Publications

We demonstrate here the wet chemical synthesis of cobalt doped γ-Fe₂O₃ nanoparticles and the subsequent effect on magnetic properties with the variation in dopant concentration. It is observed that cobalt can be homogeneously doped into the γ-Fe₂O₃ lattice up to 5 mol% without any appreciable change in the particle size (~6 nm). Further increase in cobalt concentration (10 mol% here) resulted in an increase in particle size (~9 nm) due to possible adsorption of a cobalt layer on the surface of γ-Fe₂O₃ nanoparticles rather than complete doping in the iron oxide lattice. The ac susceptibility measurements revealed an increase in blocking temperature (T_B) with percentage variation in cobalt doping (2–10%), indicating substitution of Fe³⁺ ions by Co²⁺ ions in the γ-Fe₂O₃ lattice. The dc magnetization measurements showed an increase in saturation magnetization only up to 5%, beyond which it significantly diminished. The reduction in saturation magnetization is attributed to the contribution from surface anisotropy in cobalt coated γ-Fe₂O₃ nanoparticles.

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 approach might be used to explore hydrodynamic interactions in the limit where lubrication theory is valid.

An object consisting of three spheres, linked like the spokes on a wheel, can undergo a net rotational movement when the relative positions of the spheres proceed through a four-step cycle. This rotational motion is the analogue of the two-hinged swimmer originally proposed by Purcell (1977), which has served as a prototype for mechanical implementations of swimming. We also note that the rotational motion analysed here may be useful in the design of micromachines and has similarities to molecular-scale rotational motors that have been identified recently.

Superparamagnetic particles offer a new way to probe the kinetics of adhesive processes. Two different scenarios of physical adhesion are studied. The thermal activation of van der Waals adhesion is well described by an Arrhenius model. In contrast, it is necessary to go beyond the Arrhenius description to understand the thermal activation of bridging between colloidal particles by a polymer at equilibrium adsorbance. We show that polymer bridging requires some removal of adsorbed polymer and is strongly influenced by the proximity of a glass transition within the adsorbed polymer.

We perform oil coating of hydrophobic solid surfaces via aqueous media, from emulsions, and under the presence of a shear flow. The principle of such coating is based on the use of a system at the limit of aggregation to give rise to adhesion, with asymmetrical interfaces (oil droplet/water and solid surface/water) in order to favor the oil/surface adhesion in comparison to the oil/oil adhesion. This way, droplets stick to the solid substrate, whereas they are stable and homogeneously dispersed in the bulk. We have realized coatings from two systems of emulsions made of a mixture of hydroxy-terminated silicone oil and classical silicone oil and a mixture of sunflower oil and mineral oil. The kinetics of the coating is described by a Langmuir model where the adhesion between the oil particle and the surface is modeled as a first-order reaction. The resulting coatings are formed of oil droplets uniformly covering the solid surface. The coating density can vary with the nature of the experimental systems.

In this letter, we investigate the mechanism of formation of a recently discovered new type of colloid, irreversible flexible chains of magnetic particles. The chain formation mechanism is based on magnetically induced bridging by adsorbed polymers, andweinvestigate here the associated phase diagrams, considering both thermodynamic and kinetic aspects. This phase diagram is the consequence of a balance between entropic repulsion between polymer layers at the particles surfaces, depletion forces pushing the particles together, and a short-range attractive force developing when polymers can bridge two particles. We end up with a very simple protocol allowing the formation of long, extremely regular chains, which can find numerous applications in chemistry and biology. The perspectives for the development of a new field of “macrocolloidal chemistry” are discussed.

Silica particles are dispersed in the continuous phase of bitumen-in-water emulsions. The mixture remains dispersed in quiescent storage conditions. However, rapid destabilization occurs once a shear is applied. Observations under the microscope reveal that the bitumen droplets form a colloidal gel and coalesce upon application of a shear. We follow the kinetic evolution of the emulsions viscosity at constant shear rate remains initially constant and exhibits a dramatic increase after a finite time, t. We study the influence of various parameters on the evolution of t: bitumen droplet size and volume fraction, silica diameter and concentration, shear rate, etc.

