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J. Phys. Chem. B 1997, 101, 66-72
Shear-Induced Transformations in the Lamellar Phase of Hexaethylene Glycol Monohexadecyl Ether J. Penfold,*,† E. Staples,‡ A. Khan Lodhi,‡ I. Tucker,‡ and G. J. T. Tiddy‡ ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K., and UnileVer Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, U.K. ReceiVed: July 29, 1996; In Final Form: October 10, 1996X
The shear-induced ordering of the lamellar phase of hexaethylene glycol monohexadecyl ether, C16E6, in a Couette shear cell has been observed by small angle neutron scattering, SANS. Two distinct lamellae orientations have been identified. At low shear gradients the lamellae are ordered parallel to the flowvorticity plane, whereas at higher shear gradients the lamellae order parallel to the flow-shear gradient plane corresponding to a rotation through 90° of the axis of orientation. At shear gradients intermediate to those two extremes both orientations are observed simultaneously, this being the first report of such a morphology. A novel extension of shear alignment reported here is the application of oscillatory Couette flow. Here, orientation parallel to the flow-vorticity plane and the intermediate state corresponding to components oriented in the two orthogonal directions are observed. These observations are discussed in the context of recent results of shear-induced effects observed in related systems. The additional information obtained from shear flow alignment and from measurements where the neutron beam is incident in a direction orthogonal to the shear gradient (cell side) and parallel to the flow direction has enabled a re-evaluation of the nature of the “defective” lamellar phase.
Introduction There is much current interest in shear-induced transformations and ordering in complex fluids, which include surfactant lyotropic liquid crystalline phases,1-3 thermotropic liquid crystals,4 and polymer melts.5 Owing to the wide variety and subtlety of micellar phases, a large number of shear-induced morphology transitions have been observed in amphiphilic systems. These include the shear-induced alignment of rodlike micelles6,7 and the lamellar phase to multilamellar vesicle transition.8,9 Cates and Mildner10 in theoretical work have suggested that steady shear can raise the transition temperature of the lamellarisotropic phase transition and make it less strongly first order. This is because shear reduces the influence of nonlinear fluctuations, which promote a lowered transition temperature and the first-order behavior in the static case and are not accounted for in a mean field theoretical approach. An important consequence of their theoretical work is that fluctuations in the lamellae order appear in a direction transverse to both the flow velocity and shear gradient. The shear-induced isotropic to lamellar phase transition should then give rise to lamellar orientations in that direction, that is, the lamellae order parallel to the flow-shear gradient plane (denoted orientation a in Figure 1). This form of orientational order has now been seen in a number of related systems.1-5 The different possible orientations of the lamellae director with respect to the flow velocity, shear gradient, and vorticity directions are shown in Figure 1. They are denoted throughout this paper as a, b, and c, consistent with the terminology used in other recent work.4,5 Koppi and co-workers5 have investigated the effect of shear on the lamellar ordering in the diblock copolymer melts of poly(ethylenepropylene)-poly(ethylethylene), PEP-PEE. Near the order-disorder transition temperature, TODT, of the system and at low shear, the lamellae order with the director (unit vector n †
Rutherford Appleton Laboratory. Port Sunlight Laboratory. X Abstract published in AdVance ACS Abstracts, December 1, 1996. ‡
S1089-5647(96)02285-7 CCC: $14.00
Figure 1. Schematic diagram showing vorticity, shear gradient and flow axes in Couette cell, and direction of neutron beam relative to those axes. The three possible orientations are referred to as a, b, and c in the text and follow the convention adopted in refs 4 and 5.
