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Smectic ferrofluid
Journal of Magnetism North-Holland
and Magnetic
Materials
SMECTIC
FERROFLUID
P. FABRE,
R. OBER,
Laboratoire
85 (1990) 77-81
17
M. VEYSSIE
de Physique de la Mali&e
CondensPe
Collt?ge de France,
I I Place Marcelin
Berthelot
75231
Paris Cedex
05, France
and V. CABUIL Luboratoire
de Physicochimie
Inorganique,
UniversitP Pierre et Marie
Curie,
75252
Paris Cedex
05, France
We have incorporated magnetic particles of sizes 100 A in swollen lamellar phases. It is the first time, to our knowledge, that solid grains are stabilized in ordered liquid phases. Moreover, when the grains are magnetic, this composite system realizes a periodic packing of a bidimensional magnetic liquid alternating with a non-magnetic fluid, and presents specific properties under a magnetic field.
1. Introduction The key idea of this study is to determine whether a solid dispersion of magnetic material may be confined and stabilized into a layered system. In order to check this hypothesis, we have used an oil-stabilized ferrofluid medium on one hand, and a lyotropic smectic phase on the other hand. In this article, after describing the two constitutive phases, we discuss the stability of the new composite system with regard to three parameters: +, the concentration of the ferrofluid; e, the layer thickness; and B, an external magnetic field.
2. Elaboration and characterization
of the samples
2. I. Ferrojluid The ferrofluid is a colloidal suspension of maghemite particles Fe,O,-y [l] stabilized in cyclohexane via organo-phosphorated molecules. We have studied three different volumic fractions $ in particles: + = 0.15%, $I = 1.5% and $I = 6%. The stability against aggregation of the dispersion is tested by light scattering under a magnetic field of lo4 G. As will become apparent in the following 0304-8853/90/$03.50
0 1990 - Elsevier
Science Publishers
discussion, two important characteristics of the ferrofluid are the average particle size and their polydispersity. This has been investigated by two independent methods: analysis of the magnetization curve, and small-angle X-ray scattering. In both cases, we assume a log-normal type-law for the diameter of the particles
P(D) a
1 D
- $
exp i
In’; 0 i
where lnQ, represents the mean value of In D and u is the standard deviation. The analysis of the magnetization curve has been made along the method described in ref. [2]: the comparison between the experimental data and the theoretical magnetization curve M = M,f( H) for different values of (Do, o) leads to the values Do = 76 A and (I = 0.35. As for the X-ray experiments, we use the apparatus described in ref. [3]. We analyse the scattered intensity versus the waveOvector q, q varying in the range of 0,006 to 0.12 A-‘. In the Guinier approximation (q e 0), one has I(q)
= QO)(l
- q2%/5),
with R, = Roe”* (R, = Do/2) (cf. fig. la); on the other hand, at large q values, the experimental fit of q4Z(q) =f(q) gives the Porod radius (cf. fig.
B.V. (North-Holland)
P. Fabre et al. / Smectic ferrofluid
7x
1
a 1
7
0015
I 12
Fig.
lb)
1. X-ray
data
of the ferrofluid solution for @ =1.5%. (a) Determination of R, in the Guinier (b) Determination of R, in the Porod approximation (q B- 27/R,,).
which is related to R, and u by R P = R, . From R, and R P, one may deduce the two parameters DO = 2R, = 72 A and u = 0.42. These values are in reasonable agreement with their determination by magnetization, and indicate a non-negligible polydispersity of our samples. In order to obtain an estimation of the total size of the particles, one has to take into account the e2.502
approximation
(q + 0);
adsorbed surfactant chains; this leads to a typical value for the particles diameter of 100 A. 2.2. Lyotropic smectic phases A lamellar phase is a periodic packing of alternate water and oil layers. The interfaces between the layers are constituted of both surfactant and
P. Fabre et al. / Smectlc ferrofluid
79
0
0 Fig. 2. X-ray
data
for the lamellar
1
9
phase.
