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.

The manuscript is devoted to two applications of chains of magnetic particles.
The first chapter reports the design of force sensors between colloids. Various experiments are designed and performed to characterise the particles under investigation. Our experimental setup and signal processing techniques are described in detail. Finally, we compare this technique to previous ones.
The second chapter describes the design and fabrication of the first microscopic artificial swimmers. Flexible magnetic filaments are created by aligning magnetic particles under a magnetic field and gluing them together by means of DNA strands. Their behaviour under a magnetic field is investigated. Subjected to an oscillating magnetic field, the filaments experience deformations so that a wave propagates along them, thereby generating a propulsive force, which we demonstrate through the motion of a spermatomorphic swimmer. Theoretical and numerical predictions are validated experimentally.

A composition comprising in a continuous aqueous phase a monodisperse lipid phase comprising one or more crystallizable lipids, one or more active principles and a least one dispersed phase stabilizing compound comprising two fatty acid chains and a polyethylene glycol chain, is new. - Independent claims are included for a methods of preparing the claimed composition by: - (a) introducing the active principle(s) into the crystallizable lipid; - (b) dispersing the lipid phase in the aqueous phase in the presence of an emulsifier to form an emulsion; - (c) subjecting the emulsion to shear to form a monodisperse emulsion or where the lipid phase comprises an internal aqueous phase, by: - (i) dispersing an aqueous solution comprising the active principles into the lipid (which may contain one or more active principles) in a melted state in the presence of a lipophilic surfactant; - (ii) subjecting the emulsion obtained to shear to render it monodisperse; - (iii) incorporating the monodisperse emulsion in an aqueous phase in the presence of a stabilizer to form a double emulsion; and - (iv) subjecting the double emulsion to shear to produce a monodisperse double emulsion

Producing monodisperse biodegradable microspheres (I), comprising preparing an emulsion comprising at least one polymer phase containing an active agent (A) and at least one aqueous phase, the ratio of viscosities of the organic and aqueous phases being 0.1-10, subjecting the emulsion to controlled laminar shear, removing the solvent from the polymer phase, and recovering the obtained microspheres, is new.

Assembling colloidal particles in the form of irreversibly organized chains bearing species-recognition sites different from the ligands involved in organizing the particles, is new. Independent claims are also included for: (1) producing an assembly by assembling colloidal particles in the form of linear objects and contacting the objects with an agent capable of bridging them irreversibly; (2) producing an assembly by mixing and/or grafting colloidal particles with a bridging agent or bridging agent precursor, assembling the particles in the form of linear objects, and initiating bridging between the particles; (3) detecting, analysis, purification, identification, separation or ex-vivo monitoring of at least one species using an assembly; (4) a surface element bearing an assembly; (5) a hybridization array comprising the surface element of (4); (6) a microfluidic channel or cell comprising an assembly.

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.