154 Progress of Theoretical Physics Supplement No. 175, 2008 Shear Induced Structures in Lamellar Systems From Layers to Onions to Onions and Layers Sven Koschoreck, 1 Shuji Fujii 2 and Walter Richtering 1, ∗) 1 Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056 Aachen, Germany 2 Department of Chemistry, Nagaoka University of Technology, Nagaoka 940-2188, Japan Shear flow induces the formation of multilamellar vesicles (MLV, also termed “onions”) in the lamellar phase of the nonionic surfactant C 10 E 3 in water. Depending on the applied shear rate, one can reach a state of polydisperse MLV (at intermediate shear rates) or densely packed monodisperse MLV at high shear rates. In this contribution we investigated the structure evolution when the shear rate is reduced by means of rheo-microscopy as well as rheo-small angle light and neutron scattering (SALS, SANS). Different shear quenches within the MLV structure region of 40 wt% C 10 E 3 reveal two different MLV size growth mechanisms: (i) a continuous and (ii) a discontinuous MLV size growth. In the later case, a part of the initial MLV structure transforms into planar lamellar domains leading to shear thinning. The lamellar domains perform a re-orientation process from parallel oriented lamellae into MLV in coexistence with the initial MLV structure. The pathway of the transformation of the parallel lamellae to MLV is the same as in start-up experiments, i.e. from a homeotropically aligned lamellar phase. §1. Introduction The influence of shear on the structure of complex fluids has attracted a lot of interest in the past for several reasons. Structural changes in complex fluids affect their flow properties and therefore imply consequences for various industrial applications. Closed bilayer structures are also important in biological processes resulting in a variety of pharmaceutical applications such as encapsulating drugs in liposomes made from natural lipid bilayers. Especially flow properties of lamellar phases have received a lot of attention. 1) The variety of different structures and structural transitions that can be found in lamellar phases under shear is rich. 2), 3) The lamellae can align in the flow field with their layer normal along the velocity gradient or in the vorticity direction where the first is often referred to as the parallel orientation and the latter is termed perpendicular orientation. Another morphology often found in lamellar phases exposed to shear is that of multilamellar vesicles (MLV), multi-layered closed bilayer structures, and a variety of different descriptive names like onions, liposomes, or spherulites can be found in the literature. These shear-induced structures in lamellar phases are often summarized in so-called orientation diagrams first introduced by Roux et al. 4), 5) A MLV region flanked by regions of planar lamellae dominates these diagrams in many cases. At low shear rate. the parallel orientation is found and the perpendicular orientation often dominates at higher shear rates. In some cases the transition back to lamellae ∗) Corresponding author. E-mail: Richtering@rwth-aachen.de Downloaded from https://academic.oup.com/ptps/article-abstract/doi/10.1143/PTPS.175.154/1857321 by guest on 15 June 2020