INTRQDUCTIGN
or shear field [4] is used to orient the sample, the scattering patterns are highly asymmetric and allow the monitoring of the nematic director orientation in the variou phases.
A number of investigations used in situ shear along with scattering techniques to monitor the rheology of various "soft" materials. These have included shearinduced crystal-like ordering of colloidal suspensions [5] , alignment of thermotropic liquid crystals [6] , of liquid crystal polymers [7] , and of wormlike micelles [8] as well as morphological changes in various copolymer systems [9, 10] [11] has two concentric vertical cylindrical parts (the stator and the rotor) made out of quartz (6-cm diameter and 2-mm thickness) with a 0.5-mm gap for the sample. The stator comprises a hollow aluminum block (inside the quartz cylinder) for temperature control (using a circulating bath) and allows for a horizontal neutron beam to be used either radially or tangentially to the shear cell (see Fig. 1 2 shows two examples). These peaks are either prominent or suppressed in the horizontal or vertical directions depending on the alignment of the disklike micelles. The low-Q "inner peak" (Q, ) represents the center-to-center interdistance of micelles that lie parallel to each other in an edge-to-edge configuration (see, for example, the A configuration in Fig. 3 ) while the high-Q "outer peak" (Q2) represents the interdistance between micelles that are face-to-face (see, for example, the B configuration in Fig. 3 Fig. 4(a) ], but yields distinct spots along with a weaker halo in the nematic and smectic phases. In the following, anisotropy of the outer ring will be monitored through an anisotropy ratio (ratio of the maximum intensity in the horizontal to that in the vertical directions). In the tangential beam geometry, this anisotropy ratio is constant with shear rate y in the nematic phase [ Fig. 4(b) ] and decreases with shear rate in the smectic phase [ Fig. 4(c) ]. Figure 4(c) shows that the anisotropy ratio R obeys a power law in the smectic phase (at 45 C) as R -y -in the tangential geometry.
It is interesting to note that the tangential anisotropy ratio in the smectic phase, 8 Fig. 3 ) while, in the smectic phase, a fraction of the micelles have a perpendicular orientation (A configuration in Fig. 3 ). This is observed as an increase of the value of stephaniebrown@example.com. 7'C in the tangential geometry case in Fig. 5(a) . This "Gipping" of the smectic layers was observed for other systems [4, 6, 9] . It can be attributed primarily to changes in the values of the Ericksen viscosity parameters a2 and a3 [4, 6] . Fig. 5(b) ]. Shearinduced shifts of phase transition temperatures can be attributed to the damping of critical fluctuations that become stronger close to transition lines. In block copolymers, for example, the order-disorder transition temperature is also raised and becomes closer to its mean-field prediction when reciprocating shear is applied [9] . The Contour plots of the SANS data taken at a shear rate of 5835 s ', at a sample temperature of 49.7 C, using oscillatory shear (a) with a radial beam geometry (an inner ring and two outer peaks in the vertical direction can be observed), and (b) with a tangential beam geometry (a weak outer ring and four outer peaks in the vertical and horizontal directions can be observed). Because the tangential beam geometry does not sample a uniform scattering volume, the left horizontal peak is higher than the right one.
have, however, been verified. Another observation consistent with this conclusion is shown in Fig. 6 as a plot of the outer peak intensity with varying azimuthal angle (with respect to the horizontal axis) in the detector plane using oscillatory shear and a tangential beam geometry.
For decreasing temperature one sees a gradual buildup of the horizontal peaks (located at 0' and 180'), which is consistent with an increase of the alignment parallel to the shear cell walls. The vertical peaks (located at 90 and 270'), however, first increase then decrease with decreasing temperature which is consistent with an increase then a decrease of the flipping process with decreasing temperature. This is attributable to the competition of two effects: an increase of the fraction of micelles that have "flipped" and lie in the A configuration of Fig. 3 (bulk effect) and at the same time an increase of the size of the micellar layer aligned vertically in the C configuration of Fig. 3 (wall effect) when the sample temperature is decreased. At first, ' the bulk efFect dominates whereas at low enough temperatures the wall effect takes over the whole sample volume. The anisotropy ratio for the radial beam geometry shown in Fig. 5(b) shows a behavior consistent with this conclusion.
To our knowledge, such a behavior has not been predicted or observed before.
Another conclusion is that tangential beam geometry is more useful than the radial one and that results based only on the radial beam geometry could be misleading. Fig. 7(a) alone. Actually, in all of the measurements taken in these experiments, there was not a single instance where the fiipping was complete (i.e. , corresponding to the A configuration of Fig. 3 ).
cess (seen as a contribution from vertical spots in the tangential geometry of Fig. 3 

{'paper_id': '15081040', 'title': 'Shear-induced orientational effects in discotic-liquid-crystal micelles', 's2_url': 'https://api.semanticscholar.org/CorpusID:15081040', 'source': 'S2ORC'}