Oils containing both fluorescent semiconductor and magnetic oxide nanoparticles are used to produce oil in water emulsions. This technique produces oil droplets with homogeneous fluorescence and high magnetic nanoparticle concentrations. The optical properties of the oil droplets are studied as a function of the droplet sizes for various concentrations of fluorescent and magnetic nanoparticles. For all concentrations tested, we find a linear variation of the droplet fluorescent intensity as a function of the droplet volume. For a given size and a given quantum dot (QD) concentration, the droplet fluorescence intensity drops sharply as a function of the magnetic nanoparticle concentration.Weshow that this decrease is due mainly to the strong absorption cross section of the magnetic nanoparticles and to a lesser extent to the dynamicandstatic quenching of theQDfluorescence.Therole of the iron oxide nanoparticle localization in the droplet (surface versus volume) is also discussed.

Ce mémoire contient deux parties consacrées à deux applications des chaînes de particules magnétiques.
Le premier chapitre décrit la conception d'un capteur de forces intercolloïdales. Nous présentons différentes expériences permettant de caractériser les particules utilisées. Nous décrivons ensuite la réalisation du montage expérimental et le traitement des données. Enfin, nous comparons cette technique aux autres techniques.
Le second chapitre décrit la conception et la réalisation de nageurs à l'échelle microscopique. Nous fabriquons des filaments magnétiques flexibles en alignant des particules sous l'effet d'un champ magnétique. Les particules adjacentes sont reliées par des agents pontants. Nous avons étudié le comportement de ces filaments sous champ. Sous l'effet d'un champ magnétique oscillant, le filament se déforme : une onde se propage alors le long du filament, ce qui engendre une force propulsive, dont nous démontrons l'effet sur un nageur spermatomorphe. Enfin, nous vérifions des prédictions théoriques antérieures.

L'invention concerne une composition comprenant une phase lipidique monodisperse dispersée dans une phase aqueuse continue, dans laquelle la phase lipidique comprend au moins un lipide cristallisable, au moins un principe actif et au moins un composé portant deux chaînes d'acides gras et une chaîne polyéthylène glycol.Elle concerne également le procédé de préparation de telles compositions via une émulsion simples H/E ou doubles E/H/E monodisperses.

L'invention concerne un procédé de préparation de microsphères biodégradables monodisperses comprenant les étapes de :a) préparation d'une émulsion comportant au moins une phase polymérique et au moins une phase aqueuse, le rapport des viscosités entre la phase polymérique dispersée et la phase aqueuse continue étant comprise entre 0,1 et 10 ;b) soumission de l'émulsion obtenue à un cisaillement laminaire contrôlé ;c) élimination du solvant de la phase polymérique ; etd) isolement des microsphères ainsi obtenues.Elle concerne aussi les utilisations des microsphères ainsi préparées.

La présente invention a pour objet principal un assemblage de particules colloïdales sous la forme d'une ou plusieurs chaînes caractérisé en ce que lesdites chaînes sont organisées de manière irréversible et sont porteuses d'au moins un site de reconnaissance pour une espèce, ledit site étant différent des sites impliqués dans l'organisation linéaire desdites particules.Elle vise également un procédé de préparation d'un tel assemblage et l'utilisation de cet assemblage, notamment à des fins de détection et/ou de dosage d'au moins une espèce dans un fluide.L'invention concerne également un élément de surface fonctionnalisé à l'aide d'un assemblage de chaînes colloïdales et un réseau d'hybridation comportant un tel élément de surface.

We present a new technique to measure the mechanical properties of small biomolecules. This technique uses long range repulsive colloidal forces together with magnetic attraction as a force probing tool. The biomolecules are grafted between superparamagnetic particles, which are regularly spaced within long chains maintained by an external magnetic field. Varying the magnetic field results in compression or extension of the molecules between the particles. In order to demonstrate this technique we use, as a size controlled model molecule, a short double stranded DNA (151 base pairs) for which the force-extension law is determined and found in agreement with existing predictions.

Microorganisms such as bacteria and many eukaryotic cells propel themselves with hair-like structures known as flagella, which can exhibit a variety of structures and movement patterns1. For example, bacterial flagella are helically shaped2 and driven at their bases by a reversible rotary engine3, which rotates the attached flagellum to give a motion similar to that of a corkscrew. In contrast, eukaryotic cells use flagella that resemble elastic rods4 and exhibit a beating motion: internally generated stresses give rise to a series of bends that propagate towards the tip5–7. In contrast to this variety of swimming strategies encountered in nature, a controlled swimming motion of artificial micrometresized structures has not yet been realized. Here we show that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum. The filament aligns with an external uniformmagnetic field and is readily actuated by oscillating a transverse field. We find that the actuation induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.