What are Magnetic Liquids or Ferrofluids?
Simply put, magnetic liquids are colloidal suspensions of magnetic nanoparticles. In what follows, let's see if we can define what that means.
Colloids: (From Wikipedia) In general, a colloid or colloidal dispersion is a substance with components of one or two phases. A colloid mixture is a heterogeneous mixture where very small particles of one substance are distributed evenly throughout another substance.
- Examples of natural colloidal systems include rubber latex, blood, milk, ink, paint and egg-white. When small particles of either liquid or solid are suspended in a gas, the resulting mixture is called an aerosol. Examples of aerosols include fog (water suspension in air) and smoke (solid ash particulates in air), as well as some man-made ones such as spray perfumes and deodorants, insecticides, inhalants and spray paints.
- Typically, particles within a colloidal suspension range from about 1 nanometer (one billionth of a meter, about ten times the size of an atom) to about 1 micrometer (one millionth of a meter, about 100 times smaller than the diameter of human hair). In our case, we are interested in a colloidal suspension of magnetic particles that are about 10 nanometers (nm) in diameter and suspended in either water or oil.
Magnetic nanoparticles:There are a variety of ways in which nanometer-sized magnetic particles (for instance, of iron oxide, or rusted iron) can be synthesized easily. One such straightforward reaction is the precipitation of magnetite with ammonia out of an iron solution:
2 FeCl3 + FeCl2 + 8 NH3+ 4H2O → Fe3O4 + 8 NH4Cl
This chemical reaction is in fact used in our tutorial section below to create water-based ferrofluids. If you cannot wait to find out how to create your own ferrofluid, click here to see the tutorial.
Under normal circumstances, a suspension of magnetic nanoparticles is not necessarily stable on its own. Magnetic attraction between the particles, combined with surface-based effects (such as Van der Waal's forces), will result in quick agglomeration and settling of the magnetic phase. In order to prevent the particles from coming too close to each other, various surfactants may be used for different liquid carriers.
Surfactants: (From Wikipedia) The term surfactant is coined from the phrase "surface active agent". Surfactants are typically organic molecules that have both a hydrophilic ("water-loving") head and a hydrophobic (water-repelling) tail. In the context of ferrofluids, they are used to provide a layer of coating around each nanoparticle that keeps others apart, either through electrostatic means (in the case of ionic surfactants) or steric repulsion (in the case of nonionic surfactants with long molecular structures). Fig. 1 below illustrates the concept.
Fig. 1. Surfactants help to keep colloidal suspensions stable by preventing the nanoparticles from coming too close to each other. The repulsion mechanism is typically steric within organic liquid media (a), involving a long chain polymer with a hydrophobic tail that prefers to remain away from the nanoparticles and within the liquid medium. Water, on the other hand, is a polar solvent and charged surfactant molecules (such as tetramethylammonium hydroxide (CH3)4NOH)) will work well to keep the magnetic particles apart (b). Picture adopted from University of Wisconsin MRSEC web site.
Once particles are prevented from sticking to each other, the colloidal suspension may be stabilized, provided that external forces, such as gravity, do not settle them down. Fortunately, at an average diameter of 10 nm, the magnetic nanoparticles have enough kinetic energy at room temperature to overcome gravity and be uniformly distributed within their liquid medium.
Why Do We Care about Magnetic Liquids?
If there is interest to a particular topic in scientific and engineering research, it means that there are plenty of potential industrial or healthcare applications that give the research effort a commercial significance. This is also the case for research in magnetic liquids. Plus, they are fun to play with!
Existing Applications
Below are several examples of commercial applications of ferrofluids and magnetic nanoparticles.
- Liquid seals and bearings
  
Fig. 2. Ferrofluids can be held in place simply via permanent magnets. This, in turn, enables their use as low-friction liquid seals against pressure differences (in turbo-pumps, for instance). They also act as highly efficient liquid bearings. Pictures adopted from Liquids Research and Advanced Fluid Systems (acquired by FerroTec).
  
