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!
Below are several examples of commercial applications of ferrofluids and magnetic nanoparticles.
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.
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.