Engineered ‘smart' fluids, called ferrofluids, which permit static and dynamic control by external magnetic fields of low or moderate strengths present a novel and challenging domain for scientists and technologists. The introduction of a ‘controllable' or ‘tunable' external force into the momentum transport equations opens up new fields of physical phenomena. In situations where the external field influence becomes strong enough to compete with gravitational forces or acts as a sole external agency to drive flows in the hypo-gravity environment, a new class of hydrodynamic phenomena becomes accessible. Ferrofluids exhibit special rheological properties that make them suited for a number of technological and biomedical applications. This chapter outlines the underlying physics of field-fluid interactions on one hand; on the other, it cites novel techniques in ferrofluid-actuated electronics cooling, hydrostatic & hydrodynamic bearings, extreme-boundary lubrication, damping systems, biomimetic locomotion, and medical diagnostics (e.g. magnetic drug targeting & magnetic cell sorting).
TopIntroduction
Design of applications that employ synthetic fluids (as active or passive components) gains new possibilities, if the fluids can be conveniently positioned or moved by electromagnetic forces of low or moderate field intensities (Gast & Zukoski, 1989). As magnetic forces can be generated and controlled with minimum technical effort, e.g. by varying the electric current intensity through a coil, new ideas using an additional control parameter can be realized to suit a broad range of macro-scaled and μ-scaled applications (Liu, 1993). At small length scales (e.g. in the domain of μ-fluidics), the transport of momentum and energy is largely diffusion-limited due to very low values of flow Reynolds numbers (Das, Choi, Yu, & Pradeep, 2008). A suitable magnetically-susceptible fluid proves as a viable working medium to enhance energy and species transport in these devices (Rosensweig, 1997), as its flow pattern can be influenced by external actuations. In hypo-gravity and extra-terrestrial environments, for instance, convective transport of mass, momentum and energy can only be achieved through external field actuation (without the aid of moving parts).
Effective field-fluid interaction, over a broad spectrum of technical applications, is realizable only for a magneto-rheological material with magnetic susceptibility proportional to density (Rosensweig, 1997; Liu, 1995a). The induced body force, called the Kelvin force, depends dually on the fluid magnetization and the field gradient within the medium (Rosensweig, 1997; Liu, 1995b). In paramagnetic salt solutions, for example, feeble magnetization renders the Kelvin force negligible (as compared with the g-force) even for strong magnetic fields. Liquid metals are also susceptible to Lorentz forces in strong magnetic fields; however, noticeable changes in flow-patterns are affected only by extremely high magnetic field strengths (in the order of several Tesla). In addition, liquid metals usually require a high-temperature environment. On the other hand, under-cooled metallic melts (found to exhibit magnetic ordering even in the liquid state) does not allow any handling of the liquid. Therefore, their magnetic properties cannot be used in practical designs. The viable option, then, is to produce stable suspensions of magnetic nanoparticles in appropriate carrier liquids. These suspensions (called ferrofluids) exhibit high initial susceptibility (consequently, high magnetization) even for (easily achievable) magnetic field strengths ~50mT (Liu, 1995a; Odenbach, 2002).
A ferrofluid is a colloidal suspension of single-domain magnetic nanoparticles (~3-15nm diameter) in a nonmagnetic liquid. By single-domain is meant that the magnetic moments of the individual atoms are oriented in a fixed direction. The bulk medium exhibits a magnetic behavior only when an external magnetic field is imposed, whereby the thermally-disoriented magnetic moments of the particles are aligned. The nanoparticles are coated with adsorbed surfactant layers to prevent agglomeration due to the van der Waals forces and dipole-dipole interactions. Such a stable colloid can be considered as a quasi-single-phase homogeneous liquid and the laws of continuum mechanics applied (Jiang & Liu, 1996).
The engineering utility of ferrofluids over other magneto-rheological (MR) liquids arises from the fact that their flow behavior and properties can be desirably altered in the presence of external magnetic fields of even low or moderate intensity. The possibility to exert an externally controllable force to a fluid medium, in absence of moving parts, offers broad avenues of research in basic fluid dynamics and transport phenomena. In addition, it holds promise for a wide spectrum of applications in areas of biomedicine, thermal management of miniaturized electronics applications, electrical drives and actuators, worm-like locomotion of target-delivery agents, positioning systems, hydrostatic and hydrodynamic bearings, extreme-boundary lubrication, and damping systems. Many of these concepts have been practically implemented and are currently in use (Rosensweig, 1997; Odenbach, 2002; Odenbach, 2009).