Ferrofluids are made by grinding magnetic particles together with a polar surfactant and a nonpolar solvent. The surfactant is adsorbed on the particle surfaces and acts as a coupling agent between the particles and the solvent

Rosensweig, R. E.

English

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RI1O 1313 
July60	 KSC liOS	 Brief 70-10011 
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Preparation of Magnetic Ferrofluids

in Alternative Carrier Liquids 
Ferrofluids can be made by prolonged grinding of 
particles of magnetic material such as magnetite or 
ferrite, together with a polar . surf actant and a nonpolar 
solvent. The surfactant is adsorbed on the surface of 
the submicron particles and acts as a coupling agent 
between the particles and the solvent in which the 
particles are dispersed. 
Magnetic particles in a ferrofluiô remain in suspen-
sion under the influence of applied magnetic fields 
because the submicron particle size allows particle 
motion to be maintained by thermal agitation, and 
the surfactant keeps the particles far enough apart to 
overcome the force of attraction caused by Van der 
Waals forces. 
A method for substituting one ferrofluid solvent 
for another is available. The introduction of a polar 
flocculating solvent into the ferrofluid causes the 
particles with adsorbed surfactants to come out of 
suspension. This solvent reduces forces opposing the 
Van der Waals forces so that the particles are attracted 
to one another and Waals separate from solution. 
Shrinkage of the thickness of the surfactant coating 
enables the Van der Waals forces to bring and keep 
adjacent particles together. Alternatively, the solvent 
can act to link particles, causing flocculation and
ultimately the separation of particles and solvent. 
Resuspension of the particles in another solvent com-
pletes the process. 
The invention provides a means for formulating 
ferrofluids containing solvents that are not suitable as 
grinding or manufacturing aids, and for altering the 
particle concentration in a ferrofluid. 
Note: 
Related background information on ferrofluicis 
may be found in Patent No. 3,281,403 and in an article 
entitled "Magnetic Fluids" by R. E. Rosensweig, in 
the July 1966 issue of International Science and Tech-
nology. 
Patent status: 
Title to this invention has been waived under the 
provisions of the National Aeronautics and Space Act 
[42 U.S.C. 2457 (f)], to AVCO Applied Technology 
Division, Lowell Industrial Park, Lowell, Massachu-
setts 0185i.
Source: R. E. Rosensweig of

AVCO Applied Technology Division

under contract to

Goddard Space Flight Center

(GSC-10159) 
Category 04 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned.nghts.
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Preparation of magnetic ferrofluids in alternative carrier liquids

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