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Monodisperse ferrofluid emulsions
Journal of Magnetism and Magnetic Materials 122 (1993) 37--41 North-Holland
AjI,M
Monodisperse ferrofluid emulsions J. B i b e t t e Centre de Recherche Paul Pascal / CNRS, F 33600 Pessac. France
We have produced monodisperse emulsions consisting of ferrofluid droplets. In the absence of an external magnetic field, the oil-in-water magnetic emulsion behaves like a classical oil-in-water emulsion. By applying an homogeneous field, droplet chains are observed. In some cases the chaining effect is associated with a strong modification of the optical properties of the ferrofluid emulsion.
1. Introduction
2. Preparation
Emulsions are inherently unstable dispersions of one, normally immiscible, fluid in another. With appropriate surface-active species, the emulsion can be sufficiently stable to make them very attractive for many applications [1] ranging from cosmetics to coatings, and from food to medicines. In terms of thermodynamics, emulsions are indeed unstable systems. However, the time scale for coarsening can vary tremendously depending upon the systems. In certain cases, this time can be extremely long (several years) provided that there is sufficient repulsion between droplets to prevent coalescence, and that the solubility of the dispersed phase is sufficiently small to prevent notable Ostwald ripening. Emulsions are produced by vigorously mixing oil and water; the size distribution is therefore generally very large. By applying a purification process on the initial polydisperse crude emulsion, it is possible to obtain a set of highly monodisperse samples. The purification method is analogous in principle to a fractionated crystallization process [2]. Moreover, the use of a polymerizable oil can also lead to hard magnetic particles which are easily separated by size using the same fractionated crystallization method [3].
Emulsions are metastable systems and thus require mechanical energy to be formed. A very easy and cheap method for preparing concentrated emulsions consists in slowly adding the dispersed phasc (oil) to the surfactant concentrated continuous phase under small shear (4). The surfactant used to stabilize the ferrofluid emulsion is sodium dodecyl sulfate, (SDS) and the continuous oil ferrofluid is a kerosene ferrofluid (6% by volume of F e 2 0 3) provided by Union Carbide. Ten grams of SDS are dissolved in 15 g of distilled water and 80 g of ferrofluid are slowly added to the continuous phase under mixing. To avoid the dispersion of air bubbles, the speed of the stirrer is kept very low (around 60 rpm). The concentrated emulsion is then slowly diluted under mixing by adding 800 g of distilled water. At this stage the ferrofluid emulsion is highly polydisperse with droplet diameters ranging from 0.1 to a few ~m. The oil volume fraction (4)) is about 10%. The purification method applied to this crude magnetic emulsion has been described elsewhere [2]. In principle, it is the same method used for nonmagnetic systems. The same surfactant (SDS) is added to the continuous phase, the oil volume fraction is set to roughly 10%. The role of the excess surfactant is to induce an attractive interaction between the oil droplets, which leads to a phase separation between dilute and dense phases. Since the attractive interaction is also
Correspondence to: Dr J. Bibette, Centre de Recherche Paul P a s c a l / C N R S , Avenue A. Schweizer, F-33600 Pessac, France. 0304-8853/93/$06.00 (t5 1993
Elsevier Science Publishers B.V. All rights reserved
38
.I. Bibette / Mom>di~perse l~'rrq/luM emulsion~
Fig. I. (a) A crude polydispcrsc emulsion. The droplet diameter ranges from 0.2 to a few btm. (b) A monodispcrsc cmulsion obtained by fractionatcd crystallization. The droplct diameter is about !1.7 bLm.
sensitive to the oil droplet diameter, the phase separation will lead to a densc phase which contains the larger droplets, and a dilute phase which contains the smaller droplets. The density difference between the ferrofluid and the aqueous continuous phase ensures a spatial separation between the two phases. Indeed, the dense phase or the so-called ferrofluid cream will settle, whereas thc dilute Brownian phase will remain in the uppcr part. The separation of thc two phases is simply done by pipetting the dilute phase. This leads to a first fractionation. By repeating this process several times, one can obtain a highly monodisperse fcrrofluid emulsion. The purification method is analogous in principle to a fractionatcd crystallization because thc thermodynamic of the phase transition induced by excess surfactant can be described by a liquid-solid separation [5]. The interaction generatcd by the cxccss SDS can be described by a depletion interaction. The surfactant micelles are excluded from the interdroplet spacc when the two droplets come into contact. The depletion of micelles leads to a non-compensated pressure acting on the two droplets, which is responsible for the attraction.
