Thermal expansion within a chain of magnetic colloidal particles

We study the thermal expansion of chains formed by self-assembly of magnetic colloidal particles in a magnetic field. Using video-microscopy, complete positional data of all the particles of the chains is obtained. By changing the ionic strength of the solution and the applied magnetic field, the interaction potential can be tuned. We analyze the thermal expansion of the chain using a simple model of a one dimensional anharmonic crystal of finite size.Comment: 5 pages and 3 figure

Colloidal stability for concentrated zirconia aqueous suspensions

This work started as part of an investigation into the mechanisms by which fine zirconia aqueous dispersions can be processed for ceramic materials engineering. Aqueous dispersions of TZ3Y fine zirconia particles obtained by dispersion of dry powder in acidic solutions (pH 3) have been subjected to compression through osmotic experiments. The results show a behavior that is unusual when compared with the classical behavior of colloidal dispersions. Indeed, the 50 nm particles are well dispersed and protected from aggregation by electrical double layers, with a high zeta potential (60–80 mV). Yet, during osmotic compression, the dispersion goes from a liquid state to a gel state at a rather low volume fraction, φ=0.2, whereas the liquid–solid transition for repelling particles is expected to occur only at φ=0.5. This early transition to a state in which the dispersion does not flow may be a severe drawback in some uses of these dispersions, and thus it is important to understand its causes. A possible cause of this early aggregation is the presence of a population of very small particles, which are seen in osmotic stress experiments and in light scattering. We propose that aggregation could result from the compression of this population, through either of the following mechanisms: (a) An increase in pressure causes the small particles to aggregate with each other and with the larger ones or (b) An increase in pressure induces a depletion flocculation phenomenon, in which the large particles are pushed together by the smaller ones

The Sol-Gel Process Simulated by Cluster-Cluster Aggregation

The pair-correlation function $g(r,t)$ and its Fourier transform, the structure factor $S(q,t)$, are computed during the gelation process of identical spherical particles using the diffusion-limited cluster-cluster aggregation model in a box. This numerical analysis shows that the time evolution of the characteristic cluster size $\xi$ exhibits a crossover close to the gel time $t_g$ which depends on the volumic fraction $c$. In this model $t_g$ tends to infinity when the box size $L$ tends to infinity. For systems of finite size, it is shown numerically that, when $t<t_g$, the wave vector $q_m$, at which $S(q,t)$ has a maximum, decreases as $S(q_m,t)^{-1/D}$, where $D$ is an apparent fractal dimension of clusters, as measured from the slo pe of $S(q,t)$ . The time evolution of the mean number of particles per cluster $\bar {n}$ is also investigated. Our numerical results are in qualitative agreement with small angle scattering experiments in several systems.Comment: RevTex, 13 pages + 9 postscript figures appended using "uufiles". To appear in J. of Non-Cryst. Solid

Liquid pearls

This fluid dynamics video reports how to form liquid core capsules having a thin hydrogel elastic membrane named liquid pearls. These fish-egg like structures are initially made of a millimetric liquid drop, aqueous or not, coated with an aqueous liquid film containing sodium alginate that gels once the double drop enters a calcium chloride bath. The creation of such pearls with micrometer thick membrane requires to suppress mixing until gelling takes place. Here, we show that superimposing a two dimensional surfactant precipitation at the interface confers a transient rigidity that can damp the shear induced instability at impact. Based on this, pearls containing almost any type of liquids can be created. The video focuses on the dynamics of the entry of the compound drop into the gelling bath.Comment: videos are include

Étude de la flore parodontopathogène chez des personnes obèses ayant ou non subi une chirurgie bariatrique

