A. M. Figueiredo Neto

Ferrofluids: properties and applications

Magnetic fluids may be classified as ferrofluids (FF), which are colloidal suspensions of very fine (» 10 nm) magnetic particles, and magnetorheological fluids, which are suspensions of larger, usually non-stable, magnetic particles. We review the general classification and the main properties of FF, some theoretical models and a few applications. We consider the stability of a FF in terms of various forces and torques on the magnetic particles. We discuss thermodiffusion, which is an important phenomenon in FF, and which gives rise to the Soret effect. We also consider the rotational dynamics of the magnetic moments of the particles. A large portion of this review is dedicated to applications of FF, including a few of the many technological applications. Among the uses of a FF in the study of materials, we have selected the doping of liquid crystals. Among the very promising uses in Medicine, we discuss drug targeting, hyperthermia, cell separation, and contrast in magnetic resonance imaging. We also make some comments on directions for future research on the properties of ferrofluids.

Microscopical structure of the uniaxial and biaxial lyotropic nematics

High‐resolution x‐ray measurements have been performed at the synchrotron of LURE (Orsay) to study both uniaxial (discotic and calamitic) and biaxial nematic lyomesophases of the mixture K‐laurate, 1‐decanol, and D2O. The variations of the densitometric profiles of the diffracted bands are analyzed vs temperature. They are shown to be consistent with the model of similar micellar aggregates in the three nematic phases, undergoing different orientational fluctuations. It is moreover discussed that the micelles have, on the average, the shape of biaxial platelets with a (1/2/3) anisotropy, and that they build the same pseudolamellar structure in each nematic phase.

Nonlinear optical properties of liquid crystals probed by Z-scan technique

We present a review of experimental data on the nonlinear optical properties of thermotropic and lyotropic liquid crystals probed by the Z-scan technique. Depending on the time scale, different processes originate a nonlinear optical response. Particularly, at nanosecond range, the physical processes associated with the nonlinear response in thermotropics are not completely understood. In lyotropics, the nonlinear response at ms time scale is from thermal origin and depends on the particular mesophase, relative concentration of the components, and temperature.

Impedance spectroscopy of an electrolytic cell limited by ohmic electrodes

We investigate the influence of the ohmic character of the electrodes on the impedance spectroscopy of a cell of a dielectric liquid containing ions. According to our calculations, the finite conductivity of the electrodes is responsible for an increasing of the real part of the electrical impedance in the low frequency range. It follows that the anomalous increasing of the resistance of the cell, experimentally observed by several groups, could be related also to the exchange of charge at the electrodes. Our analysis is based on a generalization of the Butler-Wolmer equation, for small values of the external voltage.

A new soap/detergent/water lyotropic liquid crystal with a biaxial nematic phase

Abstract A new lyotropic liquid crystal, potassium laurate, decylammonium chloride and water, which has two uniaxial phases and a biaxial nematic phase, is reported. A surface of the phase diagram and X-ray diffraction studies are presented. The chemical stability of this mixture is compared with the potassium laurate/decanol/water mixture. Optical measurements of the birefringence and X-ray studies indicate that this new mixture is more stable than the usual mixtures with alcohol.

Generalization of the thermal lens model formalism to account for thermodiffusion in a single-beam Z-scan experiment: determination of the Soret coefficient

A generalization of the thermal lens model formalism is proposed to introduce and characterize the thermodiffusion phenomenon. The theory treats the case in which local heating generates a temperature gradient in a single-phase binary sample (a colloid, for instance) that yields, through thermodiffusion, concentration gradients in an initially homogeneous sample. The treatment generalizes the concept of a thermal lens to a material lens as a result of the coupling of a concentration variation with the optical properties of the medium. This formalism permits the use of the Z-scan technique to determine the Soret coefficient of samples. Applying this theory to the results of a Z-scan experiment with an ionic ferrofluid sample gives values that agree with those obtained from forced Rayleigh scattering measurements of the same material.

Phase transitions in complex fluids

General theoretical approaches to phase transitions in complex fluids phase transitions in lyotropic systems phase transitions in microemulsions, colloids and gels phase transitions in polymer blends phase transitions in membranes and monolayers phase transitions in ferrofluids.

Lyotropic mixture made of potassium laurate/1-undecanol/K2SO4/water presenting high birefringences and large biaxial nematic phase domain: a laser conoscopy study

The lyotropic liquid crystalline quaternary mixture made of potassium laurate (KL), potassium sulphate, 1-undecanol and water was investigated by experimental optical methods (optical microscopy and laser conoscopy). In a particular temperature and relative concentrations range, the three nematic phases (two uniaxial and one biaxial) were identified. The biaxial domain in the temperature/KL concentration surface is larger when compared to other lyotropic mixtures. Moreover, this new mixture gives nematic phases with higher birefringence than similar systems. The behavior of the symmetric tensor order parameter invariants σ3 and σ2 calculated from the measured optical birefringences supports that the uniaxial-to-biaxial transitions are of second order, described by a mean-field theory.

Investigation of the sign of the Soret coefficient in different ionic and surfacted magnetic colloids using forced Rayleigh scattering and single-beam Z -scan techniques

The sign of the Soret coefficient S T of a large set of ionic magnetic colloids (ionic ferrofluids (IFFs)) and surfacted magnetic colloids (surfacted ferrofluids (SFFs)) is determined using forced Rayleigh scattering and the single-beam Z -scan techniques. Different samples were investigated: acid and alkaline colloids with different values of pH; colloids with different concentrations of magnetic grains; colloids with grains of different typical diameters; colloids with magnetic grains with different coating natures; colloids with different non-polar and polar liquid carriers. Our results indicate that the sign of S T depends on the sign of the surface charge of grains in IFFs. In the case of water-based SFFs, the thermodiffusive behaviour is opposite to that of IFFs; that is, grains coated with a cationic surfactant behave like negatively charged IFF (alkaline) grains and grains coated with an anionic surfactant behave like positively charged IFF (acid) grains. SFFs with grains coated with non-ionic surfactants dispersed in non-polar fluid carriers behave like SFFs with grains coated with a cationic surfactant. The nature of the liquid carrier itself is not the only determinant factor, except apparently in the case of non-polar fluids, where only S T > 0 is found. These results cannot be explained with the available theories and it is highly probable that different mechanisms are present in the thermodiffusive behaviour of these complex fluids.

The effect of hydrophobic and hydrophilic fumed silica on the rheology of magnetorheological suspensions

We investigate magnetorheological fluids (MRFs) prepared with carbonyl iron powder and different types of hydrophobic and hydrophilic fumed silica. The rheological properties of the MRF suspensions were investigated with and without an applied magnetic field. The MRF samples prepared with hydrophobic silicas presented a more pronounced thixotropic effect and a higher recovery rate than those prepared with hydrophilic silicas. The application of a magnetic field to all the MRFs samples investigated leads to an increase in the viscosity and the thixotropic effect. MRF prepared with hydrophobic silicas presented smaller values of the viscosity than those prepared with hydrophilic silicas. At low applied magnetic fields, the type of the silica used to prepare the MRF leads to noticeable differences in the shear stress. However, these differences disappear at high magnetic fields. The results obtained showed that MRF samples prepared with the hydrophobic silica with the biggest particle diameter presented bett...