Abstract
This issue of Journal of Physics: Condensed Matter is dedicated to results in the field of ferrofluid research.
Ferrofluids—suspensions of magnetic nanoparticles—exhibit as a specific feature the magnetic control of their physical parameters and of flows appearing in such fluids. This magnetic control can be achieved by means of moderate magnetic fields with a strength of the order of 10 mT. This sort of magnetic control also enables the design of a wide variety of technical applications such as the use of the magnetic forces for basic research in fluid dynamics. The overall field of ferrofluid research is already about 40 years old. Starting with the first patent on the synthesis of magnetic nanoparticle suspensions by S Papell in 1964, a vivid field of research activities has been established. Looking at the long time in which ferrofluids have been the focus of scientific interest, one can ask the question, what kind of recent developments justify a special issue of a scientific journal?
New developments in a field, which depends strongly on a certain material class and which opens research possibilities in different scientific fields will nowadays usually require an interdisciplinary approach. This kind of approach starting from the synthesis of magnetic suspensions, including research concerning their basic properties and flow behaviour and focusing on new applications has been the core of a special research programme funded by the Deutsche Forschungsgemeinschaft (DFG) over the past 6 years. Within this programme—entitled `Colloidal Magnetic Fluids: Basics, Synthesis and Applications of New Ferrofluids'—more than 30 different research groups have been coordinated to achieve new results in various fields related to ferrofluid research.
The basic approach of the program has been the assumption that new applications well beyond the typical ferrofluid techniques, for example loud speaker cooling or sealing of rotary shafts, will require tailored magnetic suspensions with properties clearly focused towards the need of the application. While such tailoring of fluids to certain well defined properties sounds like a straightforward approach one has to face the fact that it requires a clear definition of the required properties. This definition itself has to be based on a fundamental physical knowledge of the processes determining certain magnetically controlled phenomena in ferrofluids. To make this point concrete one can look into the detailed aims of the mentioned research program. The application areas identified for the future development of research and application of suspensions of magnetic nanoparticles have been on the one hand the biomedical application—especially with respect to cancer treatment—and on the other hand the use of magnetically controlled rheological properties of ferrofluids for new active technical devices.
Both directions require, as mentioned, as a basis for success the synthesis of new ferrofluids with dedicated properties. While the medical applications have to rely on biocompatibility as well as on stability of the suspensions in a biomedical environment, the use of ferrofluids in technical devices employing their magnetically controlled rheological properties will depend on an enhancement of the changes of the fluid's viscous properties in the presence of moderate magnetic fields. For both requirements ferrofluids with a make up clearly different from the usual magnetite based fluids have to be synthesized. The question of how the detailed microscopic make up of the fluids would have to look has to be answered on the basis of basic research results defining the physics background of the respective phenomena. Taking these aspects together it becomes obvious that the aforementioned research program had goals aiming far beyond the state of the art of classical ferrofluid research.
These goals as well as the basic strategy to achieve them is in a way reflected by the structure of this issue of Journal of Physics: Condensed Matter. The issue contains results emerging from the research programme as well as invited papers from researchers not participating in the programme but working in closely related areas. The issue is subdivided into five main sections dealing with synthesis, basic physical description, rheology, and both the medical and technical applications of ferrofluids. As can be expected from work done within an interdisciplinary context many of the papers would fit into more than one of these sections and catagorization is thus sometimes difficult. We have therefore tried to place them into the section reflecting the main field of research to which the respective results belong.
The first section is on synthesis and characterization of magnetic suspensions. The first paper in this section is dedicated partly to magnetite ferrofluids but with special aspects concerning the particle size tailoring them for applications especially in the field of magnetic hyperthermia. After this, three different types of `new' ferrofluids are presented. Fluids based on pure metal particles exhibiting much stronger magnetic properties than the common magnetite fluids, fluids with a temperature sensitive surfactant shell allowing a change of the particle's hydrodynamic diameter by variation of the fluid's temperature and fluids containing spheres of nonmagnetic material with embedded magnetic particles which are already used in new medical applications.
The second section is dedicated to the basic physics of ferrofluids and highlights three different topics. First the question of magnetization dynamics is discussed and different aspects of this fundamental problem, which determines the basic description of ferrofluids, are highlighted. The second topic is the well known surface instability appearing in ferrofluids in a homogeneous magnetic field perpendicular to the fluid surface. This part shows clearly how many undiscovered phenomena can be found, even in an area which is as old as the whole research field, if an appropriate measuring technique is used and fresh ideas help to find unexpected effects. The last part of this section deals with the question of dynamics and structure of ferrofluids and shows the experimental possibilities of scattering techniques in this field.
Within the third section the question of field dependent changes of the rheological behaviour of ferrofluids is discussed. The first three papers provide theoretical approaches for the understanding of the connection between the rheological properties and shear and field induced changes in the fluid's microstructure. The fourth paper provides the related experimental results showing the combination of microstructural and rheological measurements under well defined conditions. The last paper of this section does not directly belong to ferrofluid research but to a closely related field—so called magneto-rheological (MR) suspensions, which differ from ferrofluids mainly by the size of the suspended particles and the strength of the rheological effects. As modern theoretical approaches, like the one discussed by Liu et al in the second section have shown, the relation between the effects in ferrofluids and those in MR fluids is so close that it could probably be described in a common theory.
Sections four and five contain the application orientated results. In the fourth section the medical applications are the focus of interest. The section starts with a paper which could have also been placed in the synthesis section—the growth of magnetotactic bacteria and the extraction of the magnetic particles produced by these bacteria. The paper also contains information about the characterization of the particles especially with respect to their application. The characterization aspect is then continued in two papers outlining new diagnostic techniques with close relation to future biomedical application of magnetic fluids. Next in vitro applications, especially questions of cell separation using magnetic forces, are highlighted before the final papers address the therapeutic aspects of magnetic drug targeting and magnetic hyperthermia. Finally the fifth section describes three different new approaches for the technical use of ferrofluids. Again, the specialized design of the fluids themselves is an important step towards the new application goals.
Altogether the papers within this issue outline the unique potential of magnetically controlled suspensions, the interdisciplinary nature of the related research and the prospects of strongly networked and interdisciplinary activities in the field. I hope that it will give an insight into the fascination of ferrofluid research and a feeling for the advances made in the past years.