INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 17 (2005) S2923–S2928

doi:10.1088/0953-8984/17/31/019

Shear-induced structural transition in a lyotropic
lamellar phase studied using small angle neutron andlight scattering
T Kato1,3 , K Miyazaki1 , Y Kawabata1, S Komura1, M Fujii1 and M Imai2
1 Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
2 Department of Physics, Ochanomizu University, Bunkyo, Tokyo 112-0012, Japan

E-mail: kato-tadashi@c.metro-u.ac.jp

Received 30 December 2004, in final form 14 February 2005
Published 22 July 2005
Online at stacks.iop.org/JPhysCM/17/S2923
Abstract
We study the effects of shear flow on structures of the lamellar phase formed in anonionic surfactant C16 E7 (hepta(oxyethylene glycol)-n-hexadecylether)/D2O
system by using small angle neutron scattering (SANS) and small angle lightscattering (SALS) in the range of shear rate 10−3 –30 s−1 . As the shear rateincreases from 0.3 to 1 s−1 , the lamellar repeat distance for the 48 wt% sample
is decreased significantly and discontinuously. With further increase in theshear rate, d increases through a sharp minimum at 1 s−1 . The minimum value
does not depend on the surfactant concentration very much and is nearly equal
to the thickness of the bilayers. At this shear rate (1 s−1 ), the intensity of thepolarized SALS is strongly enhanced in the small angle region in the vorticity
direction. These results suggest that water layers are excluded by shear flow
and that large concentration fluctuations on the µm scale are induced, whichcorresponds to local segregation into two regions; one of these has concentrated
lamellar structures and the other is a water-rich region. As the shear rateincreases further (to 3 s−1 ), a broad diffraction peak appears in the vorticity
direction in the polarized SALS whereas a four-lobe pattern is observed in the
depolarized SALS. There results, together with the two-dimensional SANS
pattern, suggest the formation of close-packed onions elongated along the flow
direction at 3 s−1 . Comparison is made with the study of Nettesheim et al(2003 Langmuir 19 3618) who report the formation of multilamellar cylinders
as intermediate structures, between lamellae and spherical onions.

3 Author to whom any correspondence should be addressed.

0953-8984/05/312923+06$30.00© 2005 IOP Publishing Ltd

Printed in the UK

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T Kato et al

1. Introduction
It is well known that shear flow induces structural changes in materials when the product γ̇ τ
is larger than unity, where γ̇ is the shear rate and τ is the specific timescale of fluctuationsof these structures. For surfactant self-assembly, this condition is easily satisfied because τ
can be of the order of 10−9 –102 s whereas experimentally available shear rates are less than
104 s−1 . In the past ten years, effects of shear flow have been studied extensively by using
microscopy, NMR, and various kinds of scattering techniques. Among them, the lamellar
phase has become of interest because the shear-induced state is sometimes not related to an
existing equilibrium. Roux and co-workers [1–4] have found transformation from the lamellar
phase to the ‘onion phase’ where multilamellar vesicles are close packed. They have also
found a transition from ‘state II’ (the isotropic state of onions, the disordered state) to ‘state
III’ (the oriented state of lamellae) as well as to ‘state IV’ (small onions in compact planes, theordered state) and from state IV to ‘state V’ (big onions in compact planes, the ordered phase).
Following their work, various kinds of shear effects have been reported: change in orientationof membranes [5, 6], sponge-to-lamellar transformation [7, 8], multilamellar-to-unilamellar
vesicle transformation [9], reduction in the spacing [10–14], collapse of membranes [15], andformation of multilamellar cylinders or deformed onions [16–19]. It should be noted that thesestudies have been performed for the shear rates of 1–5 × 103 s−1 .
Recently, we have measured small angle neutron scattering (SANS) under shear flow for
the lamellar phase of the C16 E7 (hepta(oxyethylene glycol)-n-hexadecylether)/water system(40–55 wt% of C16 E7 ) at 70 ◦ C, paying attention to the structural change in the relatively low
shear rate range, 10−3 –30 s−1 [20]. When the shear rate increases from 0.1 to 1 s−1 , the repeat
distance (d) is decreased significantly (down to about 40% of d at rest in the most significant
case) and discontinuously. With further increase in the shear rate, d increases through a sharp
minimum. This, together with other results, strongly suggests that water layers are excluded byshear flow and that the lamellar phase segregates into two regions; one of these has concentrated
lamellar structures and the other is a water-rich region. However, macroscopic phase separation
is not observed by the naked eye.
In the present study, we measure polarized and depolarized small angle light scattering(SALS) to confirm the above prediction from the SANS results on the nm scale and to provide
a basis for discussing the structural transition on the µm scale. Preliminary results for theunpolarized mode in SALS have been reported elsewhere [21].
2. Experimental details
A rheometer (Nihon Rheology Kiki, NRM-2000) was modified for SALS measurements undershear flow. The sample cell is almost the same as that for the SANS measurements: a Couettetype shear cell consisting of two concentric cylinders made of quartz. The stator diameter andthe gap are 48 and 1 mm, respectively. The temperature of the cell is controlled to ±0.1 ◦ C
by Peltier elements and a heater in an air bath. To prevent sample evaporation, a vapour
seal is incorporated in the cell. A He–Ne laser beam (NEC GLG5360, 5 mW at 632.8 nm),which is polarized horizontally, is passed down the velocity gradient axis of the Couette cell(corresponding to the so-called radial configuration) and through an analyser. The intensity of
scattered light is detected by a CCD camera system (Hamamatsu Photonics C4880-80-22A).
A schematic presentation of the shear SALS apparatus has been reported previously [21]. In
the present study, we measure both polarized (Hh ) and depolarized (Vh ) scatterings where theanalyser is set parallel and perpendicular, respectively, to the incident polarization. The detailsof the apparatus will be presented elsewhere [22].