The contact potential between droplets is well described by thc equation [6-8]: O"
<:
~ k 7,/, ,,,
,
( ~)
(lrnl
w h c r c & , , is the micclle volume fraction, kT is the thermal encrgy, and ~r/~r,,, is the ratio of the oil droplets diamcter and the micellc diamelcr. The initial crude emulsion and one purificd samplc arc shown in fig. 1. In both cascs the oil volume fraction is about 10C',;. The monodispersc emulsion has a droplet diameter ~r of about 0.7 l.zm. 3. Structures Many different structures can bc obtained with these magnetic emulsions, in the absence of an external magnetic field, the behavior of the emulsion remains essentially unchanged, compared with nonmagnetic emulsions. Colloidal crystals made of these liquid droplets can be easily obtained by incrcasing & up to - 6 0 q . One examplc of thcse colloidal crystals is shown in fig. 2. The droplet diameter is - I p.m. Note lhat these colloidal crystals arc also directly formed during
1. Bibette / Monodisperse ferrofluid emulsions
39
obtained with different values of c/J (fig. 3(a), c/J ::::: 1%; fig. 3(b), c/J::::: 5%). In the low volume fraction limit the kinetics of aggregation is shown to follow diffusive limited cluster aggregation [12]. The monodisperse ferrofluid emulsions show interesting behavior under an external magnetic field. These systems are particularly suitable for studying the chaining effect of these magnetic droplets and ultimately some properties related to these reversible structures. To date, the kinetics of chaining and the rheological properties have been studied only on very polydisperse systems [14,15]. When the intensity of the external field is increased to a few thousand Gauss the chaining becomes irreversible and permanent rods persist even in the absence of the magnetic field. Fig. 2. Picture of a colloidal crystal. The droplet volume fraction is about 60%.
the last runs of the fractionation. Moreover, more concentrated emulsions can also be obtained by using a dialysis technique [9,10]. To do so, the emulsion is enclosed in a dialysis bag and the bag is immersed in an osmotic revervoir containing water, surfactant and polymer. The concentration of polymer and surfactant in the reservoir set the osmotic pressure and the surfactant chemical potential. It is then possible to obtain highly concentrated and stable magnetic emulsions. Higher values of c/J (still stable) can be achieved with large oil droplets [11]. The system consists of deformed droplets pressing against each other through thin water films. When the droplets are adhesive, another kind of structure can be obtained with these emulsions or magnetic emulsions. Adhesion can be induced by adding about 0.5 mol 1- I of salt (sodium chloride) and by lowering the temperature below 30°C, where a very sharp transition takes place, the droplets become very adhesive and stick as soon as they collide to build a gel [12,13]. The gel possesses a characteristic length scale which is revealed by a peak in the small-angle light scattering pattern. This length scale varies with the initial droplet volume fraction at which gelation takes place. The different gels are shown in fig. 3
4. New properties and discussion Ferrofluid are characterized by an optical anisotropy under a magnetic field. The light propagating through the magnetic fluid with its electric polarization parallel to the magnetic field is absorbed more and experiences a higher refractive index (birefringence) than light polarized perpendicularly to the field [16]. Concentrated ferrofluids exhibit very large birefringence and have led to various devices in which the intensity of the polarized light beam is controlled by a magnetic field source [17]. However the color of the ferrofluid, which is essentially black or slightly reddish when it is more dilute (for -y-Fe Z0 3 ), does not change under the applied magnetic field. The ferrofluid emulsions are essentially brown. An emulsion with a non-absorbing oil is totally white and milky, due to the strong multiple scattering phenomenon. The ferrofluid emulsion is still a strong scattering medium but with some absorption properties arising from the iron oxide particles enclosed within the droplets. Depending on the droplet diameter, the droplet volume fraction and the oxide volume fraction within a droplet, the brown color can be significantly different from one sample to another. A surprising optical effect takes place when this brownish emulsion is placed in an homoge-
J. Bibettc / Mom~di,vper.~e li'rr<~lhtid cmu/sion.~
4()
Fig. 3. E m u l s i o n
g e l s w i t h ( a ) O = 1~7;; ~r = I),6 ~ m ; ( b ) ~h = 5 ¢ ; : ~r = 1).6 ~tm.