The position
qmax of the Bragg
peak
permits
one to determine
the smectic
period
e = qmax/2n.
cosurfactant molecules. The phase is characterized by its repetition distance e = e, + e,, e, being the oil layer thickness and e, the water layer thickness. The last few years, it has been found out that it was possible to vary e, and e, independently in a very large range: from several tens to several thousands of angstroms. The lamellar phase remains stable under this oil or water ‘swelling”, provided that the interface composition, i.e. the surfactant-cosurfactant ratio, is kept constant. We have used this property to vary the oil layer thickness from distances smaller than the particle sizes to distances much larger. The phases we have studied are constituted of cyclohexane as the oil, sodium dodecylsulfate as the surfactant and pentanol as the cosurfactant. The structures of the phases are checked under a polarizing microscope: they present, between crossed polarizers, typical defects usually called “oily-streaks”. Their thickness is controlled by X-ray scattering, the Bragg peak position giving e = 2-rr/qmax (cf. fig. 2). We have studied four different values for e,: 110 A, 160 A, 210 A, 250 A; e, being kept constant and equal to 40 A.
3. Results and discussion 3.1. Existence
and stability of the ferrosmectics
The composite phases are prepared by swelling the lamellae described above with the ferrofluid solution, instead of using pure cyclohexane. They are kept in closed tubes at room temperature, and observed over long durations (of the order of several weeks). The phases that remain stable and birefringent in the tubes are put in sealed capillaries of thicknesses 100 pm: they are studied by polarized microscopy, or submitted to a magnetic field. From our observations, where we have varied the two parameters e and c$, we conclude that stable ferrosmectic phases exist, in the intermediate concentration range (C#J= 1.5%) and for the larger thicknesses (e, = 210, 250 A): the samples, uniformely colored, exhibit the typical oily-streaks textures (cf. fig. 3). Moreover, the lamellae tend to orientate spontaneously parallel to the glass boundaries (homogeneous orientation), leading to a sample with almost no defects, which is perfectly
x0
P. Fahre et al. / Smectic jerrojhid
on the particles may participate with the interfaces of the lyotropic system. In the dilute case, this causes a lack of surfacting molecules and a destabilization of the colloid: in the concentrated solution, an excess of surfactant leads to a modification of the initial phase diagram. 3.2. Magnetic field effect
Fig. 3. Observation under a polarizing microscope of the ferrosmectic phase. The sample exhibits an “oily-streak” texture, which is typical of a lyotropic lamellar phase.
black between crossed polarizers; this sample remains homogeneous and oriented over a time scale of the order of months. As for the smaller thicknesses, e, = 110, 160 A, we observe a phase separation between aggregates of particles and the lamellar phase. This behavior clearly indicates that there is a matching condition between the oil layer thickness and the particles size: one layer must be large enough to accommodate one particle (the grains are polydisperse) with its surrounding surfactant, but need not to accommodate more than one, leading to a true “ monolayer” of magnetic particles. The concentration effect seems more complicated to interpret as, for both the smaller concentration $B= 0.15% and the larger one $J = 6%, a phase separation occurs. Let us notice, however, that the processes are different in each case: for $J = 0.1591, one observes a destabilization of the particles which are expulsed from the lamellar phase; for + = 6%, one observes a biphasic system, where two phases of different concentrations in particles coexist. In fact, an interference between the two surfactants brought into play may account for this behavior: the stabilizing chains adsorbed
Supplementary evidence of the existence and stability of the ferrosmectic phases is their behavior in a magnetic field. The oriented samples have been submitted to a field B parallel or perpendicular to the lamellae and observed between crossed polarizers. Starting from a black sample. and applying a magnetic field perpendicular to the lamellae, one observes a very peculiar fan-shaped texture (cf. fig. 4) that indicates a tendancy of the lamellae to orientate parallel to the field. If the sample is then positioned parallel to B, the initial texture is recovered very shortly. This anisotropic response of the system to the magnetic field reveals the existence of a strong coupling between the magnetic grains and the lamellae. The behavior of the composite phase may thus be interpre-
Fig. 4. The “fan-shaped” texture which appears in an oriented ferrosmectic phase when a magnetic field is applied perpendicularly to the lamellae.