L obésité et la parodontite sont des pathologies touchant la population au niveau mondial. De récentes études montrent que l obésité et le traitement de celle-ci par chirurgie bariatrique seraient associés à un plus grand risque de développer une parodontite. Les mécanismes biologiques de cette association sont encore mal compris aujourd hui. Dans un premier temps un travail de synthèse des différentes études existantes sur ce sujet a été réalisé. Peu d études ont regardé les effets de l obésité et de la chirurgie bariatrique sur le parodonte au niveau bactériologique. Dans un second temps une étude a été réalisée sur l influence d une perte de poids importante par chirurgie bariatrique sur la flore parodontopathogène. Des prélèvements bactériologiques ont été réalisés sur deux groupes de patients (patient obèses et patient ayant subit une chirurgie bariatrique). Les prélèvements ont ensuite été mis en culture sur gélose WCA et les colonies obtenues ont été identifiées par une technique biochimique. Un certain nombre de bactérie appartenant à la flore parodontopathogènes ont été mis en évidence. Le nombre trop faible de sujets ayant pu intégrer l étude n a pas pu nous permettre de comparer les résultats entre les deux groupes. Mais le travail débuté invite à le poursuivre.Obesity and periodontitis are diseases affecting the population worldwide. Recent studies show that obesity and her treatement by bariatric surgery would be associated with a greater risk of developing periodontitis. But the biological mechanisms of this association are poorly understood today. At first a synthesis of various existing studies on this topic was conducted. Few studies have looked at the effects of obesity and bariatric surgery on the periodontium at bacteriological level. In a second step a study has been conducted on the influence of a significant weight loss by bariatric surgery on periodontal pathogenic flora. Bacteriological samples were performed on two groups of patients (obese patient and patient undergoing bariatric surgery). The samples were then cultured on agar WCA and the colonies obtained were identified by biochemical technique. A number of bacetria belonging to the periodontal flora have been highlighted. The low number of subjects in the study couldn't allow us to compare the results between the two groups. But the work started invite to go forward.BORDEAUX2-Bib. électronique (335229905) / SudocSudocFranceF

Rotational Diffusion in a Chain of Particles

We study the coupled rotational diffusion in a two-particle chain on the basis of a Smoluchowski equation and calculate time-correlation functions that are measurable in an experiment. This might be used to explore hydrodynamic interactions in the limit where lubrication theory is valid.Comment: 7 pages, 2 figures, to be published in J. Phys.: Condens. Matte

Control of silver-polymer aggregation mechanism by primary particle spatial correlations in dynamic fractal-like geometry

Silver nanocrystals have been prepared by reacting silver nitrate with ascorbic acid in aqueous solution containing a low concentration of a commercial polynaphtalene sulphonate polymer (Daxad 19). Various crystalline morphologies have been obtained simply by tuning the reaction temperature. We have investigated the nanoparticle formation mechanism at three different temperatures by in situ and time resolved Small Angle X ray Scattering measurements. By modeling the scattering intensity with interacting spherical particles in a fractal-like polymer-Ag matrix, we found signatures of nucleation, growth and assembly of primary particles of about 15-20 nm. We observed how the time evolution of both spatial correlations between primary particles and the dynamic fractal geometry of the polymer-Ag matrix could influence and determine both the aggregation mechanism and the morphology of forming nanostructures in solution

A Model for the Elasticity of Compressed Emulsions

We present a new model to describe the unusual elastic properties of compressed emulsions. The response of a single droplet under compression is investigated numerically for different Wigner-Seitz cells. The response is softer than harmonic, and depends on the coordination number of the droplet. Using these results, we propose a new effective inter-droplet potential which is used to determine the elastic response of a monodisperse collection of disordered droplets as a function of volume fraction. Our results are in excellent agreement with recent experiments. This suggests that anharmonicity, together with disorder, are responsible for the quasi-linear increase of $G$ and $\Pi$ observed at $\varphi_c$.Comment: RevTeX with psfig-included figures and a galley macr

Fractionation of polydisperse systems: multi-phase coexistence

The width of the distribution of species in a polydisperse system is employed in a small-variable expansion, to obtain a well-controlled and compact scheme by which to calculate phase equilibria in multi-phase systems. General and universal relations are derived, which determine the partitioning of the fluid components among the phases. The analysis applies to mixtures of arbitrarily many slightly-polydisperse components. An explicit solution is approximated for hard spheres.Comment: 4 pages, 1 figur