in Figure 1) perpendicular to the flow direction and parallel to the velocity gradient (shear gradient) direction (orientation c in Figure 1). At higher shear gradients the lamellae align with the director perpendicular to both the flow and velocity gradients (orientation a). At temperatures further away from TODT orientation a is seen for all shear. Consistent with the theory,10 they propose that the orientation a arises from shear-induced © 1997 American Chemical Society
Hexaethylene Glycol Monohexadecyl Ether disordering (close to an order-disorder transition, where the barriers to disorder are reduced) followed by reordering in the direction perpendicular to the flow and shear gradient due to the effect of vorticity, whereas the alternative orientation c is a manifestation of defect-mediated stress relaxation in the system. Safinya et al.11 have developed a novel X-ray Couette shear cell that allows access to all scattering directions. In recent measurements3,4 they have investigated the effect of shear on the orientation of the lamellar LR phase of sodium dodecyl sulfate (SDS)-pentanol-dodecanol-water and on the nematic to smectic A transition for the thermotropic liquid crystal 8CB. For the surfactant lamellar phase they report the orientation of the layers parallel to the shearing plates, orientation a, at high shear as expected. At low shear the thermotropic liquid crystal 8CB in the smectic A phase exhibits both a and c orientations in a mixed phase, whereas at higher shear only orientation a is observed. The alignment is rotated with respect to the LR alignment and is similar to the observations of Koppi et al.5 Okamoto et al.12 have observed a biaxial orientation of lamellae in the block copolymer films of polystyrene-poly(ethylenepropylene) subjected to oscillatory shear flow. The biaxial orientation of the lamellar director in the directions parallel to the shear gradient and flow directions (orientations c and b) was observed and is at variance with the shear-induced changes observed in the orientation of other lamellar phases.5,11 Berret and co-workers1 have reported a shear-induced firstorder isotropic to nematic phase transition in the “worm-like” micelles of the cetylpyridinium chloride-hexanol-0.2 M NaCl brine system. Such a shear-induced nematic phase is evident in other systems, such as the mixed surfactants of SDS and tetradecyl dimethylammonium propane sulfonate, TDPS.9 Mang et al.2 have used small angle neutron scattering (SANS) to investigate the effect of Couette shear flow on the nematic (discotic) and lamellar phases of Cesium perfluorooctanoate, Cspfo. The nematic phase aligns with the director perpendicular to the flow-vorticity plane, orientation c, whereas the LR phase aligns with the director parallel to the flow-vorticity plane in the orthogonal direction, orientation a, consistent with the observations of Koppi et al.,5 Safinya et al.,3,4 and the theoretical predictions of Cates and Mildner.10 Approaching the LR phase from above the transition (decreasing temperature) induces a change to the orthogonal orientation. Scattering techniques, both X-ray3,4 and neutron scattering,6,9 are important methods for the characterization of such shearinduced transformations. We will describe here the use of SANS in combination with Couette shear flow to characterize the shear-induced changes in the orientation order of the lamellar phase of C16E6. Crucial to this study of oriented lamellar phases has been the demonstration that, contrary to previous assertions,11 measurements can be made both through the center of the Couette cell (neutron beam orthogonal to the flow-vorticity plane) and through the side of the Couette cell (neutron beam parallel to the flow-vorticity plane). It should be noted that this feature has also been exploited elsewhere in a related study by Mang et al.2 A novel aspect of this work is the use of oscillatory Couette flow, in addition to the normal steady flow, to study the shear-induced alignment of lamellar phases. Kalus et al.13 have used oscillatory Couette flow to study shear-induced phase transitions in cetylpyridinium salicylate micelles. Experimental Details The SANS measurements were made on the LOQ diffractometer14 at the ISIS pulsed neutron source, Rutherford Appleton Laboratory, UK. The diffractometer uses the white beam timeof-flight method to record scattering in the scattering vector,
J. Phys. Chem. B, Vol. 101, No. 1, 1997 67
Figure 2. Scattered intensity dσ/dΩ (in cm-1) for 15% C12E12 in D2O (b) through cell center and (O) through cell side.
