Investigation of various magnetic fluids by means of ultrasonic methods

Journal of Magnetism and Magnetic Materials 122 (1993) 70-73 Norlh-Holland

Investigation of various magnetic fluids by means of ultrasonic methods V.V. G o g o s o v , A.V. G o r b u n o v , S.N. Tsurikov, A.A. U s a n o v and A.N. V i n o g r a d o v Institute ~l' M('clmnics, Moscow Unicersity, Michurinsky Pr. 1, 117192 Moxcow, Russia Properties of magnetic fluids on the base of dodccane and water are studied using the results of tdtrasonic propagation experiments. Deviations of various magnetic fluid properties with time and with temperature change arc found. Effects ol dilution and magnetic field action on magnetic fluid properties are studied.

1. Introduction A method to study magnetic fluid properties by means of ultrasonic propagation was suggested by Gogosov et al. in refs. [1] and [2]. The model describing magnetic fluid as a disperse medium consisting of two phases: non-magnetizable dispersive phase and magnetizable phase-aggregates, is used. It is supposed that the aggregates occupy a volume F in a unit volume of the fluid and consist of magnetic particles and surfactant with volume concentrations (per unit volume of magnetic fluid) /~ and F,. respectively, F = / ; , , + I~. In the simplest case the aggregate consists of only one magnetic particle surrounded by a surfactant cover. The effective density of aggregates toll, the volume V:.... occupied by magnetite in the aggregate (l/i, is the volume of the aggregate), the size of aggregate D and the number of aggrcgates per unit volume n are the important parameters. The suggested method allows to determine all listed parameters if the ultrasonic propagation velocity a, the absorption coefficient ~ and the magnetic fluid density p are known from cxperiments.

2. Ultrasonic propagation in magnetic fluids. Experiments Magnetic fluids on the base of dodecane and water are studied in thc present work. Three Corresl~omh'nce to: V.V. Gogosov, h/slitule of Mechanics, Moscow University, Michurinsky Pr. 1, 117192 Moscow. Russial 0304-8853/93/$06.00 ,c~ 1993

series of experiments (1, II and I11) with thc same magnetic fluid on the base of dodecanc have been conducted. The saturation magnetization M~ at a temperature of 20°( " was equal to 31.7 k A / m . The interval between cxperiments I and 11 was 20 months, the interval between cxperiments 1 and 111 was 50 months. The density of the magnetic fluid p is found to decrease with time. In experiment 11 the valuc of p is smaller by 1.4% and in experiment 111 by 3.9c4: in comparision with experiment 1. This exceeds the experimental error, which is 0.2c;;. The ultrasonic velocity and the absorption cocfficient arc measured in experiments I and 111 in the frequency range 12-132 MHz, and in experimcnt I1 in the range 12-2000 MHz. Thc measurements on the magnetic fluid on the base of dodecane have been carried out in the temperature range - 1 ( ) + IO()°C. It is shown that in cxperiments I III the wfluc of a does Ilol dcpcnd on the ultrasonic frequency ,l in the limits of thc experimental error, the wdue of a increases with time, and decreases linearly as the temperature grows. The ratio c~/,/"e of thc absorption coefficicnt a, to the square of the frequcncy decreases csscntially in experiments !-I11 as thc frequency 1 grows (the ratio (~/f-" for the common dodccanc docs not dcpcnd on ,f). It is proposed to cxplain this frequency decrease by the fact that the basic dissipative mechanism in magnetic fluids is causcd by the friction between aggregates and dispersive liquid in contrast with common liquids where it is caused by viscosity. The h)rmulae describing the dissipation arising in ultrasonic propagation due

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to the relative motion of aggregates and dispersive liquid have been derived by Gogosov et al. in refs. [2] and [3]. As the temperature T grows from 0 to 40°C in experiments I - I I I , the quantity a l l 2 is practically constant and it increases as the temperature grows from 40 to 100°C. For pure dodecane the ratio a l l 2 decreases monotonically with the temperature. Six series of experiments with magnetic fluids on the base of water have been carried out. In experiment IV a magnetic fluid with a saturation magnetization of 16.7 k A / m was used. In experiments V and V1 magnetic fluids used were prepared from the initial one by dilution with water in the ratios of 1:1 and 1:3, respectively. In experiment VII the initial magnetic fluid which had been exposed in a uniform magnetic field of 5 k A / m for 2 h was used. After 5 months the specimen VI with dissolved magnetic fluid in the ratio of 1:3 was studied again before (experiment VIII) and after (experiment IX) exposing it to a magnetic field of 5 k A / m . The treatment of these experiments allows us to obtain information about the variation of magnetic fluid properties with time, after dilution and exposure to the magnetic field. The frequency f changes in the range 12-108 MHz, the temperature T changes from 0 to 80°C. Experiments with magnetic fluids on the base of water show that the ultrasonic propagation velocity a does not depend on the frequency and changes nonmonotonically with the temperature, fig. 1. In the limits of the double experimental error the magnetic field does not affect the de-

