Journal of Magnetism and Magnetic Materials 65 (1987) 242-244
North-Holland, Amsterdam
EXPEMMENTAL METHOD FOR THE PURIFICATION AND RECONDITIONING OF FERROFLUIDS
Constantin COTAE
Physics Department, Polytechnic Institute of Jassy, Jassy 6600, Romania
The paper presents the theoretical aspects regarding the magnetogravimetric purification of ferrofluids both in the process of preparation and for their reconditioning from impurities.
An experimetal device used for magnetogravimetric purification is described together with experiments on some samples of oil-based ferrofluid that became impure with non-mixible solid, liquid, magnetic and nonmagnetic ingredients.
The experiments resulted in a complete purification of the ferrofluid samples.
1. Introduction
Due to their submicron-sized particles in suspension, the action of a surfactant and the thermal agitation, ferrofluids are quite stable in time in a magnetic field; or in a magnetic field gradient. However, a large dimensional distribution of the particles in suspension (in the process of preparation) results in the formation of particle chains and concentration germs which, in their turn, lead to a phenomenon of self-concentration aglomeration and sedimentation, respectively. This is facilitated by, the existence of a magnetic field gradient as well as by the impurities in the ferrofluid introduced during its utilization.
There are several methods for the purification and reconditioning or ferrofluids, based either upon the action of the centrifugal force - centrifugal purification [1], that of the gravitational force - the sedimentation method [2], or of the volume, levitation and gravitational forces - the magnetogravimetric method [3].
The present paper describes the magnetogravimetric method and a device for the purification and reconditioning of ferrofluids according to this method.
2. Theoretical aspects
The magnetogravimetrio force acting upon objects immersed in ferrofluids is known to represent the force resulting (F) from combining the magnetic (FMx) and the gravitational forces (FG):
Under the action of a magnetic field gradient, a volume magnetic force is induced in ferrofluids:
which is equal and contrary to the levitation force acting on nonmagnetic (or low magnetic) objects immersed in the ferrofluid [4-6] and which can be written as:
where Fli is the levitation force along direction i, Vs the volume of the immersed object, M the ferrofluid magnetization, dH/dx the magnetic field gradient (dH/dx < 0) and μ0 the magnetic permeability in vacuum (4π10-7 H/m).
The resulting magnetic force acting on the unit volume of the object in the ferrofluid is [3,4]:
where M is the ferrofluid magnetization and Ms the magnetization of the object immersed in the ferrofluid.
The gravitational force acting on the unit volume of the object in the ferrofluid is:
where ρs and ρf are the densities of the object of the ferrofluid, and j the unitary vector on the vertical.
By substituing (4) and (5) in (1) we get:
Thus when the resulting magnetic force is directed horizontally, the magnetogravimetric force (F) acting upon the impurities in the ferrofluid (fig. 1) results in their moving at an angle to the vertical, which depends on M, Ms, ρf, ρs, dH/dx, and in a ferrofluid volume free of impurities.
3. Experimental method and device
Fig. 2 shows the device used in our experiments. It consists of a nonmagnetic (plexiglas) container, a feeding pipe (a) and an exit section (b) placed in a horizontally oriented magnetic field gradient. The container is provided with rooms for impurities in both its upper and lower parts.
The magnetic field gradient is achieved with a wedge - shaped electromagnet with pole pieces.
The ferrofluid used in our experiments was an oil-based one of magnetization 24x103 A/m, density 1.25x103 kg/m3 and viscosity 4.5x10 -3 Ns/m2. •
The impurities were introduced artificially and consisted of nonmixible ingredients, namely copper, aluminum, iron powder and water.
During the experiments we also used a comparison sample of the same ferrofluid which went through the same purification treatment without having got any impurities.
The experiments were carried out separately for each type of impurity as well as simultaneously, with all impurities introduced together.
The ejection speed of the purified ferrofluid was 0,2 l/h and it was controlled through the feeding pressure and the top on the exit section.
4. Results and discussion
By treating the ferrofluid samples successively at a magnetic field gradient 1.6, 1.3 and 1.0 kOe/cm, for the samples treated magnetogravimetrically the ferrofluids obtained at the end were of density 1.2x103 kg/m3, magnetization 22.4x103 A/m, and viscosity 4.5x10-3 Ns/m2.
The same characteristics were also obtained for the comparison sample after it had gone through the .same process of magnetic purification.
As the characteristics of the ferrofluid obtained after magnetogravimetric purification are the same both for the impure samples and for the comparison sample it results that the ferrofluid purification is complete.
We take it that the lower value for the ferrofluid magnetization and density in all cases is due to the separation of a fraction in the magnetic (solid), unstabilized phase in suspension. The large dimensional distribution of the particles hi suspension results in the formation of particle chains and germs of concentration and sedimentation which, in a magnetic field gradient, are moved to the sections of maximum field and taken out of the ferrofluid, which leads to lower values of ferrofluid density and magnetization.
References
[1] R. Kaiser and G. Miskolczy, J. Appl. Phys. 41 (1970) 1064.
[2] I.Mindru and D.M. Cecareanu, Chimia coloizilor si suprafetelor, metode experimentale (Editura Technica, Bucureşti (1976) p. 417.
[3] C. Cotae and Gh. Calugaru, Patent R.S.R. 75 690, (1979).
[4] R.E. Rosensweig, AIAA J. 4 (1966) 1751.
[5] S.A. Khalafalla, IEEE Trans, on Magn. MAG-12 (1976) 455.
[6] E. Luca, Gh. Calugaru, R. Badescu, C. Cotae and V.Badescu, Ferofluidele si aplicatiile lor industriale (Editura Tehnica, Bucureşti, 1978) p. 336.