Q, with a range 0.005-0.25 Å-1, in a single measurement. Incident neutron wavelengths in the range 2-10 Å were used, and the instrumental resolution, ∆Q/Q, was ∼8%. The scattered neutrons are recorded on a two-dimensional detector placed at 4.3 m from the sample (in the Qx-Qy plane as defined in Figure 1), corrected for background scattering and detector response and converted to a scattering cross section in absolute units (cm-1) using standard procedures.14 A Couette shear-flow cell (of radius 25.25 mm and with a 0.5 mm gap), specifically designed for SANS measurements,15 was used with shear gradients, G, in the range 0-5000 s-1. The design and performance of the Couette cell is described in detail elsewhere.15 In the normal scattering geometry the neutron beam is incident normal to the axis of rotation of the Couette cell, that is, perpendicular to the flow direction and parallel to the shear gradient and the vorticity. Measurements have also been made through the side of the cell in the direction orthogonal to the shear gradient (note that in this case the effective path length will be substantially longer than 1 mm). The Couette cell drive mechanism has been modified to provide the capability of oscillatory (sinusoidal) flow in addition to steady Couette flow. Not only can the period of the flow be adjusted in the range 0-75 oscillations s-1 and the amplitude up to 3.6 mm (corresponding to a mean shear rate of 540 s-1) but also the phase of the oscillation relative to the neutron pulse can be varied. We report here results of alignment of lamellar phases in oscillatory flow as well as in steady Couette flow. An important aspect of this work is that, for the study of lamellar phase dispersions (particularly when the lamellae are oriented in the flow-vorticity plane (orientation c)), it is essential to make measurements through the cell side in addition to the cell center, as described earlier. The Couette cell used here is of a design that, as recently demonstrated,9 makes such measurements through the cell side feasible for neutron scattering. The inner stator and outer rotating cup are constructed of quartz and contribute little to the background scattering. We have used the scattering from an isotropic micellar solution, 15% C12E12 in D2O (see Figure 2), to normalize the scattering through the cell side and to correct for the variable path length through the cell in that scattering geometry. All the data presented here for the “cell side” configuration have all been corrected using that geometry. The finite beam size compared to the gap width for the measurements through the cell side means that the flow and shear gradient directions will be less well defined in that situation. This will cause a smearing of the data additional to instrumental resolution so that any detailed quantitative analysis would require the complex integrals over the directional variations to be included. At this stage we are reporting only qualitative changes and have not included such
68 J. Phys. Chem. B, Vol. 101, No. 1, 1997
Figure 3. Intensity contours for 50.6 wt % C16E6 in D2O through cell side at G ) 0 and T ) 42 °C. (For reasons of clarity we have not labeled the individual intensity contours, which vary in absolute value from figure to figure.)
Penfold et al.
Figure 4. As in Figure 3 but for G ) 5000 s-1.
calculations. However, the change in direction of the flow and shear gradient within the beam aperture is not severe ((10°). Hence, this does not affect our qualitative interpretation of the data. The measurements reported here were made on a lamellar phase sample of C16E6 (50.6 wt % in D2O) predominantly at the temperatures of 37 and 42 °C. Some further measurements were made in the temperature range 26-52 °C. The D2O was obtained from BDH, the C16E6 was from Nikkol, Japan, and the C12E12 was synthesized at Unilever Research. The surfactants were used without further purification. Results and Discussion (i) Shear-Induced Orientation. Measurements were made on a 50.6% lamellar phase solution of C16E6 in D2O (42 °C) at different shear gradients from 0 to 5000 s-1, both through the cell center (neutron beam orthogonal to the flow-vorticity plane) and the cell side (neutron beam orthogonal to the shear gradient-vorticity plane). At zero or low shear gradients a pronounced anisotropic scattering is observed for measurements through the cell side (see Figure 3) with distinct peaks in the Qx direction superimposed upon a diffuse ring of scattering of much lower intensity. This pattern is characteristic of an aligned lamellar phase. The peak position at Qx ≈ 0.1 Å corresponds to a d spacing of ∼63 Å. The data are consistent with the lamellae being ordered parallel to the flow-vorticity plane orientation a, as intuitively expected. For measurements made through the cell center the scattering at zero shear is much weaker, corresponding to scattering only from a disordered component. At a shear gradient of 5000 s-1 the scattering pattern (through the cell side) is similar in form to that at low or zero shear except that is rotated through 90°, and the pronounced peaks in the scattering are now in the Qy direction (see Figure 4). This corresponds to a change in the orientational order of the lamellae. The lamellae are now ordered parallel to the flow-shear gradient plane orientation c. The changes in orientational order with increasing shear are consistent with the observations of Koppi et al.,5 Mang et al.,2 Safinya et al.,3,4 and with the theoretical predictions of Cates and Mildner.10 At the inter-
Figure 5. As in Figure 3 but for G ) 1250 s-1.
