Electroactive elastomers are composites made of solid particles embedded in an elastomeric network whose mechanical or optical properties can be changed by the application of an electric or a magnetic field. These materials have obviously a strong connection with ER and MR fluids and can be more appropriated for some applications. We present recent results concerning two kinds of filled elastomer, one based on carbonyl iron particles and the second one on silica particles. In the first case we show that that change of elastic properties obtained by the application of a magnetic field depend dramatically on the way we have structured the suspension before the polymerization. We explain quantitatively these experimental results with the help of finite element calculation to predict the magnetic forces between the particles. In the second case we show how it is possible to modulate the transmission of a laser beam by shearing a thin elastomeric film whose particles have been initially aligned with the help of an electric field. Some applications related to the organization of the filler particles by the application of a field or a combination of a field and a flow before polymerization will be discussed.

16 pagesInternational audienceWe investigate the sedimentation of concentrated suspensions at low Reynolds numbers to study collective particle effects on local particle velocity fluctuations and size segregation effects. Experiments are carried out with polymethylmetacrylate (PMMA) spheres of two different mean diameters (190 and 25 μm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labelled PMMA spheres and a particle image velocimetry method. We also report measurements of the interstitial fluid pressure during settling. Experiments show that size segregation effects can occur during the sedimentation of concentrated suspensions of either quasi-monodisperse or bidisperse spheres. Size segregation is correlated to the organi-sation of the sedimentation velocity field into vortexlike structures of finite size. A loss of size segregation together with a significant decrease of the fluid pressure gradient in the bulk suspension is observed when the size of vortex-like structures gets on the order of the container size. However, the emergence of channels through the settling zone prevents a complete loss of size segregation in very concentrated suspensions

Structures formed by a colloidal suspension of silica particles in 4-methyl cyclohexanol have been analyzed in the presence of an electric field. The formation of chains of particles was detected using an elliptical mirror to collect scattered light and a nearly matched refractive index between particles and solvent. A numerical method has been developed to obtain the size distribution of chains and their kinetics of formation from the record of a two-dimensional map of scattered light. We have compared the experimental size distribution to the prediction of a statistical theory based on a minimization of the free energy of a gas of chains. This theory quite well reproduced the experimental results for small chains but overestimates the tail of the distribution at high field. A saturation of the average size of chains versus the electric field was observed experimentally instead of a continuous growth as would be expected from aggregation under dipolar forces. A kinetic model, taking into account both capture and escape rates of a particle at the extremity of a chain, was shown to reproduce well the experimental growth of the average size of chains with time.

We study a theoretical model describing a laser with a modulated parameter, concentrating on the appearance of extreme events, also called optical rogue pulses. It is shown that two conditions are required for the appearance of such events in this type of nonlinear system: the existence of generalized multi-stability and the collisions of chaotic attractors with unstable orbits in external crisis, expanding the attractor to visit new regions in phase space.

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We have developed a new Ewald summation for a three-dimensional dipolar system with two-dimensional periodicity in a uniaxial field and a rotating field in a horizontal plane. Under a constant pressure and temperature, Monte Carlo simulation has been carried out; phase transitions are found and chainlike structure for a uniaxial field and monolayer or multilayer for rotating field are obtained, which are well consistent with experiments.

Suspensions of solid colloidal particles presenting a large mismatch of magnetic permeability or dielectric permittivity with the suspending fluid are interesting systems to correlate structure and rheology by tuning attractive dipolar forces with the help of an external field. The beginning of phase separation with formation of chainlike structure is experimentally studied in a suspension of silica particles using light scattering and in a suspension of magnetic colloidal polystyrene by means of light transmission. For a silica volume fraction of 10% we find that the mean distance between particles inside chains drops off suddenly with the increase of field. An analysis of evolution of size distribution with the field is attempted but does not show evidence for a strong increase of average sizes of chains with field. We determine the phase diagram of the magnetic suspension by measuring light transmission and show that the experimental critical volume fraction is much lower than that predicted with a theory based on transition in an isotropic medium.

In a number of applications, the use of portable X-ray fluorescence (pXRF) instruments offers a time and cost-saving alternative to standard laboratory instruments. This is particularly true in a mining context where decisions must be taken quickly in the field. However, pXRF is a technique known to be efficient, provided that samples are well prepared, i.e., dried and finely ground. On the mine face, little-to-no sample preparation is conceivable as mining vehicles must be able to operate continuously. Therefore, solutions have to be found even for raw materials and one of the most critical problems is the sample water content, in particular in the context of open pit mines in a tropical area. A large number of analysis shows that knowledge of humidity enables the measured concentration to be effectively corrected for the three instruments used (Niton, X-met, Titan). It is possible to overcome the difficulty of measuring water content in the field by fixing it to its maximum value (saturation). The results show that the saturation method is reliable, or at least, promising.