J. Bibette / Monodisperse ferrofluid emulsions
neous magnetic field of about 100 G. The color changes instantaneously from brown to yellow, green, red or blue. Preliminary results, which are essentially qualitative at this stage, do not give a final explanation of this phenomenon, but they are of interest. The colors are only visible in the backscattering direction, which is also parallel to the applied magnetic field. In other directions colors are not observed. The color varies with the intensity of the magnetic field but it seems that for a single droplet diameter a particular color dominates: the largest droplets appear red and the smallest more green or blue. The appearance of these colors is correlated with the formation of very short chains (doublets of droplets). In the absence of these short chains, but in the presence of the field, this optical effect is absent. One explanation for this phenomenon could be a diffraction effect of the chain since the droplet diameter is of the order of A, the wavelength of light. Nevertheless this effect is much smaller if the droplet core is polymerized, with the iron oxide grains trapped within the droplets. At this stage, it could be supposed that the spacing within the chain would change by varying the intensity of the magnetic field because the droplets are deformable. Quantitative experiments to explore the surprising properties of these magnetic emulsions are currently under way.
41
Helpful discussions with Paul Chaikin are gratefully acknowledged.
References [l] K.J. Lissant, Emulsions and Technology, vol. 6 (Marcel Dekker, New York, 1974). [2] J. Bibette, J. Colloid Interface Sci. 147 (1991) 474. [3] J. Bibette, D. Charmot and G. Schorsch, Patent, France, 90-01725. [4] M.P. Aronson, Langmuir 5 (1989) 494. [5] J. Bibette, D. Roux and F. Nallet, Phys. Rev. Lett. 65 (1990) 2470. [6] A. Vrij, Pure Appl. Chem. 48 (1976) 471. [7] J. Bibette, D. Roux and B. Pouligny, J. Physique II 2 (1992) 401. [8] P. Richetti and P. Kekicheff, Phys. Rev. Len. 68 (1992) 1951. [9] V.A. Parsegain, R.P. Rand, N.L. Fuller and D.C. Rau, in: Methods in Enzymology, vol. 127, Biomembranes, Part 0 (Academic Press, New York, 1986). [10] J. Bibette, Langmuir 8 (1992) 3180. [11] J. Bibette, D. Morse, T. Winen and D. Weitz, Phys. Rev. Len. 69 (1992) 2439. [12] J. Bibene, T. Mason, H. Gang and D. Weitz, Phys. Rev. Len. 69 (1992) 981. [13] J. Bibette, T. Mason, H. Gang and D. Weitz, submitted to Langmuir. [14] M. Fermigier, J. Colloid Interface Sci., to appear. [15] E. Lemaire and G. Bossis, J. Phys. D: Appl. Phys. 24 (1992) 1473. [16] P.C. Scholten, IEEE Trans. Magn. MAG-16 (1980) 221. [17] S. Taketomi, US Patent 4, 812, 167, March 14 (1989).
AjI,M
Monodisperse ferrofluid emulsions J. B i b e t t e Centre de Recherche Paul Pascal / CNRS, F 33600 Pessac. France
We have produced monodisperse emulsions consisting of ferrofluid droplets. In the absence of an external magnetic field, the oil-in-water magnetic emulsion behaves like a classical oil-in-water emulsion. By applying an homogeneous field, droplet chains are observed. In some cases the chaining effect is associated with a strong modification of the optical properties of the ferrofluid emulsion.
1. Introduction
2. Preparation
Emulsions are inherently unstable dispersions of one, normally immiscible, fluid in another. With appropriate surface-active species, the emulsion can be sufficiently stable to make them very attractive for many applications [1] ranging from cosmetics to coatings, and from food to medicines. In terms of thermodynamics, emulsions are indeed unstable systems. However, the time scale for coarsening can vary tremendously depending upon the systems. In certain cases, this time can be extremely long (several years) provided that there is sufficient repulsion between droplets to prevent coalescence, and that the solubility of the dispersed phase is sufficiently small to prevent notable Ostwald ripening. Emulsions are produced by vigorously mixing oil and water; the size distribution is therefore generally very large. By applying a purification process on the initial polydisperse crude emulsion, it is possible to obtain a set of highly monodisperse samples. The purification method is analogous in principle to a fractionated crystallization process [2]. Moreover, the use of a polymerizable oil can also lead to hard magnetic particles which are easily separated by size using the same fractionated crystallization method [3].
