J. Phys.: Condens. Matter 8 (1996) 9513–9517. Printed in the UK The shear-induced transition between oriented textures and layer-sliding-mediated flows in a micellar cubic crystal J-F Berret†, F Molino†, G Porte†, O Diat‡ and P Lindner§ † Groupe de Dynamique des Phases Condens´ ees, UMR 5581, Universit´ e de Montpellier II, F-34095 Montpellier C´ edex 05, France ‡ European Synchrotron Radiation Facility, BP 220, F-38042 Grenoble C´ edex, France § Institute Laue–Langevin, BP 156, F-38042 Grenoble C´ edex 9, France Received 15 July 1996 Abstract. A series of experiments has been performed in order to analyse the shear-induced structures for a micellar cubic phase using small-angle neutron and x-ray scattering techniques. Steady shear was applied in a Couette cell to the (EO) 127 (PO) 48 (EO) 127 triblock copolymer system dissolved in water. At rest, the system crystallizes into a long-range ordered mesophase of face-centred symmetry (lattice parameter ∼300 ˚ A). The good resolution of the x-ray technique enables us to study in detail the transition between shearing flows dominated by oriented textures at low shear rates ( ˙ γ< 1s −1 ) and flows mediated by the mechanisms of layer sliding at higher rates ( ˙ γ> 100 s −1 ). Since the pioneering work of Hoffman [1] and Ackerson and Clark [2] on shear-induced transitions in suspensions of charged particles, much effort has been made to identify the structural properties of sheared colloidal crystals [3, 4]. More recently, it has been shown that large-size monodisperse particles can be prepared by using the selective solvent properties of diblock or triblock copolymers [5–7], yielding ordered mesophases of cubic symmetry. As far as the structure under shear flow is concerned, a link has been established between the former conventional (charge-stabilized) colloidal crystals and the latter micellar cubic phases [7]. When a colloidal crystal is subjected to a shearing field, layers of maximum compacity and hexagonal symmetry (hereafter referred to as 2D hcp layers) are formed in such a way that the close-packed direction is parallel to the flow velocity v, whereas the layers are stacked perpendicular to the shear gradient ∇v. For a face-centred cubic (fcc) symmetry, the 2D hcp layers are the (111) planes and the close-packed direction is the [110] twofold axis [3]. During the last decade, neutron scattering at small angles has proven to be a very powerful tool in the identification of sheared structures. However, the major drawback of such experiments is the poor resolution (limited by the 10% accuracy on the incident neutron wavelength), which is not sufficient to characterize quantitatively ordered structures. The system that we have investigated under shear is a ‘soft solid’, resulting from the aggregation of self-assembling triblock copolymers and crystallizing in an fcc structure. In this communication, we report on shear-induced structures observed using both neutron (SANS) and x-ray (SAXS) scattering at small angles. The resolution of the SAXS spectrometer located on the ‘high-brilliance beam line’ (BL4) at the European Synchrotron Radiation Facility (ESRF) [8] enables us to distinguish between shear flows mediated by oriented textures or by layer sliding. 0953-8984/96/479513+05$19.50 c  1996 IOP Publishing Ltd 9513