A ferrofluid, influenced by a magnet underneath.
Ferrofluids comprise microscopic ferromagnetic nano-particles, usually magnetite, hematite or some other compound containing iron. The nano-particles are typically on the order of 10 nm. This is small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them to contribute to the overall magnetic response of the fluid. This is analogous to the way that the ions in an aqueous paramagnetic salt solution (such as an aqueous solution of copper sulfate or manganese chloride) make the solution paramagnetic.
True ferrofluids are stable. This means that the solid particles do not agglomerate or phase separate even in extremely strong magnetic fields. However, the surfactant tends to break down over time (a few years), and eventually the nano-particles will agglomerate, and they will separate out and no longer contribute to the fluid's magnetic response. The term magnetorheological fluid (MRF) refers to liquids similar to ferrofluids (FF) that solidify in the presence of a magnetic field. Magnetorheological fluids have micrometre scale magnetic particles that are 1–3 orders of magnitude larger than those of ferrofluids.
When a paramagnetic fluid is subjected to a sufficiently strong vertical magnetic field, the surface spontaneously forms a regular pattern of corrugations; this effect is known as the normal-field instability. The formation of the corrugations increases the surface free energy and the gravitational energy of the liquid, but reduces the magnetic energy. The corrugations will only form above a critical magnetic field, when the reduction in magnetic energy outweighs the increase in surface and gravitation energy terms. Ferrofluids have an exceptionally high magnetic susceptibility and the critical magnetic field for the onset of the corrugations can be realised by a small bar magnet.
Magnetite: 3-15 % by volume
Oil Soluble Dispersant: 6-30 % by volume
Carrier Liquid: 55-91 % by volume
CHEMICAL AND PHYSICAL PROPERTIES
Boiling Point (°F): 401-491
Specific Gravity: 0.92 to 1.47
Vapor Pressure (mm Hg.): 1 at 100°F
Percent Volatile by Volume: 55-91 %
Vapor Density (AIR = 1): 6.4
Solubility in Water: Negligible
Evaporation Rate at: <0.1
Appearance & Odor: Black liquid, Mild odor
FIRE AND EXPLOSION HAZARD AREA
Flash Point (°F): 160°
Flammable Limits: uel: 0.6 lel : 7.0 at 77°F
Extinguishing Media: Co2, Foam, dry chemical, water spray.
Special Fire Fighting Procedure: Avoid smoke inhalation. Water spray may
Unusual Fire and Explosion Hazard: None
HEALTH HAZARD AREA
Threshold Limit Value: 5mg/m3 for oil mist in air (OSHA
Regulations 29 CFR 1910-1000)
Effects of Overexposure: No experience with overexposure.
Prolonged or repeated contact with skin or
eye contact may cause irritation. Inhalation
of mist or vapor at high temperature may
irritate respiratory passages.
Emergency and First Aid Procedures:
Skin Contact: Wash with soap and water.
Eyes: Flush with water and consult a physician for
Inhalation of Smoke or Mist: Move to fresh air and refer to physician for
Ingestion: The material has minimal toxicity, but fluid
aspirated into the lungs during ingestion
could cause severe pulmonary injury.
You should not induce vomiting and should
seek medical attention if the material is
Ferrofluid under the influence of a strong vertical magnetic field.
Ferrofluids are used to form liquid seals (ferrofluidic seals) around the spinning drive shafts in hard disks. The rotating shaft is surrounded by magnets. A small amount of ferrofluid, placed in the gap between the magnet and the shaft, will be held in place by its attraction to the magnet. The fluid of magnetic particles forms a barrier which prevents debris from entering the interior of the hard drive. However, the ferrofluid is still similar enough in properties to a true liquid that it will not interfere with the spinning of the shaft.
Ferrofluids have friction-reducing capabilities. If applied to the surface of a strong enough magnet, such as one made of neodymium, it can cause the magnet to glide across smooth surfaces with minimal resistance.
Magnetorheological dampers of various applications have been and continue to be developed. These dampers are mainly used in heavy industry with applications such as heavy motor dampening, operator seat/cab dampening in construction vehicles, and more.
As of 2006, materials scientists and mechanical engineers are collaborating to develop stand-alone seismic dampers which, when positioned anywhere within a building, will operate within the building's resonance frequency, absorbing detrimental shock waves and oscillations within the structure, giving these dampers the ability to make any building earthquake-proof, or at least earthquake-resistant.
The United States Air Force introduced a Radar Absorbent Material (RAM) paint made from both ferrofluidic and non-magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce the Radar Cross Section of aircraft.
NASA has experimented using ferrofluids in a closed loop as the basis for a spacecraft's attitude control system. A magnetic field is applied to a loop of ferrofluid to change the angular momentum and influence the rotation of the spacecraft.
Magnetorheological Finishing, a magnetorheological fluid-based optical polishing method, has proven to be highly precise. It was used in the construction of the Hubble Space Telescope's corrective lens.
Ferrofluids have numerous optical applications due to their refractive properties; that is, each grain, a micromagnet, reflects light. These applications include measuring specific viscosity of a liquid placed between a polarizer and an analyzer, illuminated by a helium-neon laser.
In medicine, a compatible ferrofluid can be used for cancer detection. There is also much experimentation with the use of ferrofluids to remove tumors. The ferrofluid would be forced into the tumor and then subjected to a quickly varying magnetic field. This would create friction, yielding heat, due to the movement of the ferrofluid inside the tumor which could destroy the tumor.