mediate shear value of 1250 s-1 there is pronounced scattering in both orthogonal directions (Qx and Qy) as shown in Figure 5. There are now marked peaks in the scattering in both the Qx and Qy directions of similar intensity. Random orientations would of course give rise to a uniform diffuse ring of scattering. Hence, these data imply that there are now domains within the sample with both distinct orientations. Similar biaxial orientation has been observed by Okamoto et al.12 for a block copolymer lamellar phase subjected to oscillatory shear. An important difference between this work and that of Okamoto et al. is that here the lamellae directors are oriented in the directions parallel to the shear gradient and vorticity directions, whereas Okamoto et al. observed the director oriented in the flow and shear gradient directions. Measurements were also made through the cell center and cell side for the same lamellar phase solution sample subjected to oscillatory flow in the range 0-180 s-1. Orientational order, similar to that observed with conventional Couette flow, was observed for the oscillatory flow measurements. At low values of oscillatory flow, 15 s-1 and 3.6 mm amplitude, alignment of the lamellae in the flow-vorticity plane was observed. At
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J. Phys. Chem. B, Vol. 101, No. 1, 1997 69
Figure 6. As in Figure 3 but for oscillatory flow of 22 s-1 and 3.6 mm amplitude.
higher values of oscillatory flow, 22 s-1 and 3.6 mm amplitude, the marked scattering peaks were observed in both the Qx and Qy directions (Figure 6), a scattering pattern similar to that observed in Figure 5 for conventional Couette flow. As discussed earlier, this implies that there are now distinct and approximately equal components aligned in both orientations. In contrast to the measurements in conventional Couette flow, it was not possible to induce the totally orthogonally aligned state where the lamellae are ordered parallel to the flow-shear gradient plane. This is probably because the mean shear gradient was not sufficiently high. Measurements were also made where the oscillation of the Couette cell was locked to the pulsed neutron beam (at 50 Hz) so that the phase of the oscillation relative to the neutron pulse could be controlled. This enables any relaxation processes that are occurring on a time scale of e20 ms to be probed. No differences in the orientational patterns or in the time dependence of the orientational order were observed. The shear dependence observed here differs from that observed by Koppi et al.5 No hysteresis in the transition between the two different orientations is observed here, whereas this was not the case for the diblock copolymer melts studied by Koppi et al.5 However, note that at low shear rates, the initial degree of alignment of the lamellar phase does partially depend upon the immediate shear history of the sample. As the temperature is lowered (some limited measurements were also made at 37 °C), a lower shear rate is required to induce the change from orientation c to a. This is opposite to the observations of Safinya et al.3,4 for the thermotropic liquid crystal 8CB at the nematic to smectic A transition (TNA f SMA), where a larger shear is required to maintain orientation a as the temperature is lowered below the TNA f SMA transition. Figure 7 shows isometric plots of the data from the measurements through the cell side at shear gradients. The differences in the patterns clearly show the changes in the orientation of the lamellae. Figure 7 also gives an indication of the extent to which both orientations are present even at the extremes of shear. That is, there remains a small component of the other orientation in each of the orthogonal directions. Furthermore, in Figures 3 and 4 there is clear evidence of a more diffuse and less intense component, seen most clearly in the direction orthogonal to the main lamellar peaks. This “defect” contribution has a different
Figure 7. Isometric intensity plot for 50.6 wt % C16E6 in D2O for measurements through the cell side at T ) 42 °C: (a) G ) 0 s-1; (b) G ) 5000 s-1.
“d” spacing (∼73 Å). The intensity and spacing of this “defect” component are not affected by shear or reorientation. This will be discussed in more detail in the next section. It is probable that the mechanisms proposed by Cates and Mildner10 and supported by Koppi et al.5 for block copolymer lamellar phases are responsible for the reorientation of the lamellae with increasing shear observed here. The novel aspect of the shear-induced reorientation reported here is the observation of the biaxial orientation, with the lamellar director oriented parallel to both the flow and vorticity directions. (ii) “Defect” Scattering. Figure 8 shows the scattered intensity in the Q⊥ (Qx) and Q| (Qy) directions integrated over a segment of width ∆θ ) (10° for the neutron beam incident through the cell side. The position of the scattering peak corresponds to a lamellar d spacing of 63 Å at a temperature of 42 °C. A similar representation of the data obtained with the neutron beam incident through the cell center (Figure 9) gives rise to a much lower scattered intensity at G ) 0. This arises only from any ordered fraction that has a component in that direction. Here, two distinct peaks in the scattering are observed, corresponding to d spacings of 63.0 and 73.0 Å. This is also seen (to a lesser extent) at G ) 5000 s-1, where the lamellar orientation has rotated through 90° and which in this case gives rise to a more intense peak in the Q| (Qy) direction. There is also evidence for the double peak in the measurements through the cell side, but it is more difficult
70 J. Phys. Chem. B, Vol. 101, No. 1, 1997
Figure 8. Scattering intensity in the directions Q⊥ (Qx) (O), Q| (Qy) (b) integrated over a sector of ∆θ ) (10° for 50.6 wt % C16E6 in D2O for measurements through the cell side at T ) 42 °C: (a) G ) 0 s-1; (b) G ) 5000 s-1.