Fig. 3. Ferrofluids are also utilized in loudspeakers to enable enhanced thermal contact (for cooling) and better damping for the voice coils. Over 50 million loudspeakers sold in the US each year use ferrofluids in this context. Pictures adopted from Liquids Research and Advanced Fluid Systems (acquired by FerroTec).
 
Fig. 4. Magnetic liquids with larger particles (typically, ~ 100 nm or so) will change their viscosity significantly (sometimes, orders of magnitude) under applied magnetic fields. This happens because the particles align with the applied field and form chains across the width of the fluid container, impeding shear forces within the fluid. This phenomenon is utilized in magnetic inertial dampers and active shock absorbers. Pictures adopted from FerroTec.
 
Fig. 5. Magnetic nanoparticles functionalized (i.e., coated on the surface) with specific receptors or antibodies are used as contrast agents in transmission electron microscopy (TEM) studies of individual cells. Particles within the cells (as shown in the picture on the left) are taken up typically within small vesicles in a process called endocytosis. Pictures adopted from Europhysics News .
Fig. 6. Functionalized magnetic nanoparticles can also be used as contrast agents in magnetic resonance imaging (MRI). Seen here as bright spots are the major lymph nodes in mice -- such visualization could help in earlier and accurate diagnosis of cancer. Pictures adopted from Kobayashi, et. al., Cancer Research (63), p. 271-276, 2003. If your institution has access to the electronic version of that journal, you can find the paper here.
- Transformer coolant liquids
Fig. 7. Oil-based ferrofluids are used in large power transformers to improve both their dielectric properties and thermal performance.
A non-magnetic material within a ferrofluid displaces the magnetic medium. As such, it creates a hole inside the ferrofluid. When external magnetic fields are applied, this magnetic hole acts as if it possesses the negative of the magnetization of the surrounding medium, and is repelled by the magnetic field. This phenomenon has been used to separate mine ores based on their density differences. The approach is a clean alternative to chemical separation. We use the same principle in our own laboratory for cellular and particle manipulation.
Fun with Ferrofluids
Ferrofluids are liquids that respond to magnetic fields. As such, they offer not only immense opportunities for cutting-edge research in Nanotechnology, but also they are fun to play with!
Perhaps the most notable aspect of ferrofluids is their ability to form spikes on their surface under strong magnetic fields.
 
 
Fig. 8. Some examples of spike patterns on a ferrofluid surface. These images were taken using strong permanent magnets in the vicinity of the liquid surface. Pictures are adopted from Minako Takeno's web site about the Appearance of Magnetism. A separate movie showing ferrofluid spikes in action may be found here.
The spikes form due to surface instabilities: when a high strength magnetic field gradient is present near the ferrofluid surface, tiny microscopic ripples on the liquid surface get attracted by that field; as they rise from the surface, they feel even more attractive forces. Eventually, stable spike patterns form on the ferrofluid surface. Their attraction towards the field is balanced by surface tension and gravity. Changing either the surface tension of the ferrofluid carrier medium (e.g., by using different liquid solvents) or altering buoyancy forces (by immersing the ferrofluid inside a secondary, immiscible liquid) will change the density of these spike patterns.
The patterns get even more interesting if the ferrofluid is constrained to reside in a basically two dimensional topography inside a Hele-Shaw cell. See Fig. 9 below and click on the picture for a very interesting movie that depicts the kinds of interesting shapes that can be created inside such an enclosure.
Fig. 9. A drop of ferrofluid within a secondary, immiscible liquid is placed between two parallel plates with a narrow gap (a Hele-Shaw cell). As the intensity of the externally applied magnetic field is turned up, the ferrofluid drop transmogrifies into very interesting shapes and forms. Click on the picture above to see it in real-time. Picture adopted from the American Institute of Physics Gallery of Fluid Motion.
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