Progr Colloid Polym Sci (2001) 117: 131±135 Ó Springer-Verlag 2001
V. Socoliuc D. Bica
V. Socoliuc (&) National Institute for Research and Development in Electrochemistry and Condensed Matter, Tirnava 1, 1900 Timisoara, Romania e-mail:
[email protected] D. Bica Center for Fundamental and Advanced Technical Research, Romanian Academy, Bd. Mihai Viteazul 24, Timisoara, Romania
NANOSTRUCTURED MATERIALS
Experimental investigation of magneticinduced phase-separation kinetics in aqueous ferro¯uids
Abstract In this article we report microscopy and light scattering investigation of phase-separation phenomenon in ferro¯uids. The in¯uence of the temperature and an external magnetic ®eld on the kinetics and on the quantitative extent of the phase separation was investigated.
Introduction Ferro¯uids, or magnetic colloids, are suspensions of magnetic nanoparticles dispersed in a carrier liquid. In order to prevent agglomeration due to attractive van der Waals or magnetic dipole±dipole interactions, in addition to the Brownian motion, a repulsive force between the particles is created by means of steric hindrance or electrostatic repulsion. The ideal ferro¯uid is a homogenous dispersion of isolated particles. Phase separation is one of the main phenomena that leads to radical changes in the ferro¯uid colloidal stability and structure [1]. When the temperature or the external magnetic ®eld strength exceed their critical values, the formation of highly concentrated phase droplets may occur in the ferro¯uid. In this article we present the results of research on the in¯uence of the temperature and an external magnetic ®eld on the kinetics of phase-separation phenomena in an aqueous ferro¯uid as well as on the size, shape and space con®guration of the condensed-phase drops.
Sample description An aqueous ferro¯uid was prepared for the investigations with a volume fraction of 3% magnetic particles
Key words Ferro¯uids á Colloidal stability á Phase transition á Phase separation
stabilized with dodecyl benzene sulfonic acid produced by means of a chemical coprecipitation method [2, 3].
Experimental Optical microscopy investigations of the ferro¯uid condensation phenomena were carried out. A special electromagnet and a sample thermostat were designed and adapted to a metallographic microscope with a maximum magni®cation of 1500 ´. The magnetic ®eld range was 0±1.5kOe, the ®eld gradient in the sample region was less than 10Oe/mm at 0.5 kOe and about 50Oe/mm at 1.5kOe and the magnetic ®eld transient time was less than 1 s. Wide-angle forward scattering of light was investigated by recording the scattering pattern projected on a screen with a charge-coupled-device camera (Fig. 1). This technique allows the instantaneous sampling of the entire scattering pattern and the recording of its time evolution. A He±Ne laser beam polarized perpendicular to the magnetic ®eld direction was used as incident light. Detection of the light transmitted in the incident beam direction was performed (Fig. 2). The He±Ne laser incident beam was polarized at 54.7° with respect to the ®eld direction in order to annul the dichroism induced by the magnetic particle orientation and agglomeration [4]. The scattered light was ®ltered out with an iris. In order to compensate the laser emission ¯uctuations the incident beam was split and both beams were ®ltered and focused on two identical large-area photodetectors (IPL10530DAW Integrated Photomatrix) with incorporated operational ampli®ers. The signals from the photodetectors were fed into a National Instruments LabPC+ data acquisition board; data processing and signal
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Fig. 1 Wide-angle light scattering experiment setup
Fig. 2 Forward scattering experiment setup
referencing were made with a virtual instrument developed with LabView. The ferro¯uid was contained in a 10-lm light-path cell with a detachable window. The magnetic ®eld was perpendicular to the incident light beam and parallel to the cell plates. A Weiss electromagnet was used to produce a static magnetic ®eld in the range 0±4kOe with less than 10Oe/cm gradient. The magnetic ®eld strength was measured with 10Oe precision. The sample temperature was measured with 0.1 °C precision and was controlled with a water thermostat system and a specially designed cell mount.
temperature decrease. At low temperatures the density of the drops increases signi®cantly with increasing ®eld intensity. At temperatures above 30 °C no condensed-phase drops were observed even at the highest ®eld value (1.5kOe), while at temperatures below 20 °C condensedphase drops were observed for ®eld values as low as 10Oe.