Figure 9. As in Figure 8 but for measurements through the cell center. The scattered intensity in the Q⊥ (Qx) direction (O) is multiplied by 10.
to observe, since the scattering is now dominated by the intense diffraction peak. We have remarked earlier upon this weaker
Penfold et al.
Figure 10. Scattering intensity in (a) Qx and (b) Qy directions with temperature for measurements through the cell side for 50.6 wt % C16E6 in D2O at G ) 30 s-1 at (-) 34, (‚‚‚) 38, (- - -) 42, (-‚-) 46, (-‚‚) 48, and (-‚‚‚) 52 °C.
more diffuse contribution to the scattering in the context of the data in Figures 3, 4, and 7. From those data it was evident that the “defect” scattering corresponded to a different “d” spacing and was invariant with shear and the orientational order of the lamellae. These observations are qualitatively consistent with the data of Funari et al.,16 who have made measurements in a conventional quartz cuvette with no preferential orientation in the sample for C16E6 in the concentration range 28-70 wt %. At 50.6 wt % and 42 °C Funari et al.16 obtained d spacings of 57.0 and 66.0 Å, which compares with our values of 63 and 73 Å. Funari et al. describe the larger d spacing contribution to a defective lamellar phase (denoted LHR ), which they describe as planes broken by irregular water-filled defects. We have made further measurements at a shear flow of 30 s-1 and through the cell side for the 50.6% lamellar phase solution of C16E6 in D2O in the temperature range 36-52 °C. This encompasses the transition from LHR to LR,16 and the evolution of the scattering in the Qx and Qy directions is shown in Figure 10. The measurements in the Qy direction (Figure 10a) clearly show the additional scattering peak at low Q (larger d spacing), attributed to the “defective” lamellar phase by Funari et al.16 Its evolution with temperature is such that the peak disappears at a temperature ∼48 °C. For the peak in the Qx direction there is an evolution of the position and hence the d spacing with temperature, similar to that observed by Funari et al. In addition, there is a variation in the anisotropy of the scattering (see Figure 11), that is, it is less readily aligned. The transition from the “defective” to conventional lamellar phase is also accompanied by an abrupt change in the temperature dependence of these parameters. The almost complete loss of anisotropy in the scattering is reminiscent of an order-disorder transition. Figure 12 shows the evolution of the “d” spacing of the defect structure and the pure lamellar spacing.
Hexaethylene Glycol Monohexadecyl Ether
Figure 11. Variation of Qx(Imax) (b), lamellar “d” spacing (O), and anistropy (Qx/Qy (at Imax) (+) with temperature for 50.6 wt % C16E6 in D2O at G ) 30 s-1 and for measurements through the cell side.
J. Phys. Chem. B, Vol. 101, No. 1, 1997 71 Å at 53 °C. If we assume that the thickness of the individual lamellae is invariant with temperature and that at 53 °C the lamellar phase is defect-free, we can obtain the phase volume associated with the defects, at any temperature, from the fractional change in the main d spacing. This results in the maximum phase volume associated with the defects at 36 °C being on the order of 10%. Between 36 and 48 °C the change in the main “d” spacing is consistent with a 4% change in water content of the lamellae while the secondary “d” spacing changes from 66 to 88 Å. We consider now in more detail different possible models for the observed “defective” lamellar phase. First, we assume that the surfactant alkyl chain mixing in opposite layers is largely preserved. If the defects are in the form of holes in the bilayer, then from the previous arguments the data are only consistent with a small amount of water distributed between a large number of relatively monodisperse defects. The “defect” peak can be identified with the mean defect separation within the lamellae, and a crude estimate of the number density can be obtained by assuming a simple square lattice. This calculation is consistent with a 10 Å radius tube defects (holes) through the lamellae. Similarly, if the origin of the defect peak arises from a lamellar sheet of close packed rod micelles (of infinite length) with a minor dimension close to that of a pure lamellar phase, the alkyl regions would be again separated by unreasonable distances. An alternative model would be the existence of sheets of infinite rods with no orientational order between adjacent layers. The measured d spacing would now correspond to the layer spacing within each layer. For rods with circular cross section the radius of the alkyl core is given by
R ) xσsdpds Figure 12. Variation in “defect” (O) and main lamellar (b) “d” spacing with temperature for 50.6 wt % C16E6 in D2O at G ) 30 s-1 and for measurements through the cell side.