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pendence a versus T: a w = a v . , avm = a~x. The quantity a remains constant within 5 months: avl = awu. Dilution of magnetic fluid results in an increase of a and the fact that the dependence a(T) tends to a similar one for pure liquid. The ratio a / f 2 decreases monotonically with the frequency and depends on temperature nonmonotonically. As the temperature changes from 0 to 20-30°C the ratio a / f 2 decreases; it then remains invariable, and begins to increase with the growth of temperature from 60 to 100°C. Dilution causes an essential reduction of a / f 2 ; it is connected with the reduction of the number of aggregates per unit volume. For the magnetic fluids exposed to the magnetic field the quantity a / f 2 decreases. The diminution is significant at low frequencies and it reduces as the frequency and the temperature grow. The density of the magnetic fluid after interaction with the magnetic field is reduced by an order of 1%, which is the same for diluted and undiluted magnetic fluids. 3. D e t e r m i n a t i o n of the characteristic parameters of magnetic fluids

The method used to analyze the experimental data is the one described in ref. [2], and thus will not be described here. It results from this analysis that the part of volume occupied by magnetite in the aggregate, VmJV~,, in experiments I-1I increases significantly as the temperature grows, fig. 2. The quantity F m decreases very slowly as the temperature grows, fig. 3. These phenomena may be explained by surfactant transfer from the

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KId Gogosov et al. / lm,estigation of magnetic fluids

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bulk of aggregates to dispersive liquid as the temperature grows. This transfer is verified also by the increase of the effective density of aggregates and an essential diminution of F~, fig. 3, as the temperature grows. In the temperature range 0-50°C the behavior of Vma/Va, jOaq~and /7, in experiment III coincides with the data of experiment II within the double experimental error. At temperatures higher than 50°C the quantities Vma/Va ' p0 and F~ remain constant in experiment III while they increase significantly in experiment II, figs. 2 and 3. So we can come to the conclusion that the temperature effects on surfactant transfer from the aggregates to the dispersive liquid weaken with time. Let us consider the variation of parameters of the magnetic fluid with time at constant temperature. In experiment II the quantity F m is less than in experiment I and in experiment III this quantity is less than in experiment II, fig. 3. This may be explained by settlement of aggregates with time. It is worth noting that the value of F m at temperature 20°C calculated on the basis of the treatment of experiment lI is equal to 8.3%. At the same time the value of /],1, calculated from the independent measurements of the saturation magnetization of magnetic fluid is equal to 7.1%. The quantities V m J V a (fig. 2) and p2 in experiment II are less significant (F~ is more, fig. 3) than the same quantities in experiment I. In experiments III and II in the range 0-80°C they are practically equal, figs. 2 and 3. The deviation of the magnetic fluid properties may be explained

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by surfactant transfer from dispersive liquid to aggregates in the time interval between experiments I and II. This transfer is absent in the time interval between experiments 11 and III - the magnetic fluid became more stable. The effective aggregate size calculated using the absorption coefficient decreases with the growth of the temperature, from 360 ,~ at T = - 1 0 ° C to 130 A at T = 100°C. The number of particles increases significantly as the temperature grows. These phenomena can be explained by the formation of micelles from surfactant in dispersive liquid and fragmentation of the aggregates as the temperature grows. The behavior of the quantities Vm:,/V., Pll, 1~, /],, D and n in magnetic fluids on the base of water coincides qualitatively with the corresponding one in magnetic fluids on the base of dodecane, figs. 4-6. With temperature growth the parameters Vma/]/Ta (fig. 4), Pl~ and n increase. and the parameters I~ (fig. 5), I m and D (fig. 6)

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decrease. It is worth noting that dilution weakens the dependencies of these parameters on the temperature. At T = 20°C the quantity /'m (/'s) in magnetic fluids is equal to 8.7% (2.5%, experiment l) and 5.97% (9.96%), respectively. The surfactant coating in the magnetic fluid on the base of water is essentially larger and less stable with the change of the temperature in comparison with the magnetic fluid on the base of dodecane. The part of aggregate volume V m J V d occu-

73

pied by magnetite and the effective density p2 decreased after dilution, fig. 4. This means that the sufactant has transferred from the dispersive liquid to aggregates after dilution. The quantities VmJV,, and p0 remained practically the same after dilution in proportions of 1 : 1 and 1 : 3, fig. 4. It is seen from figs. 4 and 5 that the exposing of the magnetic fluid to the magnetic field does not affect the magnetic fluid parameters. No significant difference between undiluted and diluted (in the proportion of 1:3) magnetic fluids is observed in experiments VIII and IX in comparison with experiment V1. References [1] V.V. Gogosov, G.A. Shaposhnikova, S.N. Tsurikov, A.A. Usanov and A.N. Vinogradov, Magn. Gidrodin. 4 (1989) 29. [2] V.V. Gogosov, G.S. Nikolsky, S.N. Tsurikov, A.A. Usanov and A.N. Vinogradov, in: Electromagnetic Forces and Applications, eds. J. Tani and T. Takagi (Elsevier, Amsterdam, London, New York, Tokyo, 1992) p. 95. [3l V.V. Gogosov, S.I. Martynov, G.A. Shaposhikova and S.N. Tsurikov, Magn. Gidrodin. 2 (1987) 19.