P. Fabre et al.
ted in terms of the demagnetizing field, which favors the orientation of an anisotropic object in the B-direction. However, a striking effect still remains to be explained: in contrast to what happens for ordinary smectic phases (“ghost transition”) [5], the value of the field required to observe the orientation is very low (less than a 100 Cl). In order to interpret this result, we have attributed to the medium an effective anisotropy of the susceptibility, derived from the energy of the demagnetizing field parallel or perpendicular to B. Due to the superparamagnetism of the medium [6], the order of magnitude of this anisotropy is several orders larger than in usual smectics. This leads to a much smaller threshold field for the instability. This effect in a magnetic field, which is typical of the new phase, deserves more study in order to be quantitatively interpreted. In particular, it is important to determine the elastic and viscous properties of the doped system,
Smectic ferrofluid
81
using, for instance, magnetically induced birefringence relaxation [7], and quasi-elastic light scattering.
References 111R. Massart. IEEE Trans. Magn. 17 (1981) 1247. PI J.C. Bacri, R. Perzynski, D. Salin. V. Cabuil and R. Massart, J. Magn. Magn. Mat. 62 (1986) 36-46. R. Ober, M. Veyssie and H. Finkelmann, 131 H. Mattoussi, Europhys. Lett. 2 (3) (1986) 233. and G. Fournet, Small Angle Scattering of (41A. Guinier X-Rays, (John Wiley, New York, 1955); 0. Glatter and 0. Kratky, Small Angle X-Ray Scattering (Academic Press. New York, 1984). [51P.G. de Gennes, The Physics of Liquid Crystals (Clarendon Press, Oxford, 1974). Ferrohydrodynamics (Cambridge Univer[61 R. Rosensweig, sity Press, Cambridge, 1985). [71J.C. Bacri, R. Perzynski, D. Salin and J. Servais, J. Phys. 48 (1987) 1385.
and Magnetic
Materials
SMECTIC
FERROFLUID
P. FABRE,
R. OBER,
Laboratoire
85 (1990) 77-81
17
M. VEYSSIE
de Physique de la Mali&e
CondensPe
Collt?ge de France,
I I Place Marcelin
Berthelot
75231
Paris Cedex
05, France
and V. CABUIL Luboratoire
de Physicochimie
Inorganique,
UniversitP Pierre et Marie
Curie,
75252
Paris Cedex
05, France
We have incorporated magnetic particles of sizes 100 A in swollen lamellar phases. It is the first time, to our knowledge, that solid grains are stabilized in ordered liquid phases. Moreover, when the grains are magnetic, this composite system realizes a periodic packing of a bidimensional magnetic liquid alternating with a non-magnetic fluid, and presents specific properties under a magnetic field.
1. Introduction The key idea of this study is to determine whether a solid dispersion of magnetic material may be confined and stabilized into a layered system. In order to check this hypothesis, we have used an oil-stabilized ferrofluid medium on one hand, and a lyotropic smectic phase on the other hand. In this article, after describing the two constitutive phases, we discuss the stability of the new composite system with regard to three parameters: +, the concentration of the ferrofluid; e, the layer thickness; and B, an external magnetic field.