The additional scattering peak with a larger d spacing has been described in terms of lamellar phases broken by irregular water-filled defects.16-18 Diffuse scattering associated with structural defects in other lyotropic lamellar phases has been attributed alternatively to heterophase fluctuations of rods in the lamellar phase.19-21 Furthermore, this diffuse scattering can in some circumstances and for some systems have distinct inplane order associated with it.21 The shear-induced order of the C16E6 lamellar phase enables the nature of the “imperfections” responsible for the diffuse scattering to be examined in more detail. Some additional constraints on the nature of these imperfections can be obtained from inspection of the data in Figures 3 and 9a. In this instance the lamellae are highly oriented with the director (almost completely) normal to the cell wall. With the incident beam along the director (see Figure 9a) an isotropic scattering pattern is obtained with a well-defined peak arising from the “imperfections”. With the beam traversing the cell side (Figure 10) the scattering from the imperfections, again in the form of a welldefined peak in I(Q), is restricted to the vertical plane symmetric about the incident beam. The scattering is therefore consistent with the imperfections being constrained within the lamellar sheet; that is, there is no evidence for correlations between imperfections present in adjacent lamellae. The existence of a well-defined peak in I(Q) for the “defect” scattering is indicative of entities with little polydispersity, while the absence of scatter in the Q range 0.02-0.08 Å-1 suggests that the structure persists over dimensions on the order of 500 Å. The main d spacing evolves from 60.4 Å at 36 °C to 67.5
where A1 is the cross-sectional area of the rod, σs the alkyl phase volume fraction, and dp and ds the main lamellar and defect “d” spacings. The total rod (alkyl) area per unit volume is given by
A ) 2σsR/(dpds) This corresponds to an area per molecule of 47 Å2 as expected for a rod micelle close to the lamellar phase transition. At higher temperatures the changes in cross section can be accommodated by, for example, an elliptical cross section. Such a model can result in very small “intermicelle” dimensions along the director of the lamellar phase. However, this can also be varied by adjusting the ellipticity within the constraints imposed by the “defect” “d” spacing. C16E6 is intermediate in phase behavior compared to the more complex mesophase structures found for the longer alkyl chain nonionics (C30E6) and the more simple and conventional progression of mesophases of C12E6. We have earlier inferred from the form of the data that the origin of the “defect” peak must be in a local, rather than all pervading, structure. Kekicheff et al.19 have suggested that defects in the form of domains of rods coexist with the lamellae. The structures observed here are robust with respect to shear and readily equilibrate at a given temperature, suggesting that their origin lies in dehydration-induced changes in spontaneous curvature. This would suggest that, close to the hexagonallamellar phase boundary, raising the temperature does not initially produce a “continuous” lamellar phase, domains of “interdigitated” lamellar phase with rod-like structural defects, or that the hexagonal phase evolves into sheets of rod-like micelles (with no orientational correlation between adjacent sheets) prior to the formation of a true lamellar phase.
