Concentrated Ferrofluids Research Articles

Measuring the rotational viscosity of ferrofluids without shear flow

The shear free solid-body rotation of a ferrofluid in a magnetic field experiences a damping due to internal rotational friction. A device based on the principle of a torsional pendulum is presented. It allows us to perform direct and precise measurements of the field dependence of the genuine rotational viscosity without a disturbing shear flow. Experimental results for a moderately concentrated ferrofluid compare reasonably well with theoretical models for monodisperse ferrofluids of noninteracting rigid dipoles.

Dynamics of concentrated ferrofluid with labelled particles

In the study of ferrofluid dynamics the separation of self and pair correlation is the essential problem at both intermediate and high concentrations of the magnetic phase. The neutron spin-echo (NSE) total scattering function St(q,t)=βSp(q,t)+(1−β)Ss(q,t) contains both contributions, where the parameter β=σp/(σp+σs) depends on the corresponding cross sections σs,p. To distinguish these different kinds of dynamics we eliminated one of them by using a mixture of magnetic particles with different scattering amplitudes and concentrations. The NSE experiments (LLB, CE-Saclay) at momentum transfer q1≈0.4nm ∼1/Dp (Dp is the particle diameter) and q2≈0.7nm>1/Dp have been carried out for these opposite situations. The functions Sp(q,t),Ss(q,t), from these data behave quite differenly. The autocorrelations reveal a stretched relaxation at low q: Ss(q,t)=exp[−q2Γ(t)/2], where Γ(t)=2Dtα is the squared particle displacement, the exponent α=0.5±0.1 at q1, and α=0.8±0.1 at q2. The magnitude α<1 indicates the strong influence of dipole forces on the motion of a particle. It resembles the segmental relaxation in a polymer chain. On the other hand, the pair correlations show the oscillations (period∼40ns, amplitude∼1nm) mixed with diffusion.

Agglomerate formation in moderately concentrated ferrofluids from static magneto-optical measurements

In this paper the stability of ferrofluids based on several organic solvents with magnetite particles having different stabilization layers from static linear dichroism and birefringence has been studied. Based on the theory of formation it has been found that the mean number of particles per chain for most samples is very low; the mean ellipticity of chains computed from the formation theory turns out to be that of individual particles. A good agreement with the results of image processing of an electron micrography was found.

Preparation and Properties of Monodisperse Magnetic Cobalt Colloids Grafted with Polyisobutene

A single-step method to synthesize monodisperse metallic cobalt particles of diameter around 8 nm is described. The particles are sterically stabilized by modified polyisobutene and form stable ferrofluids in toluene. The core–shell cobalt spheres have a narrow size distribution and are nonaggregated. Various techniques, such as infrared spectroscopy, X-ray diffraction, and elemental analysis establishes the presence of grafted modified polyisobutene on the surface of cobalt particles. The hydrodynamic thickness of the grafted polyisobutene calculated from sedimentation and viscosity measurements and independently from the dynamic light scattering is ca. 10 nm. The dependence of magnetization has been measured for ferrofluid, showing superparamagnetic behavior above 247 K. Due to anisotropy, superposition of magnetic curves is not observed below this temperature. The concentration dependence of the sedimentation coefficient agrees qualitatively with the theory for dipolar spheres. SAXS data on concentrated ferrofluid dispersion indicate interpenetration of polyisobutene chains grafted to the cobalt particles.

Long-time self-diffusion in a model ferrofluid

The long-time self-diffusion coefficients of a concentrated ferrofluid are determined theoretically with the use of the generalized Langevin equation approach. The dependences of the translational and rotational diffusions on the concentration and the dipolar strength are studied. These dependences are expressed through the static structural information of the ferrofluid suspension. It is found that the effect of the dipolar interaction leads to the strong suppression of both long-time coefficients compared to the free diffusion coefficients of the particles in the infinite dilute case.

An effective correction to the theoretical curve of magnetobirefringence of magnetic fluid

Previously theoretical models for magnetically induced birefringence (M-B) of magnetic fluid ignored interparticle interactions and accordingly resulted in considerable deviation from the experiments at low field. Under such circumstances, we measure the Curie-Weiss behaviors of two ferrofluid samples from −50 °C to 80 °C and correct the M-B models by means of mean-field approximation (Weiss model). Experiments show that the corrected model coincides with the experiments quite well even for concentrated ferrofluid samples.

Rotational viscosity in ferrofluids

We have measured the rotational viscosity ηr of a concentrated ferrofluid by observing the increase of critical angular velocity in a Taylor cell with two different magnetic field configurations. The results are compared to theoretical calculations for diluted ferrofluids at different relative orientations of the field and the vorticity of the fluid. The observed deviations are attributed to breakdown of the theory for concentrated ferrofluids.

Magnetically induced dielectric anisotropy in concentrated ferrofluids

Magnetically induced dielectric anisotropy (MDA) has been studied in concentrated suspensions of magnetite in paraffin (n-hexadecane). The volumetric fraction was phi =0.3. The maximum value for the applied magnetic field was 48 kA m-1. A clear trend to the saturation in MDA was found for applied fields larger than 32 kA m-1. In this case, the dielectric anisotropy is up to 15% of the dielectric constant at zero field. A constant value for g=1.23 was found for magnetic fields higher than 12 kA m-1 (g=( epsilon /sub ///- epsilon (0))/( epsilon (0)- epsilon perpendicular to ) where epsilon (0) is the dielectric constant at null field). Below this field, a transition to g=1 was attained at 4 kA m-1. These results disagree with the value g=2 theoretically deduced for ideally monodispersive suspensions in the overall field range. The present results are tentatively explained qualitatively and quantitatively taking into account the mechanism of particle aggregation.

Rotational viscosity of ferrofluids and the Taylor instability in a magnetic field

We have measured the rotational viscosity Δη of a concentrated ferrofluid by observing the increase of the first critical angular velocity in a Taylor cell with longitudinal magnetic field. The method is suitable for a very accurate determination of Δη because of the sensitivity of the onset of Taylor instabilities to the effective viscosity of the fluid. The results are in agreement with recent theoretical investigations.

Determination of Ferrofluid Structure by Neutron Polarization Analysis

Neutron polarization analysis is combined with small-angle scattering for the study of a concentrated cobalt ferrofluid. The geometry of the cobalt particle core and surrounding surfactant layer has been determined and an anomalous magnetic layer found at the cobalt surface. The interparticle structure factor, measured for the first time in a magnetically aligned ferrofluid, agrees well with a recent theory. Magnetic fluctuations, measured as a function of applied field, are found to be predominantly paramagnetic.