Magnetic colloids, also known as a ferrofluids (FFs), are colloidal suspensions of single-domain magnetic nanoparticles (ferro- or ferri-magnetic), with typical dimensions of about 10 nm, dispersed in a polar or non-polar liquid carriers like water, benzene, toluene or hexane1. These are often coated with surfactants to prevent agglomeration due to magnetic forces or van der Waals forces. The magnetic properties of these magnetically controllable nanofluids is of great scientific interest and is intensively researched2-4.
Ferrofluids contain about 1017 particles/mL and is opaque to visible light4. Ferrofluids posses magnetism and fluid behavior incorporated in them5. Since the rheological behavior of ferrofluids can be controlled by means of magnetic field, they can instantaneously change from a liquid to a solid like state and vice versa. They do not retain magnetism in the absence of an externally applied field and also form spikes in the magnetic field due to induced dipoles in the superparamagnetic nanoparticles aligning in the direction of applied magnetic field which can be observed only at or above a certain concentration typically beyond 10 vol %4.
The important applications of ferrofluids include semiactive shock absorbers, dampeners for seismic damage controls in civil engineering, in seals, valves, robotics and microelectronic devices5,6. Many of the ferrofluid applications require high magnetization capacity and long-term stability. The dimension of the nanoparticles and the steric and coulombic interactions between these particles are of prime importance in determining the colloidal stability7. According to Rosensweig (1985), an ideal ferrofluid is a colloidal suspension of nanoparticles of ferro- or ferrimagnetic materials dispersed in to a liquid carrier that does not settle out, even after long exposure to a force field (gravitational or magnetic)8.
1. C. Scherer and A. M. F. Neto, Brazilian Journal of Physics 2005, 35, 718.
2. P. Berger, N. B. Adelman, K. J. Beckman, D. J. Campbell, A. B. Ellis and G. C. Lisensky, J. Chem. Edu 1999, 76, 943.
3. P. Bain and T. J. McCarthy, Langmuir 2010, 26, 6145.
4. N. Jain, Y. Wang, S. K. Jones, B. S. Hawkett and G. G. Warr, Langmuir 2010, 26, 4465.
5. N. Jain, X. Zhang, B. S. Hawkett and G. G. Warr, ACS Appl. Mater. Interfaces 2011, 3, 662.
6. J. Fang, H. Wang, Y. Xue, X. Wang and J. Lin, ACS Appl. Mater. Interfaces 2010, 2, 1449.
7. J. A. Gomes, M. H. Sousa, F. A. Tourinho, R. Aquino, G. J. da Silva and R. Perzynski, J. Phys. Chem. C 2008, 112, 6220.
8. R. E. Rosensweig. Ferrohydrodynamics, Cambridge University Press, Cambridge. 1985.