72 J. Phys. Chem. B, Vol. 101, No. 1, 1997 Several similar systems have been analyzed successfully using the “defective” lamellar phase description, where the lamellar phase is broken by irregular water-filled holes of defects16-18 and which, for some systems, shows distinct correlations.21 For these systems, unlike here, the experimental evidence for “holes” is unambiguous. In the current system the small amount of water associated with defects and the large number of defects implied by the defect “correlation” peak result in geometrical conflicts when simple geometries are considered. The defect dimensions clearly depend on the local geometry (curvature) within the lamellae, but we have not found an “interdigitated” geometry consistent with both the scattering and accepted packing constraints. However, the “hole” defect description has been successfully applied to several different lamellar phase systems,17,18 and further measurements are required here to establish the exact form of the defects in this instance. More detailed measurements as a function of surfactant concentration, temperature, and shear rate will be necessary before a more quantitative analysis and description of the nature of the defects observed here can be made. Summary We have observed two different shear-induced orientations of the lamellar phase of C16E6 orthogonal to each other. At low shear the lamellae are ordered parallel to the flow-vorticity plane. At high shear gradients the lamellae order in the orthogonal direction, parallel to the flow-shear gradient plane, is consistent with the theoretical predictions of Cates and Mildner.10 At intermediate shear we observe for the first time a biaxial orientation with the lamellar director parallel to both the flow and vorticity directions. Oscillatory flow is also able to induce the coexistence of the two orthogonally aligned states but does not induce a complete transition from one to the other as observed for steady shear. The results are analogous to those reported by other workers for different but related systems, and we have highlighted those differences. Our results are in qualitative agreement with recent work on the phase behavior of C16E6 by Funari et al.16 However, the shear-induced ordering has offered the opportunity to re-evaluate the nature of the “defective” lamellar phase, and different defect structures are discussed.
Penfold et al. Finally, our measurements confirm in more detail an earlier report9 on the ability to make neutron-scattering measurements with a Couette cell having the neutron beam orthogonal to the shear flow-vorticity plane (that is, through the cell side). Moreover, we demonstrate the application of an oscillatory flow in addition to the normal steady Couette flow. References and Notes (1) Berret, J. F.; Roux, D. C.; Porte, G.; Lindner, P. Europhys. Lett. 1994, 25, 521. (2) Mang, J. T.; Kumar, S.; Hammouda, B. Europhys. Lett. 1994, 28, 489. (3) Safinya, C. R.; Sirota, E. B.; Bruinsima, R. F.; Jeppersen, C.; Plano, R. J.; Wenzel, L. J. Science 1993, 261, 588. (4) Safinya, C. R.; Sirota, E. B.; Plano, R. J. Phys. ReV. Lett. 1991, 66, 1986. (5) Koppi, K. A.; Tirrell, M.; Bates, F. S.; Almdal, K.; Colby, R. H. J. Phys. II (France) 1992, 2, 1941. (6) Penfold, J.; Staples, E. J.; Cummins, P. G. AdV. Colloid Interface Sci. 1991, 34, 451. (7) Hoffman, H.; Hoffman, S.; Rauscher, A.; Kalus, J. Prog. Colloid Polym. Sci. 1991, 85, 24. (8) Diat, O.; Roux, D. J. Phys. II 1993, 3, 9. (9) Penfold, J.; Staples, E. J.; Khan Lodhi, A.; Tucker, I. Int. J. Thermophys. 1995, 16, 1109. (10) Cates, M.; Mildner, S. F. Phys. ReV. Lett. 1989, 62, 1865. (11) Plano, R. J.; Safinya, C. R.; Sirota, E. B.; Wenzel, L. J. ReV. Sci. Instrum. 1993, 64, 1309. (12) Okamoto, S.; Saijo, K.; Hashimoto, T. Macromolecules 1994, 27, 5547. (13) Kalus, J.; Lindner, P.; Hoffmann, H.; Ibel, K.; Munch, C.; Sander, J.; Schmelzer, U.; Selbach, J. Physica B 1991, 174, 164. (14) Heenan, R. K.; Osborn, R.; Stanley, H. B.; Mildner, D. R. F.; Furusaka, M. J. To be submitted for publication. (15) Cummins, P. G.; Staples, E. J.; Millen, B.; Penfold, J. J. Meas. Sci. Technol 1990, 1, 179. (16) Funari, S. S.; Holmes, M. C.; Tiddy, G. J. T. J. Phys. Chem. 1994, 98, 3015. (17) Holmes, M. C.; Leaver, M. S.; Smith, A. M. Langmuir 1995, 11, 356. (18) Leaver, M. S.; Holmes, M. C. J. Phys. II 1993, 3, 105. (19) Kekicheff, P.; Cabane, B.; Rawiso, M. J. Phys., Lett. 1984, 45, L813. (20) Hendrikx, Y.; Charvolin, J.; Kekicheff, P.; Roth, M. Liq. Cryst. 1987, 2, 677. (21) Kekicheff, P.; Cabane, B. Acta Crystallogr. 1988, B44, 395.