Results and discussion Optical microscopy investigation After the sudden onset of the 1.5kOe magnetic ®eld, a high density of small acicular drops (primary drops) of the condensed state form in the ferro¯uid, aligned parallel to the magnetic ®eld direction. Their length is about 5 lm and the thickness was estimated to be less than 1 lm. From the beginning of their formation the primary drops move chaotically inside the cell, while the formation of larger drops (secondary drops about 20 lm long and 1 lm thick) begins (Fig. 3). The primary drops vanish as the number of secondary drops increases. The secondary drops also drift within the uncondensed ferro¯uid but their motion is much slower than that of the primary drops. Owing to their large magnetic moment aligned in the ®eld direction and to the magnetic dipole attraction, the secondary drops stick together head to tail, thus leading to the formation of very elongated drops (some of them exceeding 100 lm). Similar evolution resulted from theoretical modeling of phase condensation in ferro¯uids [5] and was also observed experimentally [6]. After about 1 min the drops no longer grew and they settled in a fairly stable space con®guration owing to magnetic lateral repulsion. The temperature was found to have little in¯uence on the dimensions of the drops but their density increases with a
Light scattering experiment The pasted (negative) images of the scattering pattern at several moments of time after the ®eld settling subsequent to its sudden onset are shown in Fig. 4. The spots at the bottom of the picture are the images of the unscattered light beam projection. The sample temperature was 27 °C, the applied ®eld was 3kOe and the ®eld transient time was less than 1 s. The pronounced lack of circular symmetry of the scattering pattern is characteristic for very elongated scatterers [7]. One can observe that immediately after the onset of the ®eld the diraction pattern presents a minimum in the vicinity of the central spot. Over time the minimum moves toward the central spot and the scattered light intensity increases over the entire angular range. The angular dependence of the scattered light intensity is plotted in Fig. 5 for several moments after the onset of the ®eld. The data were obtained from vertical gray-level distribution sampling of the patterns plotted in Fig. 4. Once again one can observe the displacement of the minimum toward the central spot and also the increase in the scattered intensity at the minimum corresponding angle. An important question arises regarding the origin of the local minim in the scattered light pattern (Fig. 5). It is well known that light scattering in ferro¯uids with droplike aggregates originates from the light diraction on the drops of the condensed phase which, since their
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Fig. 3 Optical microscopy image of the condensed phase drops elongated in the external magnetic ®eld direction for H 1.5kOe at a 21 °C and b 28 °C
particle density is much greater than the uncondensed state, are opaque to visible light. Following Ref. [7], in the case of very elongated scatterers, the scattered light angular dependence can be well approximated as sin u 2 I
h ;
1 u
where u 2pns/kásin(h/2), where s is the drop thickness, n is the refractive index of the uncondensed phase, k is the wavelength of light in a vacuum and h is the scattering angle. If one assumes that the light scattered by the ferro¯uid sample is the result of the incoherent superposition of light diracted by individual drops, the diraction function (sinu/u)2 should ®t the experimental
Fig. 4 Time evolution of the scattered light patterns after the ®eld setup as recorded with a charge-coupled-device camera (27 °C, H 3kOe)
Fig. 5 Angular distribution of the scattered light at several moments of time after the onset of the ®eld
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data quite well. The diraction function with a ®rstorder minimum angle and a ®rst-order maximum amplitude corresponding to the experimental data at t 0.3 s is plotted in Fig. 4. The experimental angle of the ®rst-order maximum is nearly twice the theoretical value and both the width and magnitude of the experimental zero-order maximum are much smaller then the theoretical values. On the other hand, the drops thickness that results from Eq. (1) with the minimum corresponding angle and the refractive index of water (n 1.5) is about 10 nm, which is an order of magnitude greater than the value estimated from microscopy investigations; therefore, we infer that the minimum in the scattered light originates from the coherent interference of the light diracted by individual drops. The space con®guration of the condensed-phase drops in the ferro¯uid could be modeled as the superposition of the light scattered by a series of ideal diraction gratings with the space between neighboring drops distributed over a certain range of values. Thus, one can model the angular dependence of the scattered light as the incoherent superposition of the light coherently scattered by pairs of neighboring drops as 0 2pns sin
h=212 sin k A I
h cos4 h@ 2pns sin
h=2 k
Z D
! 2 sin pNnDk sin h g
D; d0 ; wdD : sin pnDksin h
2
The term cos4(h) describes the perpendicular polarization of light relative to the elongation direction of the drops, the second term describes the angular dependence of the light diracted by individual drops and the integral describes the incoherent summation of the interference term of pairs of drops (N 2) over the distribution g(D,d0,w) of the distance between neighboring drops, where d0 and w are the distribution parameters. In Fig. 6 the angular dependence of the interference integral is plotted for d0 2.5 lm and several values of w assuming a Gaussian distribution of the distance between drops and for d0 5 lm and w 2 lm. As the distribution width increases, the integral becomes smother and constant at high values of the scattering angle, while the ®rst-order minimum remains dependent on d0. The minimum corresponding angle is related to the mean distance between drops by the following approximate equation nD