Additionally heavy metals used in MRI could be enclosed in carbon "cages" to protect the body from these possibly harmful metals.
An external magnetic field imposed on a ferrofluid with varying susceptibility, e.g., due to a temperature gradient, results in a nonuniform magnetic body force, which leads to a form of heat transfer called thermomagnetic convection. This form of heat transfer can be useful when conventional convection heat transfer is inadequate, e.g., in miniature microscale devices or under reduced gravity conditions.
Ferrofluids are commonly used in loudspeakers to sink heat between the voice coil and the magnet assembly, and to passively damp the movement of the cone. They reside in what would normally be the air gap around the voice coil, held in place by the speaker's magnet. Since ferrofluids are paramagnetic, they obey Curie's law, thus become less magnetic at higher temperatures. A strong magnet placed near the voice coil (which produces heat) will always attract colder ferrofluid towards it more than warmer ferrofluid thus forcing the heated ferrofluid away, towards the heat sink. This is an efficient cooling method which requires no additional energy input.
If the shock absorbers of a vehicle's suspension are filled with ferrofluid instead of plain oil, and the whole device surrounded with an electromagnet, the viscosity of the fluid (and hence the amount of damping provided by the shock absorber) can be varied depending on driver preference or the weight being carried by the vehicle - or it may be dynamically varied in order to provide stability control. The MagneRide magnetic ride control or active suspension is one such system which permits the damping factor to be adjusted once every millisecond in response to conditions. As of 2007, BMW manufactures cars using their own proprietary version of this device, while GM (the first auto manufacturer to do so), Audi, and Ferrari offer the MagneRide on various models.
General Motors and other automotive companies are seeking to develop a magnetorheological fluid based clutch system for push-button four wheel drive systems. This clutch system would use electromagnets to solidify the fluid which would lock the driveshaft into the drive train.
*Above reference material from Wikipedia, the free encyclopedia
Ferrofluid: Magnetic Liquid Technology
A ferrofluid is a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier. The particles, which have an average size of about 100Å (10 nm), are coated with a stabilizing dispersing agent (surfactant) which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid. The surfactant must be matched to the carrier type and must overcome the attractive van der Waals and magnetic forces between the particles. The colloid and thermal stabilities, crucial to many applications, are greatly influenced by the choice of the surfactant. A typical ferrofluid may contain by volume 5% magnetic solid, 10% surfactant and 85% carrier.
Magnetic Behavior of Ferrofluid
In the absence of a magnetic field, the magnetic moments of the particles are randomly distributed and the fluid has no net magnetization.
When a magnetic field is applied to a ferrofluid, the magnetic moments of the particles orient along the field lines almost instantly. The magnetization of the ferrofluid responds immediately to the changes in the applied magnetic field and when the applied field is removed, the moments randomize quickly.
In a gradient field the whole fluid responds as a homogeneous magnetic liquid which moves to the region of highest flux. This means that ferrofluids can be precisely positioned and controlled by an external magnetic field. The forces holding the magnetic fluid in place are proportional to the gradient of the external field and the magnetization value of the fluid. This means that the retention force of a ferrofluid can be adjusted by changing either the magnetization of the fluid or the magnetic field in the region.
Ferrofluid Properties and Their Application
Ferrofluid is designed as a component of a device and therefore it must meet specific performance objectives of the device. The selection of ferrofluid depends on many factors such as environments, operating life, etc. There are many different combinations of saturation magnetization and viscosity resulting in a ferrofluid suitable for every application.
The performance and operating life of a product that uses ferrofluid can be significantly affected by the characteristics of the ferrofluid. From ferrofluids with low evaporation rate or vapor pressure to ferrofluids with viscosity-optimized products, the characteristics of ferrofluid can dramatically shape the capabilities of the end product.
Thermal conductivity of a ferrofluid depends linearly on the solid loading. Fluorocarbon based ferrofluids have the lowest thermal conductivity of all commercial ferrofluids, therefore they are the least desirable materials for heat transfer applications.
In devices, ferrofluids come in contact with a wide variety of materials. It is necessary to ensure that ferrofluids are chemically compatible with these materials. The fluids may be exposed to hostile gases, such as in the semiconductor and laser industries; to liquid sprays in machine tool and aircraft industries; to lubricant vapors in the computer industry; and to various adhesives in the speaker industry. Furthermore, ferrofluids may be in contact with various types of plastics and plating materials. The surface morphology can also affect the behavior of the fluid. The selection of ferrofluid is carefully engineered to meet application requirements.
Additionally, ferrofluids may be expected to perform at temperature of 150°C continuously or 200°C intermittently, in winter conditions (-20°C) and space environments (-55°C). They may also be required to withstand nuclear radiation without breakdown.
Characteristics of Ferrofluid that Affect Performance
The thermal stability of a ferrofluid is related to particle density. The particles appear to behave like a catalyst and produce free radicals, which lead to cross linking of molecular chains and eventual congealing of the fluid. Catalytic activity is higher at elevated temperatures and, therefore, ferrofluids congeal more rapidly at these temperatures.
High magnetization ferrofluids are of interest as they produce volumetric efficiencies of magnetic circuit designs leading to lightweight and lower cost products. They can also be used to reduce reluctance of magnetic circuits and fringing field thus increasing useful flux density in the air gap. The domain magnetization of magnetite ultimately limits the maximum magnetization value that can be realized in a ferrofluid.
**NOT RECOMMENDED FOR CHILDREN UNDER THE AGE OF 12
Ferrofluid Video: www.dansdata.com/images/magnets/ferrofluid.mpg