistant to displacement by dithiothreitol than mercaptoundecanoic acid monolayers. The presence of DHLA on gold particles allows their functionalization by the electroluminescent luminol through amine coupling reactions assisted by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide. As a luminol-functionalized particle is nine times as bright as a single luminol molecule, the use of the particles as a biological probe with a lower threshold of detection is envisaged.

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PubMed2004

Universal depinning force fluctuations of an elastic line: application to finite temperature behavior.

The depinning of an elastic line in a random medium is studied via an extremal model. The latter gives access to the instantaneous depinning force for each successive conformation of the line. Based on conditional statistics the universal and nonuniversal parts of the depinning force distribution can be obtained. In particular the singular behavior close to a (macroscopic) critical threshold is obtained as a function of the roughness exponent of the front. We show, moreover, that the advance of the front is controlled by a very tenuous set of subcritical sites. Extension of the extremal model to a finite temperature is proposed, the scaling properties of which can be discussed based on the statistics of depinning force at zero temperature. In particular a new temperature-dependent correlation length is shown to become relevant.

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PubMed2004

Large-scale numerical simulations of ultrametric long-range depinning.

The depinning of an elastic line interacting with a quenched disorder is studied for long-range interactions, applicable to in-plane crack propagation or wetting. An ultrametric distance is introduced instead of the Euclidean distance, allowing for a drastic reduction of the numerical complexity of the problem. Based on large-scale simulations, two to three orders of magnitude larger than previously considered, we obtain a very precise determination of critical exponents which are shown to be indistinguishable from their Euclidean metric counterparts. Moreover, the scaling functions are shown to be unchanged. The choice of an ultrametric distance thus does not affect the universality class of the depinning transition and opens the way to an analytic real-space renormalization-group approach.

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PubMed2004

Dual multifractal spectra.

The multifractal formalism characterizes the scaling properties of a physical density rho as a function of the distance L. To each singularity alpha of the field is attributed a fractal dimension for its support f(alpha). An alternative representation has been proposed by considering the distribution of distances associated to a fixed mass. Computing these spectra for a multifractal Cantor set, it is shown that these two approaches are dual to each other, and that both spectra as well as the moment scaling exponents are simply related. We apply the same inversion formalism to exponents obtained for turbulent statistics in the Gledzer-Ohkitani-Yamada shell model and observe that the same duality relation holds here.

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PubMed2003

Shearing of loose granular materials: a statistical mesoscopic model.

A two-dimensional lattice model for the formation and evolution of shear bands in granular media is proposed. Each lattice site is assigned a random variable which reflects the local density. At every time step, the strain is localized along a single shear band which is a spanning path on the lattice chosen through an extremum condition. The dynamics consists of randomly changing the "density" of the sites only along the shear band, and then repeating the procedure of locating the extremal path and changing it. Starting from an initially uncorrelated density field, it is found that this dynamics leads to a slow compaction along with a nontrivial patterning of the system, with high-density regions forming which shelter long-lived low-density valleys. Further, as a result of these large density fluctuations, the shear band, which was initially equally likely to be found anywhere on the lattice, gets progressively trapped for longer and longer periods of time. This state is, however, metastable, and the system continues to evolve slowly in a manner reminiscent of glassy dynamics. Several quantities have been studied numerically which support this picture and elucidate the unusual system-size effects involved.

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PubMed2003

Slow relaxation due to optimization and restructuring: solution on a hierarchical lattice.

Motivated by the large strain shear of loose granular materials, we introduced a model that consists of consecutive optimization and restructuring steps leading to a self-organization of a density field. The extensive connections to other models of statistical physics are discussed. We investigate our model on a hierarchical lattice that allows an exact asymptotic renormalization treatment. A surprisingly close analogy is observed between the simulation results on the regular and the hierarchical lattices. The dynamics is characterized by the breakdown of ergodicity, by unusual system size effects in the development of the average density, as well as by the age distribution, the latter showing multifractal properties.

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PubMed2002

Multiscale displacement field measurements of compressed mineral-wool samples by digital image correlation.

We propose a multiscale approach to determine the displacement field by digital image correlation. The displacement field is first estimated on a coarse resolution image and progressively finer details are introduced in the analysis as the displacement is more and more securely and accurately determined. Such a scheme has been developed to increase the robustness, accuracy, and reliability of the image-matching algorithm. The procedure is used on two different types of examples. The first one deals with a representative image that is deformed precisely and purposefully to assess the intrinsic performances. In particular, the maximum measurable strain is determined. The second case deals with a series of pictures taken during compression experiments on mineral-wool samples. The different steps ofthe procedure are analyzed and their respective role is assessed. Both reflection and transmission images are tested.

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PubMed2002

Extremal model for amorphous media plasticity.

An extremal model for the plasticity of amorphous materials is studied in a simple two-dimensional antiplane geometry. The steady state is analyzed through numerical simulations. Long-range spatial and temporal correlations in local slip events are shown to develop, leading to nontrivial and highly anisotropic scaling laws. In particular, the plastic strain is shown to concentrate statistically over a region which tends to align perpendicular to the displacement gradient. By construction, the model can be seen as giving rise to a depinning transition, the threshold of which (i.e., the macroscopic yield stress) also reveals scaling properties reflecting the localization of the activity.

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PubMed2002

Turbulentlike fluctuations in quasistatic flow of granular media.

We analyze particle velocity fluctuations in a simulated granular system subjected to homogeneous quasistatic shearing. We show that these fluctuations share the following scaling characteristics of fluid turbulence in spite of their different physical origins: (i) scale-dependent probability distribution with non-Gaussian broadening at small time scales; (ii) spatial power spectrum of the velocity field showing a power-law decay, reflecting long-range correlations and the self-affine nature of the fluctuations; and (iii) superdiffusion of particles with respect to the mean background flow.

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PubMed2002

Correlation image velocimetry: a spectral approach.

A method, believed to be new, is introduced to evaluate displacement fields from the analysis of a deformed image compared with a reference image. In contrast to standard methods, which determine a piecewise constant displacement field, the present method gives direct access to spectral decomposition of the displacement field. A minimization procedure is derived and used twice: first, to determine an affine displacement field and, then, the spectral components of the residual displacement. Although the method is applicable to any space dimension, only cases dealing with one-dimensional signals are reported: First, a purely synthetic example is discussed to estimate the intrinsic performance of the method, and a second case deals with a profile extracted from a sample of compressed glass wool.

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