ABSTRACT
Magnetic particle suspensions have an important potential for application and therefore a variety of studies on these functional fl uids have been conducted in various fi elds including the traditional fl uid engineering fi eld and the recent bioengineering fi eld where there is interest in drug delivery systems. In the fi eld of magnetic recording material, which is a typical application fi eld for magnetic particles, high-density recording materials [1, 2] and optical units [3-7], as shown in Fig. 1.1, have been actively studied. In these applications, suspensions composed of magnetic particles function as an intermediary medium to obtain an ultimate goal material. In another typical application fi eld of fl uid engineering, as shown in Fig. 1.2, the magneto-rheological effect, which implies that the apparent viscosity of magnetic suspensions varies due to the strength of an external magnetic and fl ow fi elds, has been applied in order to develop mechanical actuators and dampers [8, 9]. Functional particles responding to an external magnetic fi eld, which may be developed by coating materials with magnetic particles, have a feasible potential for innovative application [10]. This kind of composite particle is designed to be controlled by an applied magnetic fi eld whilst a non-magnetic material part of the composite particle has been synthesized to possess another specifi c function. In this application, the latter function constitutes the main function of the composite particle and the former function is used for transporting the particle to a specifi c site. Currently these types of magnetic composite particles are regarded as a key material in their application to the biomedical engineering fi eld [11-13]. Hence, there have been vigorous attempts to synthesize multi-functionalized composite particles that can include medicines as a goal in the application to a drug
delivery system [14-27], as shown in Fig. 1.3. From a suspension physics engineering point of view, the application of these higher functionalized magnetic particles to a drug delivery system is a signifi cantly hopeful application fi eld. From an academic point of view, it is a challenging subject to develop a technology employing a gradient magnetic fi eld for guiding composite particles to the site of a specifi c cancer cell and to administer the drug agents from the particle [28-36]. Moreover, their application to the fi eld of resource engineering and environmental engineering is also hopeful and challenging [37-39]. In these applications the precious metals or harmful substances dissolved in sea water are captured and recovered by magnetic composite particles using a gradient applied magnetic fi eld. As clearly demonstrated in these application examples, using the characteristic feature that their physical properties and behavior in a
fl ow fi eld can be controlled by an external non-uniform magnetic fi eld, functional suspensions composed of new composite magnetic particles offer signifi cant potential for expanding their application in various fi elds.
New magnetic particles (magnetic materials) such as magnetic recording materials have actively been developed by combining different kinds of molecules by many researchers [40], and representative magnetic recording materials are the FePt particles [1, 2]. In addition to the ordinary Fe2O3 or Fe3O4-based particles, there have been vigorous attempts to generate multi-functionalized magnetic particles in a variety of application fi elds. Such particles are synthesized by coating non-magnetic base particles with smaller magnetic particles or another magnetic coating material,