Shear-induced structural transition in a lyotropic lamellar phase

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◦

Figure 1. Shear rate dependence of the repeat distance for the vorticity ( ), transverse (),and parallel () orientations previously reported [20]. The different symbol sizes correspond todifferent runs.

SALS measurements have been performed for the 48 wt% sample at 70 ◦ C. The shear
history of the sample is almost the same as that for the previous SANS measurements at
48 wt% [20]; the shear rate has been changed in the order 0.3 s−1 (2 h), 1 s−1 (2 h), and 3 s−1(1 h).
3. Results and discussion
Figure 1 shows shear rate dependence of the repeat distance at 48 wt% obtained by SANS
measurements, which has been previously reported [20]. For the lamellar structure, there arethree principal orientations, perpendicular, transverse, and parallel orientations, with the layer
normal along the vorticity, flow, and velocity gradient directions, respectively. These threeorientations can be detected by using two kinds of configurations; one is the so-called radialconfiguration where the beam is directed through the centre of the cell (along the velocity
gradient direction), and the other is the tangential configuration where the beam is directed
through the end of the cell (along the flow direction). The different symbols in figure 1
correspond to the different orientations of lamellae. In the range 0–0.03 s−1 , the repeat distanceis almost independent of the shear rate. As the shear rate increases from 0.3 to 1 s−1 , d isreduced suddenly from 6.6 to 5.2 nm. With the further increase in the shear rate, d increases
again through a minimum (referred to as d ∗ , hereafter). This figure also demonstrates thatsuch a specific shear rate dependence of d is obtained for all the principal orientations oflamellae. Similar shear rate dependences have been obtained for other concentrations. As
the concentration of C16 E7 decreases from 55 to 40 wt%, d increases from 6.5 to 8.5 nm
at rest whereas d ∗ remains almost constant (∼5 nm) and nearly equal to the thickness of thebilayers at rest obtained from the line shape analysis of small angle x-ray scattering [23]. These
results strongly suggest that water layers are excluded by shear flow and that the lamellar phasesegregates into surfactant-rich and water-rich regions.
Theoretical prediction of the shear-induced collapse has been reported by Bruinsma andRabin [24] and Ramaswamy [25]. According to these theories, the critical shear rate above
which the collapse occurs can be expressed in terms of the interlayer spacing (d), the viscosityof the solvent (η), and the bending constant of a surfactant layer (κ). For η ∼ 1 cp = 10−3 Pa s,d ∼ 10 nm, and κ ∼ kB T , the critical shear rate is calculated to be 106 s−1 which is more thansix orders of magnitude higher than the shear rate giving d ∗ in the present study. It should beS2926

T Kato et al

SANS

shear
rate

SALS

tangential

radialat restHh

Vh

0.3 s –1

1 s –1-2

0

2

q / nm-1

gradient

vorticity

vorticity

3 s –1-2

0

2

q / nm-1

-2

0

2 -2

q / µm-1

02q / µm-1

flow

Figure 2. Evolutions of 2D patterns of SANS [20] and polarized (Hh ) and depolarized (Vh ) SALS
with increasing shear rate, from 0 to 3 s−1 . No significant scattering pattern is observed at rest for
SALS.

