Magnetic Nanoparticles in Fluid Suspension: Ferrofluid Applications
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Ferrofluids are synthesized colloidal mixtures of nonmagnetic carrier liquids, typically water or oil, containing single-domain, permanently magnetized particles, typically magnetite, with diameters on the order of 5–15 nm and volume concentration up to about 10%. A 10% by volume magnetite ferrofluid would have a saturation magnetization of about 0.0560 T = 560 G (1 T = 10,000 G). Brownian motion keeps the nanoscopic particles from settling under gravity and a surfactant layer, such as oleic acid, surrounds each particle to provide short-range steric hindrance and electrostatic repulsion between particles to prevent particle agglomeration. A polymeric layer surrounding each nanoparticle may also serve this purpose. These nanoparticle coatings allow ferrofluids to maintain fluidity even in intense, high-gradient magnetic fields. The study and application of ferrofluids, invented in the mid-1960s, involves interdisciplinary science and technology integrating chemistry, fluid mechanics, and magnetism. Because of the small particle size, ferrofluids involved nanoscience and nanotechnology from their inception.
Conventional ferrofluid applications use d.c. magnetic fields from permanent magnets for use as liquid O-rings in rotary and exclusion seals, bearings, as dampers in stepper motors and shock absorbers, in magnetorheological fluid composites, for heat transfer in loudspeakers, in inclinometers and accelerometers, for grinding and polishing, and in magnetocaloric pumps and heat pipes. Ferrofluid is used in over 50 million loudspeakers each year. Almost every computer disk drive uses a magnetic fluid rotary seal for contaminant exclusion and the semiconductor industry uses silicon crystal-growing furnaces that employ ferrofluid rotary shaft seals. A representative seal can withstand a pressure difference of up to 1.5 MPa (∼ 15 atm) at a speed of 7000 rpm. Ferrofluids are also used for the separation of magnetic from nonmagnetic materials and for the separation of materials by their density using a nonuniform magnetic field to create a magnetic pressure distribution in the ferrofluid that causes the fluid to act as if it has a variable density that changes with height. Magnetic materials are attracted to the regions of strongest magnetic field, whereas nonmagnetic materials are displaced to the regions of low magnetic field with matching effective density. Magnetomotive separations use this selective buoyancy for sink–float separation of materials such as ore minerals, one novel application being the separation of diamonds from beach sand.
Ferrofluids are a multifunctional medium that allow applications in each of its constituent disciplines of chemistry, fluid mechanics, and magnetism. With modern advances in understanding nanoscale systems, current research focuses on synthesis, characterization, and functionalization of nanoparticles with magnetic and surface properties tailored for application as microelectromechanical/nanoelectromechanical sensors, actuators, in microfluidic/nanofluidic devices, as nanobiosensors, as targeted drug delivery vectors, in magnetocytolysis of cancerous tumors, in hyperthermia, in separations and cell sorting, for magnetic resonance imaging (MRI), and in immunoassays.