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INTRODUCTION
Prior to recent years the engineering applications of fluid mechanics were restricted to systems in which electric and magnetic fields play no role. However, the interaction of electromagnetic fields and fluids has been attracting increasing attention with the promise of applications in areas as diverse as controlled nuclear fusion, chemical reactor engineering, medicine, and high-speed silent printing. The study of various field and fluid interactions may be divided into three main categories:
1. electro hydrodynamics (EHD), the branch of fluid mechanics concerned with electric force effects;
2. magnetohydrodynamics (MHD), the study of the interaction between magnetic fields and fluid conductors of electricity; and
3. ferrohydrodynamics (FHD), the subject of this work, which has become of interest owing to the emergence in recent years of magnetic fluids.
1.1 Scope of ferrohydrodynamics
Ferrohydrodynamics deals with the mechanics of fluid motion influenced by strong forces of magnetic polarization. Developing an understanding of the consequences of these forces occupies most of this book. It will be well at the outset to emphasize the difference between ferrohydrodynamics and the relatively better-known discipline of magnetohydrodynamics. In MHD the body force acting on the fluid is the Lorentz force that arises when electric current flows at an angle to the direction of an impressed magnetic field. However, in FHD there need be no electric current flowing in the fluid, and usually there is none. The body force in FHD is due to polarization force, which in turn requires material magnetization in the presence of magnetic field gradients or discontinuities. Likewise, the force interaction arising in EHD is often due to free electric charge acted upon by an electric force field. In comparison, in FHD free electric charge is normally absent, and the analog of electric charge, the monopole, has not been found in nature. An analogy between EHD and FHD arises, however, for charge-free electrically polarizable fluids exposed to a gradient electric field. A major difference from FHD is the magnitude of the effect, which is normally much smaller in the electrically polarizable media. This work is concerned exclusively with FHD; however, the reader interested in EHD or MHD will find excellent starting points in the references cited at the end of this chapter.
Ferrohydrodynamics began to be developed in the early to mid-1960s, motivated initially by the objective of converting heat to work with no mechanical parts. However, as colloidal magnetic fluids (ferrofluids) became available, many other uses of these fascinating liquids were recognized. Many of these ideas are concerned with the remote positioning and control of magnetic fluid using magnetic force fields. An aspect of this behavior is illustrated in the photograph of Figure 1.1.
Ferrohydrodynamics has inherent interest if for no other reason than the uniqueness of fluid having giant magnetic response. As a result, a number of striking phenomena are exhibited by the magnetic fluids in response to impressed magnetic fields. These responses include the normal field instability, because of which a pattern of spikes appears on the fluid surface; the spontaneous formation of intricate labyrinthine patterns in thin layers; the generation of body couple in rotary fields, which is manifested as antisymmetric stress; unusual buoyancy relationships, such as the self-levitation of an immersed magnet; and enhanced convective cooling in ferrofluids having a temperature-dependent magnetic moment. It is a major objective of this work to build a significant understanding of the subject, based on the continuum-mechanical approach as augmented, where needed, by the microscopic description.
Demonstrated applications of ferrofluids span a very wide range. Actual commercial usage presently includes novel zero-leakage rotary shaft seals used in computer disk drives (Bailey 1983), vacuum feedthroughs for semiconductor manufacturing and related uses (Moskowitz 1975), pressure seals for compressors and blowers (Rosensweig 1979a), a...