Emulsions are metastable systems and thus require mechanical energy to be formed. A very easy and cheap method for preparing concentrated emulsions consists in slowly adding the dispersed phasc (oil) to the surfactant concentrated continuous phase under small shear (4). The surfactant used to stabilize the ferrofluid emulsion is sodium dodecyl sulfate, (SDS) and the continuous oil ferrofluid is a kerosene ferrofluid (6% by volume of F e 2 0 3) provided by Union Carbide. Ten grams of SDS are dissolved in 15 g of distilled water and 80 g of ferrofluid are slowly added to the continuous phase under mixing. To avoid the dispersion of air bubbles, the speed of the stirrer is kept very low (around 60 rpm). The concentrated emulsion is then slowly diluted under mixing by adding 800 g of distilled water. At this stage the ferrofluid emulsion is highly polydisperse with droplet diameters ranging from 0.1 to a few ~m. The oil volume fraction (4)) is about 10%. The purification method applied to this crude magnetic emulsion has been described elsewhere [2]. In principle, it is the same method used for nonmagnetic systems. The same surfactant (SDS) is added to the continuous phase, the oil volume fraction is set to roughly 10%. The role of the excess surfactant is to induce an attractive interaction between the oil droplets, which leads to a phase separation between dilute and dense phases. Since the attractive interaction is also
Correspondence to: Dr J. Bibette, Centre de Recherche Paul P a s c a l / C N R S , Avenue A. Schweizer, F-33600 Pessac, France. 0304-8853/93/$06.00 (t5 1993
Elsevier Science Publishers B.V. All rights reserved
38
.I. Bibette / Mom>di~perse l~'rrq/luM emulsion~
Fig. I. (a) A crude polydispcrsc emulsion. The droplet diameter ranges from 0.2 to a few btm. (b) A monodispcrsc cmulsion obtained by fractionatcd crystallization. The droplct diameter is about !1.7 bLm.
sensitive to the oil droplet diameter, the phase separation will lead to a densc phase which contains the larger droplets, and a dilute phase which contains the smaller droplets. The density difference between the ferrofluid and the aqueous continuous phase ensures a spatial separation between the two phases. Indeed, the dense phase or the so-called ferrofluid cream will settle, whereas thc dilute Brownian phase will remain in the uppcr part. The separation of thc two phases is simply done by pipetting the dilute phase. This leads to a first fractionation. By repeating this process several times, one can obtain a highly monodisperse fcrrofluid emulsion. The purification method is analogous in principle to a fractionatcd crystallization because thc thermodynamic of the phase transition induced by excess surfactant can be described by a liquid-solid separation [5]. The interaction generatcd by the cxccss SDS can be described by a depletion interaction. The surfactant micelles are excluded from the interdroplet spacc when the two droplets come into contact. The depletion of micelles leads to a non-compensated pressure acting on the two droplets, which is responsible for the attraction.
The contact potential between droplets is well described by thc equation [6-8]: O"
<:
~ k 7,/, ,,,
,
( ~)
(lrnl
w h c r c & , , is the micclle volume fraction, kT is the thermal encrgy, and ~r/~r,,, is the ratio of the oil droplets diamcter and the micellc diamelcr. The initial crude emulsion and one purificd samplc arc shown in fig. 1. In both cascs the oil volume fraction is about 10C',;. The monodispersc emulsion has a droplet diameter ~r of about 0.7 l.zm. 3. Structures Many different structures can bc obtained with these magnetic emulsions, in the absence of an external magnetic field, the behavior of the emulsion remains essentially unchanged, compared with nonmagnetic emulsions. Colloidal crystals made of these liquid droplets can be easily obtained by incrcasing & up to - 6 0 q . One examplc of thcse colloidal crystals is shown in fig. 2. The droplet diameter is - I p.m. Note lhat these colloidal crystals arc also directly formed during
1. Bibette / Monodisperse ferrofluid emulsions
39
obtained with different values of c/J (fig. 3(a), c/J ::::: 1%; fig. 3(b), c/J::::: 5%). In the low volume fraction limit the kinetics of aggregation is shown to follow diffusive limited cluster aggregation [12]. The monodisperse ferrofluid emulsions show interesting behavior under an external magnetic field. These systems are particularly suitable for studying the chaining effect of these magnetic droplets and ultimately some properties related to these reversible structures. To date, the kinetics of chaining and the rheological properties have been studied only on very polydisperse systems [14,15]. When the intensity of the external field is increased to a few thousand Gauss the chaining becomes irreversible and permanent rods persist even in the absence of the magnetic field. Fig. 2. Picture of a colloidal crystal. The droplet volume fraction is about 60%.
