SPECIAL SECTION - COMPLEX FLUIDS
Complex and supermolecular fluids (liquid crystals, ferrofluids, polymers, colloids, microemulsions, biological membranes) are fascinating examples of systems where the concepts and experimental techniques of condensed matter physics can be used to investigate collective properties of matter. These complex fluids have been studied by a variety of experimental techniques (linear and non-linear optical techniques, scattering and diffraction of X-rays and neutrons, X-ray absorption fine structure, calorimetry, electron and atomic force microscopy, photo-thermal effects, nuclear magnetic resonance).
Some of these materials remained as laboratory curiosities for several years. More recently, they have been used to test fundamental ideas in physics, chemistry, and biology. They are also used in several objects and devices of our daily life.
Liquid crystals, which are sometimes called mesophases, are intermediate states of matter between a crystalline solid and an isotropic liquid. Thanks to their fluidity and viscosity, associated with electro-optical properties, liquid crystals are widely used in displays of low energy cost. Their thermo-optical properties lead to many applications (as in tumoral diagnosis). Some phases of liquid crystals (and especially of lyotropic crystals, which are mixtures of amphiphilic molecules and a solvent under proper temperature and concentration conditions) present up to three different refraction indices along the orthogonal directions of space. Biaxial lyotropic nematic liquid crystals and smectic mesophases provide examples of systems where the critical regions in the vicinity of phase transitions can be accessed and studied in great experimental detail. In general, the phase diagrams of liquid crystals are very rich, with first and second-order transition lines, Landau and Lifshitz points, and many reentrant phases, as a function of the relevant parameters (temperature, pressure, relative concentrations of the components of the mixture). As proposed by de Gennes, another interesting feature of liquid crystals is the analogy between the phase transitions in smectics and the superfluid transition in Helium .
Magnetic fluids (also called ferrofluids) are colloidal suspensions of magnetic grains with typical dimensions of 10nm. These materials present the fluidity of an isotropic liquid and a high magnetic susceptibility. The grains are usually made by maghemite (g-Fe2O3) or ferrites, and the stability of the colloid is achieved either by coating with surfactant agents (surfacted ferrofluids) or by charging the surfaces of the grains (ionic ferrofluids). The fluid carrier can be either polar or non-polar, depending on the type of the grains of the ferrofluid. Due to their unusual properties, these fluid materials have been extensively investigated from the points of view of physicists, chemists, and engineers. There are many technological applications of ferrofluids, as in the rotating seals of hard-disks and in heat dissipation devices. In the next few years, applications (including in medicine and biology) are expected to increase in number and complexity.
As it is well known, polymers are another class of complex fluids with an enormous impact on the world economy due to their mechanical, thermal and rheological properties.
It has been a delicate task to co-edit a special issue of the BJP dedicated to complex fluids. First, it was important to choose some of the Brazilian groups which would be able to give an up-to-date description of the scientific activity in this area in our country. Also, it was necessary to invite scientists from abroad to give an idea of some of the most interesting problems faced by the international community. The result of this effort has gone beyond our initial expectations. The contributions in this issue represent a fair amount of the scientific activity in the area in Brazil. Also, they give an overview of the interests of some of the more active foreign groups.
A. M. Figueiredo Neto,
Guest Editor
Instituto de Física,
Universidade de São Paulo
Publication Dates
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Publication in this collection
26 Aug 1999 -
Date of issue
Dec 1998
