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.
Reference
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.