2. Elaboration and characterization
of the samples
2. I. Ferrojluid The ferrofluid is a colloidal suspension of maghemite particles Fe,O,-y [l] stabilized in cyclohexane via organo-phosphorated molecules. We have studied three different volumic fractions $ in particles: + = 0.15%, $I = 1.5% and $I = 6%. The stability against aggregation of the dispersion is tested by light scattering under a magnetic field of lo4 G. As will become apparent in the following 0304-8853/90/$03.50
0 1990 - Elsevier
Science Publishers
discussion, two important characteristics of the ferrofluid are the average particle size and their polydispersity. This has been investigated by two independent methods: analysis of the magnetization curve, and small-angle X-ray scattering. In both cases, we assume a log-normal type-law for the diameter of the particles
P(D) a
1 D
- $
exp i
In’; 0 i
where lnQ, represents the mean value of In D and u is the standard deviation. The analysis of the magnetization curve has been made along the method described in ref. [2]: the comparison between the experimental data and the theoretical magnetization curve M = M,f( H) for different values of (Do, o) leads to the values Do = 76 A and (I = 0.35. As for the X-ray experiments, we use the apparatus described in ref. [3]. We analyse the scattered intensity versus the waveOvector q, q varying in the range of 0,006 to 0.12 A-‘. In the Guinier approximation (q e 0), one has I(q)
= QO)(l
- q2%/5),
with R, = Roe”* (R, = Do/2) (cf. fig. la); on the other hand, at large q values, the experimental fit of q4Z(q) =f(q) gives the Porod radius (cf. fig.
B.V. (North-Holland)
P. Fabre et al. / Smectic ferrofluid
7x
1
a 1
7
0015
I 12
Fig.
lb)
1. X-ray
data
of the ferrofluid solution for @ =1.5%. (a) Determination of R, in the Guinier (b) Determination of R, in the Porod approximation (q B- 27/R,,).
which is related to R, and u by R P = R, . From R, and R P, one may deduce the two parameters DO = 2R, = 72 A and u = 0.42. These values are in reasonable agreement with their determination by magnetization, and indicate a non-negligible polydispersity of our samples. In order to obtain an estimation of the total size of the particles, one has to take into account the e2.502
approximation
(q + 0);
adsorbed surfactant chains; this leads to a typical value for the particles diameter of 100 A. 2.2. Lyotropic smectic phases A lamellar phase is a periodic packing of alternate water and oil layers. The interfaces between the layers are constituted of both surfactant and
P. Fabre et al. / Smectlc ferrofluid
79
0
0 Fig. 2. X-ray
data
for the lamellar
1
9
phase.
The position
qmax of the Bragg
peak
permits
one to determine
the smectic
period
e = qmax/2n.
cosurfactant molecules. The phase is characterized by its repetition distance e = e, + e,, e, being the oil layer thickness and e, the water layer thickness. The last few years, it has been found out that it was possible to vary e, and e, independently in a very large range: from several tens to several thousands of angstroms. The lamellar phase remains stable under this oil or water ‘swelling”, provided that the interface composition, i.e. the surfactant-cosurfactant ratio, is kept constant. We have used this property to vary the oil layer thickness from distances smaller than the particle sizes to distances much larger. The phases we have studied are constituted of cyclohexane as the oil, sodium dodecylsulfate as the surfactant and pentanol as the cosurfactant. The structures of the phases are checked under a polarizing microscope: they present, between crossed polarizers, typical defects usually called “oily-streaks”. Their thickness is controlled by X-ray scattering, the Bragg peak position giving e = 2-rr/qmax (cf. fig. 2). We have studied four different values for e,: 110 A, 160 A, 210 A, 250 A; e, being kept constant and equal to 40 A.
3. Results and discussion 3.1. Existence
and stability of the ferrosmectics
The composite phases are prepared by swelling the lamellae described above with the ferrofluid solution, instead of using pure cyclohexane. They are kept in closed tubes at room temperature, and observed over long durations (of the order of several weeks). The phases that remain stable and birefringent in the tubes are put in sealed capillaries of thicknesses 100 pm: they are studied by polarized microscopy, or submitted to a magnetic field. From our observations, where we have varied the two parameters e and c$, we conclude that stable ferrosmectic phases exist, in the intermediate concentration range (C#J= 1.5%) and for the larger thicknesses (e, = 210, 250 A): the samples, uniformely colored, exhibit the typical oily-streaks textures (cf. fig. 3). Moreover, the lamellae tend to orientate spontaneously parallel to the glass boundaries (homogeneous orientation), leading to a sample with almost no defects, which is perfectly
x0
P. Fahre et al. / Smectic jerrojhid
on the particles may participate with the interfaces of the lyotropic system. In the dilute case, this causes a lack of surfacting molecules and a destabilization of the colloid: in the concentrated solution, an excess of surfactant leads to a modification of the initial phase diagram. 3.2. Magnetic field effect
Fig. 3. Observation under a polarizing microscope of the ferrosmectic phase. The sample exhibits an “oily-streak” texture, which is typical of a lyotropic lamellar phase.
