Ferromagnetic Liquids: Meaning, Properties and Uses

Ferromagnetic fluids are substances NASA first invented in 1963 by scientist Steve Papell. He created a liquid rocket fuel that could be attracted towards a fuel pump by exposing it to a magnetic field in a weightless environment. Later, the advancement of technology was based on the efforts of R.E. Rosensweig and his colleagues, who improved the quality and process of making ferromagnetic liquids. Recently in 2019, the University of Massachusetts and Beijing University were successful in making a permanently magnetic ferrofluid that remains magnetic even after removing the external magnetic field.

Ferromagnetic liquids

Ferromagnetic fluid is a colloidal liquid that is made of nanoscale ferromagnetic substances. The particle size of the ferromagnetic substance can be defined as a single domain particle with diameters from 40 to 200 Å. These particles have variable magnetic and hydrodynamic properties. They are suspended in a carrier fluid which commonly is organic solvents. Each particle is surfactant coated, which helps prevent the formation of clumps and decreases surface tension. That’s why it is said that the magnetic attraction between the nanoparticles is weaker than the Van der Waals force of the surfactant. The magnetization may be achieved through Brownian rotation, i.e., due to the physical rotation of the particle, or Neel’s rotation, i.e., within a static particle. Some examples of ferromagnetic fluids can be fluids composed of Iron, Nickel and Cobalt.

Properties Of Ferromagnetic Fluids

A typical ferrofluid comprises 5% solid component, 85% liquid component and the rest 10% is the surfactant. The major properties of the ferromagnetic fluid resemble the properties of ferromagnetic substances used in the process. Some of the general properties include:

1. The atomic dipoles of ferromagnetic substances have the same orientation as the external magnetic field.
2. There is a permanent dipole moment present in the ferromagnetic substances’ atoms that lie in their domain.
3. The magnetization intensity is large, positive and proportional to the magnetising field. The saturation of the magnetization varies from material to material.
4. The dipole moment of the ferromagnetic substances is very large and is in the same direction as the magnetising field.

1. The magnetic flux of the ferromagnetic substance is very large and positive as the field lines inside the material are very dense. Magnetic flux can be calculated as B =  μ0 (H + M)

 μ0 = magnetic permittivity of free space

 H = strength of the applied magnetic field

 M = intensity of magnetization

6. The magnetic susceptibility of ferromagnetic materials is also significant and positive which can be calculated by Xm = M / H

 M = intensity of magnetization

 H = strength of the applied magnetic field

1. The ferromagnetic substances lose their magnetic properties due to exposure to high temperatures on liquidation.
2. The relative permeability of a ferromagnetic substance is also significant and proportional to the magnetising field present inside the material.
3. The magnetising field inside the material is more powerful than the field applied.
4. As the ferromagnetic substances generate an attractive force, they tend to stick at the poles when kept in a non-uniform magnetic field as their strongest at that point.
5. When ferromagnetic substances are placed in a watch glass on two poles pieces kept at a distance, the substance accumulates at the edges, leaving a depression in the middle as the field is strongest at the poles.

Applications Of ferromagnetic liquids

A differentiating feature of ferromagnetic liquids is their multiple and important uses in various industries. That’s why many scientists have put in their efforts to produce stable magnetic fluids which can be used for technological advancements. Some of the major applications of ferromagnetic liquids in different fields are:

1. Technological uses

• Dynamic sealing: In multiple machines, two or more functional parts need to be isolated from each other but are in proximity. They have to carry energy in some form, like rotation from one part to another. These are then isolated with ferromagnetic fluids as they provide less resistance to the rotation but obstruct any passage between the two due to magnetic attraction.
• Heat dissipation: Some equipment heat during functioning and needs to be cool by using a good heat conductor and connecting it to a larger surface to dissipate maximum heat. The extraction can’t be done using solid material in some cases. In these cases, ferromagnetic liquids are used for, e.g., a loudspeaker.

2. Biomedical uses

• Magnetic drug targeting: The concept behind this application is that a ferrofluid bounded drug can be injected into a tumour and kept for some time using a focused magnetic field. The drug will perform its action with minimal side effects and in minimal dose, as the amount required will be significantly less in targeted therapy.
• Hyperthermia: Ferromagnetic liquids absorb electromagnetic energy at different frequencies from the water. This allows us to generate heat in a targeted part of the body without affecting surrounding tissues where the fluid is injected.
• Contrast-Enhanced MRI: One of the very popular uses of ferromagnetic fluids is the MRI, a powerful aid in diagnostic medicine. But experiments and research are being conducted to make biocompatible ferrofluid that can be selectively absorbed by a particular tissue, thus making it visible in MRI.

Conclusion

The invention of ferromagnetic fluids, i.e., the fluids which can generate attractive magnetic force when kept in a strong magnetic field, proved to be a boon for the scientific industry. Ferromagnetic fluids were initially unstable, but with the efforts of many scientists, a stable combination was also discovered. They are composed of three main components, i.e., the nanoparticles of a ferromagnetic substance, the surfactant and the carrier fluid. Ferromagnetic fluids have gained popularity as it has wide applications like rotator shafts, loudspeakers, rocket fuel, etc. Some examples of ferromagnetic fluids can be iron oxide, ferromagnetic liquids with cobalt and Nickel, etc.