the last runs of the fractionation. Moreover, more concentrated emulsions can also be obtained by using a dialysis technique [9,10]. To do so, the emulsion is enclosed in a dialysis bag and the bag is immersed in an osmotic revervoir containing water, surfactant and polymer. The concentration of polymer and surfactant in the reservoir set the osmotic pressure and the surfactant chemical potential. It is then possible to obtain highly concentrated and stable magnetic emulsions. Higher values of c/J (still stable) can be achieved with large oil droplets [11]. The system consists of deformed droplets pressing against each other through thin water films. When the droplets are adhesive, another kind of structure can be obtained with these emulsions or magnetic emulsions. Adhesion can be induced by adding about 0.5 mol 1- I of salt (sodium chloride) and by lowering the temperature below 30°C, where a very sharp transition takes place, the droplets become very adhesive and stick as soon as they collide to build a gel [12,13]. The gel possesses a characteristic length scale which is revealed by a peak in the small-angle light scattering pattern. This length scale varies with the initial droplet volume fraction at which gelation takes place. The different gels are shown in fig. 3
4. New properties and discussion Ferrofluid are characterized by an optical anisotropy under a magnetic field. The light propagating through the magnetic fluid with its electric polarization parallel to the magnetic field is absorbed more and experiences a higher refractive index (birefringence) than light polarized perpendicularly to the field [16]. Concentrated ferrofluids exhibit very large birefringence and have led to various devices in which the intensity of the polarized light beam is controlled by a magnetic field source [17]. However the color of the ferrofluid, which is essentially black or slightly reddish when it is more dilute (for -y-Fe Z0 3 ), does not change under the applied magnetic field. The ferrofluid emulsions are essentially brown. An emulsion with a non-absorbing oil is totally white and milky, due to the strong multiple scattering phenomenon. The ferrofluid emulsion is still a strong scattering medium but with some absorption properties arising from the iron oxide particles enclosed within the droplets. Depending on the droplet diameter, the droplet volume fraction and the oxide volume fraction within a droplet, the brown color can be significantly different from one sample to another. A surprising optical effect takes place when this brownish emulsion is placed in an homoge-
J. Bibettc / Mom~di,vper.~e li'rr<~lhtid cmu/sion.~
4()
Fig. 3. E m u l s i o n
g e l s w i t h ( a ) O = 1~7;; ~r = I),6 ~ m ; ( b ) ~h = 5 ¢ ; : ~r = 1).6 ~tm.
J. Bibette / Monodisperse ferrofluid emulsions
neous magnetic field of about 100 G. The color changes instantaneously from brown to yellow, green, red or blue. Preliminary results, which are essentially qualitative at this stage, do not give a final explanation of this phenomenon, but they are of interest. The colors are only visible in the backscattering direction, which is also parallel to the applied magnetic field. In other directions colors are not observed. The color varies with the intensity of the magnetic field but it seems that for a single droplet diameter a particular color dominates: the largest droplets appear red and the smallest more green or blue. The appearance of these colors is correlated with the formation of very short chains (doublets of droplets). In the absence of these short chains, but in the presence of the field, this optical effect is absent. One explanation for this phenomenon could be a diffraction effect of the chain since the droplet diameter is of the order of A, the wavelength of light. Nevertheless this effect is much smaller if the droplet core is polymerized, with the iron oxide grains trapped within the droplets. At this stage, it could be supposed that the spacing within the chain would change by varying the intensity of the magnetic field because the droplets are deformable. Quantitative experiments to explore the surprising properties of these magnetic emulsions are currently under way.
41
Helpful discussions with Paul Chaikin are gratefully acknowledged.
References [l] K.J. Lissant, Emulsions and Technology, vol. 6 (Marcel Dekker, New York, 1974). [2] J. Bibette, J. Colloid Interface Sci. 147 (1991) 474. [3] J. Bibette, D. Charmot and G. Schorsch, Patent, France, 90-01725. [4] M.P. Aronson, Langmuir 5 (1989) 494. [5] J. Bibette, D. Roux and F. Nallet, Phys. Rev. Lett. 65 (1990) 2470. [6] A. Vrij, Pure Appl. Chem. 48 (1976) 471. [7] J. Bibette, D. Roux and B. Pouligny, J. Physique II 2 (1992) 401. [8] P. Richetti and P. Kekicheff, Phys. Rev. Len. 68 (1992) 1951. [9] V.A. Parsegain, R.P. Rand, N.L. Fuller and D.C. Rau, in: Methods in Enzymology, vol. 127, Biomembranes, Part 0 (Academic Press, New York, 1986). [10] J. Bibette, Langmuir 8 (1992) 3180. [11] J. Bibette, D. Morse, T. Winen and D. Weitz, Phys. Rev. Len. 69 (1992) 2439. [12] J. Bibene, T. Mason, H. Gang and D. Weitz, Phys. Rev. Len. 69 (1992) 981. [13] J. Bibette, T. Mason, H. Gang and D. Weitz, submitted to Langmuir. [14] M. Fermigier, J. Colloid Interface Sci., to appear. [15] E. Lemaire and G. Bossis, J. Phys. D: Appl. Phys. 24 (1992) 1473. [16] P.C. Scholten, IEEE Trans. Magn. MAG-16 (1980) 221. [17] S. Taketomi, US Patent 4, 812, 167, March 14 (1989).