(This figure is in colour only in the electronic version)noted that these theories assume direct suppression of undulation fluctuations of membranes
by shear flow. In this case, the reduction of d should be observed only for a limited orientation(probably the parallel orientation). In the present study, however, the shear rate dependencesof d are almost the same for all the principal directions, as can be seen from figure 1.
Figure 2 shows evolutions of 2D (two-dimensional) SALS patterns with increasing shear
rate for Hh (polarized) and Vh (depolarized) scattering. For comparison, previously reported
SANS patterns for both tangential and radial configurations are also included. The SANS
pattern at rest for the radial configuration demonstrates that the orientations of the lamellae
are isotropic in the vorticity–flow plane. In the vorticity–velocity gradient plane, on the other
hand, the lamellae are oriented in the parallel direction, as can be seen from the pattern for
the tangential configuration. So the contributions of the three principal orientations are in thefollowing order: parallel > perpendicular ∼ transverse.
The 2D SALS data in figure 2 were reduced to 1D (one-dimensional) data in the vorticity
directions by integrating the scattering intensity over a segment of width φ = ±10◦ where
φ is the azimuthal angle. The 1D patterns obtained for Hh and Vh scattering are shown infigures 3(a) and (b), respectively. For the shear rate of 1 s−1 where the repeat distance takes
a minimum, the Hh scattering intensity in the small angle region is strongly enhanced. This
indicates large concentration fluctuations on the µm scale and is therefore consistent with theprediction from the SANS results on the nm scale. As the shear rate increases up to 3 s−1 , abroad diffraction peak appears along the vorticity direction, indicating that some sort of regularstructure is formed. The repeat distance calculated from the peak position is about 3.8 µm.

Shear-induced structural transition in a lyotropic lamellar phase

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(b)
scattering intensity

scattering intensity

(a)

1

2

q / µm-1

1

2

q / µm-1

Figure 3. 1D polarized (Hh ) and depolarized (Vh ) SALS patterns for the shear rates of 0.3 s−1 (),1 s−1 ( ), and 3 s−1 ( ) obtained by integrating the 2D scattering intensity over a segment ofwidth φ = ±10◦ where φ is the azimuthal angle.

•◦

In the Vh scattering, on the other hand, a four-lobe pattern is observed at 3 s−1 . It is wellknown that such a four-lobe pattern is observed for optically anisotropic spheres like polymer
spherulites [26, 27]. The radius of the anisotropic sphere (R) is calculated to be about 5.0 µm
from the observed peak position (qmax ) of the 1D Vh scattering pattern using the relation
qmax R = 4.09 [26, 27]. In surfactant systems, a multilamellar vesicle (or onions) may be
the most probable candidate, as already reported for several systems [1–4, 16–19]. However,spherical onions are inconsistent with the SANS results; the 2D pattern shows a pronounced
diffraction peak in the vorticity direction (see figure 2), which indicates that the perpendicularorientation (the normal vector of the lamellae is along the vorticity direction) dominates. If
the onions are elongated along the flow direction, on the other hand, the experimental results
can be explained because only their hemispherical parts contribute to the four-lobe pattern in
the SALS and because the cylindrical part gives a diffraction peak in the vorticity direction in
the SANS. It should be noted that the SANS pattern in the tangential configuration indicatesthat the parallel orientation (the normal vector of the lamellae is along the velocity gradientdirection) dominates compared to the perpendicular orientation (see figure 2). Therefore, thecross section perpendicular to the flow direction may be not circular but elongated along thevorticity direction.
Nettesheim et al [19] have reported a shear-induced transition from a lamellar phase tomultilamellar vesicles using SANS and SALS for 40 wt% samples of C10 E3 and C12 E4 in
D2 O in the shear rate range 1–100 s−1 . They have found that there is the formation of anintermediate structure oriented in the flow direction, corresponding to long, multilamellar
cylinders (or deformed onions) before the formation of spherical onions. According to theirresults, however, the four-lobe pattern is observed in the SALS only for the shear rate where
the SANS gives an isotropic 2D pattern4 . Moreover, the repeat distance remains constantthroughout the transition process (lamellar → multilamellar cylinders → onions). In thepresent study, on the other hand, the four-lobe pattern in the SALS and the anisotropic SANS
4 In [16, 17], the four-lobe pattern in the depolarized SALS and the anisotropic SANS patterns have been observedat the same shear rate. In these studies, however, they demonstrate that spherical onions already formed at low shear
rates become deformed when the shear rate increases further.

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T Kato et alpattern have been observed at the same shear rate (3 s−1 ) and the repeat distance is decreased
significantly, as previously described. Nettesheim et al have also shown that the transition
from lamellae to multilamellar cylinders is controlled by the shear stress. For the systems
C10 E3 and C12 E4 in D2 O, the transition occurs at the shear stress 40–100 Pa. In our system, on
the other hand, the shear stress in the steady state is less than 1.5 Pa above 1 s−1 [28], which
is much lower than their results.
Although theoretical studies of the transition from the lamellar phase to the onion phasehave been reported by several workers [2, 29, 30], the transition mechanism is still unclear. The
present results suggest that the local segregation may play an important role in the transition
processes.
Acknowledgment
This research was partially supported by the Ministry of Education, Science, Sports andCulture, Grant-in-Aid for Scientific Research (B), 15340140, 2003.
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