k ; 2 sin hmin
0 2p
ns sin
h=212 sin k I
h cos4 h@ 2p
ns sin
h=2 A :
4
k
Following the previous discussion one can use Eq. (3) to compute the mean distance between drops and Eq. (4) to determine the mean thickness of the drops times the uncondensed phase refractive index by ®tting the experimental data at high scattering angles. The time dependence of the mean thickness and spacing of the drops times the matrix refractive index is plotted in Fig. 7. At the beginning of the condensation process the drops thickness is constant at about 0.8 lm for about 5 s and afterwards it slowly and asymptotically increases toward about 1.1 lm. The spacing between the drops increases constantly from 3 to 8 lm, mainly owing to the process of coalescence of primary drops as observed in the microscopy investigations. Although it was not possible to determine the refractive index of the condensed phase, it is reasonable to assume that it is independent of the drop size. Light extinction experiment The time dependence of the scattered light intensity (normalized to the transmitted intensity in the absence of the ®eld) was measured between 15 and 35 °C, after the sudden onset of the magnetic ®eld for several values of its magnitude (the data measured at 23 °C are plotted in Fig. 8). The magnetic ®eld transient time was less than 1 s. It was found, as an example, that for ®eld values below 0.5kOe no light scattering occurs at 27 °C, while at 23 °C light scattering occurs at ®eld values as low as 0.15kOe. The lower the temperature and the higher the magnetic ®eld value, the more pronounced the light scattering.
3
while at high scattering angles the experimental data are approximately well described by the polarization term times the diraction term, since the integral of the interference term is independent of the scattering angle:
Fig. 6 The interference term for a pair of drops integrated over the Gaussian distribution of the spacing between drops for several values of the parameter w
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Fig. 7 Time dependence of the mean thickness of the drops and the mean spacing between drops
Depending on temperature, above a certain value of the magnetic ®eld the scattered light intensity reaches a local maximum. The time until the scattered light intensity reaches the local maximum increases with temperature and decreases with magnetic ®eld value. One can divide the time evolution of the scattered light into two stages: prior and subsequent to the local maximum corresponding moment. On basis of the microscopy observations one can infer that at the beginning of the ®rst stage light scattering is mainly due to the formation of primary drops. The transient time of the scattered light was found to be independent of ®eld intensity but it increases with decreasing temperature. At temperatures below 20 °C light scattering occurs for magnetic ®eld values as low as 10Oe and above 30 °C no scattering was observed even at the highest magnetic ®eld value (3kOe), in good agreement with microscopy observations. For several temperature values the critical ®eld value (Hc) was determined at which the drops begin to scatter the light. The data was ®tted with the equation derived by Cebers [8] for the dependence on temperature of the critical ®eld at which phase condensation occurs: H c
t
t
1 ; t1
5
Fig. 8 Time dependence of the scattered light intensity normalized to the transmitted intensity in the absence of the ®eld at 23 °C for several values of the magnetic ®eld intensity
where t1 is the temperature above which phase condensation does not occur no matter how strong the magnetic ®eld is. As a result of the ®tting of the experimental data with Eq. (5), a value of 31.7 °C was found for t1 , which is in good agreement with the observations from microscopy and scattering investigations.
Conclusions The eect of magnetic-induced phase condensation on the aqueous ferro¯uid investigated is that the ferro¯uid becomes a biphasic system i.e. condensed-phase droplets form in equilibrium with the uncondensed phase matrix. The density of the condensed-phase droplets increases with decreasing temperature and increasing magnetic ®eld intensity. While the length of the droplets at equilibrium increases with ®eld intensity, their thickness is independent both of temperature and ®eld intensity. Above 32 °C, magnetic-induced phase condensation does not occur. The kinetics of phase condensation was found to be in¯uenced mainly by the temperature. The growing process of the condensed-phase drops was found to evolve in two stages: small primary drops grow and stick together into large secondary drops, similar to the theoretical predictions reported in Ref. [4].
References 1. Berkovski B, Bashtovoy V (1996) Magnetic ¯uids and applications handbook. Begell House, New York 2. Bica D (1985) Romanian Patent 90078 3. Bica D (1995) Rom Rep Phys 47:265
4. Kopcansky P, Koneracka M, Tomasovicova N, Tomco L (1999) J Magn Magn Mater 201:204 5. Yu Zubarev A, Ivanov AO (1997) Phys Rev E 55:7192 6. Jayedevan B, Nakatani I (1999) J Magn Magn Mater 201:62
7. van de Hulst HC (1957) Light scattering by small particles. Wiley, New York 8. Cebers A (1991) Physical properties and models of magnetic ¯uids. European Advanced Course of UNESCO, Minsk, April 1991
