We report on the shear-induced transition from an oriented lamellar phase (Lα phase) to multilamellar vesicles (MLV) in two nonionic surfactant systems, namely, a 40 wt % sample of triethylene glycol monodecyl ether (C10E3) and a 40 wt % sample of tetraethylene glycol monododecyl ether (C12E4) in D2O. This transition was studied by time-resolved small-angle neutron and light scattering under shear. Within a range of shear rates from 2 to 100 s-1 at 25 °C the transition from the Lα phase to MLVs in the C10E3 system apparently is controlled by strain. This transition involves an intermediate structure with cylindrical scattering symmetry. This can be interpreted as multilamellar cylinders (MLCs) or as a coherent stripe buckling with the wave vector of the undulation in a neutral direction. The intermediate structures found along the transition path are stable for long times, when shear is turned off. This allows for studies on trapped intermediate structures and experiments where different positions within the gap of a couette shear cell were examined in so-called gap-scan experiments. These experiments revealed that the transition from planar lamellae to MLVs is homogeneous throughout the gap. A temperature increase to 32 °C changes neither the pathway nor the strain control in comparison with experiments run at 25 °C. Upon a further increase in temperature to 38 °C, the transition leads to a mixture of MLC and planar lamellae or a weakly buckled state. With C12E4 as surfactant, and therefore with changed bilayer properties, a strain control is still observed, but less strain is needed for the transition compared to that of the C10E3 system. A comparison of the transition for the two systems, their transient as well as their steady-state viscosities, indicates that the transition is controlled by the stress.

The mechanism and kinetics of a shear-induced formation of multi-lamellar vesicles in a lyotropic lamellar phase of C10E3 (Triethyleneglycol-decylether) was investigated by rheology and time-resolved small-angle neutron and light scattering (SANS, SALS). Starting from a well-defined, macroscopically oriented lamellar phase, the transition occurs in two steps. First, there is a formation of an intermediate structure oriented in the flow direction which scatters only perpendicular to the flow. This is compatible with long, multi-lamellar cylinders (tubuli). Comparing results from three different shear rates shows that the formation of this intermediate structure is strain controlled. As shear is continued, multilamellar vesicles are formed.Introduction. -The influence of shear on the structure and orientation of complex fluids has attracted much interest in recent years [1,2]. Following the pioneering work of Diat et al. [3], particular attention has been given to lyotropic lamellar phases which exhibit two special features under shear. One is the flipping of aligned planar lamellae [4-10] and the second concerns the formation of multi-lamellar vesicles (MLVs) [3,[11][12][13][14][15][16][17][18]. While the shearinduced formation of MLVs has been demonstrated in many different systems, both with surfactants and with block copolymers [19], it is neither yet fully understood why they are formed, nor what the mechanism of the formation is. In the present paper we focus on the latter problem and present data from the so-called start-up experiments, where a well-defined, oriented lamellar state, having the bilayers aligned with their normal in the gradient direction, is suddenly exposed to shear of a constant rate. The structural evolution from lamellae to MLVs is followed by time-resolved i) viscosity measurements, ii) small-angle light scattering (SALS) and iii) small-angle neutron scattering (SANS).The system contains the surfactant C 10 E 3 (Triethyleneglycol-decylether) dissolved in heavy water (D 2 O). In this system, a lamellar (L α ) phase is stable at room temperature over a wide range of concentration. Below about 50 wt.% the lamellar phase coexists with an L 3 (sponge) phase at higher temperatures. In this lamellar phase, MLVs can be formed upon shearing, however, the stability, under shear, of the MLV state relative to a planar bilayer state depends

The influence of shear on lyotropic lamellar phases in the system sodium dodecyl sulfate (SDS)/decanol/water has been studied using small angle neutron and light scattering (SANS, SALS), birefringence and rheology. Eight different samples with a constant water content of 67.4%, but different surfactant-cosurfactant ratio were studied. Static SANS measurements showed that replacing of SDS with decanol leads to a transition from a defective lamellar phase, characterized first by a ribbon like structure and then by a pore like structure, to a classical lamellar phase. An orientation diagram was obtained from SANS, SALS and birefringence measurements under shear. For samples with low decanol content, shear flow leads to an alignment of lamellae but in addition to previous studies, we found two reorientations, from a parallel (at low shear rates) to a perpendicular alignment of the lamellae (with respect to the walls of the shear cell) and to a parallel alignment again at the highest shear rates available. At intermediate decanol content, a shear induced formation of multilamellar vesicles was observed in a certain shear rate region. Samples with classical lamellar structure at high decanol content exhibited no shear induced vesicle formation.