black between crossed polarizers; this sample remains homogeneous and oriented over a time scale of the order of months. As for the smaller thicknesses, e, = 110, 160 A, we observe a phase separation between aggregates of particles and the lamellar phase. This behavior clearly indicates that there is a matching condition between the oil layer thickness and the particles size: one layer must be large enough to accommodate one particle (the grains are polydisperse) with its surrounding surfactant, but need not to accommodate more than one, leading to a true “ monolayer” of magnetic particles. The concentration effect seems more complicated to interpret as, for both the smaller concentration $B= 0.15% and the larger one $J = 6%, a phase separation occurs. Let us notice, however, that the processes are different in each case: for $J = 0.1591, one observes a destabilization of the particles which are expulsed from the lamellar phase; for + = 6%, one observes a biphasic system, where two phases of different concentrations in particles coexist. In fact, an interference between the two surfactants brought into play may account for this behavior: the stabilizing chains adsorbed
Supplementary evidence of the existence and stability of the ferrosmectic phases is their behavior in a magnetic field. The oriented samples have been submitted to a field B parallel or perpendicular to the lamellae and observed between crossed polarizers. Starting from a black sample. and applying a magnetic field perpendicular to the lamellae, one observes a very peculiar fan-shaped texture (cf. fig. 4) that indicates a tendancy of the lamellae to orientate parallel to the field. If the sample is then positioned parallel to B, the initial texture is recovered very shortly. This anisotropic response of the system to the magnetic field reveals the existence of a strong coupling between the magnetic grains and the lamellae. The behavior of the composite phase may thus be interpre-
Fig. 4. The “fan-shaped” texture which appears in an oriented ferrosmectic phase when a magnetic field is applied perpendicularly to the lamellae.
P. Fabre et al.
ted in terms of the demagnetizing field, which favors the orientation of an anisotropic object in the B-direction. However, a striking effect still remains to be explained: in contrast to what happens for ordinary smectic phases (“ghost transition”) [5], the value of the field required to observe the orientation is very low (less than a 100 Cl). In order to interpret this result, we have attributed to the medium an effective anisotropy of the susceptibility, derived from the energy of the demagnetizing field parallel or perpendicular to B. Due to the superparamagnetism of the medium [6], the order of magnitude of this anisotropy is several orders larger than in usual smectics. This leads to a much smaller threshold field for the instability. This effect in a magnetic field, which is typical of the new phase, deserves more study in order to be quantitatively interpreted. In particular, it is important to determine the elastic and viscous properties of the doped system,
Smectic ferrofluid
81
using, for instance, magnetically induced birefringence relaxation [7], and quasi-elastic light scattering.
References 111R. Massart. IEEE Trans. Magn. 17 (1981) 1247. PI J.C. Bacri, R. Perzynski, D. Salin. V. Cabuil and R. Massart, J. Magn. Magn. Mat. 62 (1986) 36-46. R. Ober, M. Veyssie and H. Finkelmann, 131 H. Mattoussi, Europhys. Lett. 2 (3) (1986) 233. and G. Fournet, Small Angle Scattering of (41A. Guinier X-Rays, (John Wiley, New York, 1955); 0. Glatter and 0. Kratky, Small Angle X-Ray Scattering (Academic Press. New York, 1984). [51P.G. de Gennes, The Physics of Liquid Crystals (Clarendon Press, Oxford, 1974). Ferrohydrodynamics (Cambridge Univer[61 R. Rosensweig, sity Press, Cambridge, 1985). [71J.C. Bacri, R. Perzynski, D. Salin and J. Servais, J. Phys. 48 (1987) 1385.