Influence of shear on the transient and the steady state properties of lyotropic bilayer systems
The work presented in this thesis comprises in-situ structural determination under shear using time-resolved scattering techniques and flow birefringence. These techniques were used to study transient structures occurring in surfactant bilayer systems under shear on length scales from a few nanometers up to several micrometers. The major concern of this thesis is the lamellar-to-MLV dynamic transition under shear, which is one of the unsolved mysteries in the area of complex fluids. In particular, details of the structural transition from a state of well aligned lamellae in parallel orientation to multilamellar vesicles (MLV) under the influence of a shear field were studied by a combination of time resolved small-angle neutron (SANS) and small-angle light scattering (SALS) using a nonionic surfactant lamellar phase. The study revealed five distinct transient states of bilayer organization and yielded strong support for a stress rather than a rate control of the transition. Reversible and irreversible parts of the transition, i.e., elastic storage of deformation on one hand and dissipation, which thus contributes to MLV formation, on the other, were determined in flow reversal experiments using the above mentioned time-resolved scattering techniques. Furthermore, the intriguing problem of particle inclusion in concentrated surfactant systems and its effect of the dynamic properties of such a sample was addressed by studying the incorporation of clay in a nonionic surfactant lamellar phase. Flow-SANS revealed a drastic decrease in lamellar spacing leading to the conclusion, that a micro phase separation takes place in such systems. The influence of shear on the bilayer organization was furthermore studied in a sponge- (or L3-) phase using flow-birefringence and transmission measurements. Here, the critical shear rates for the transition from the disordered sponge- to the shear aligned lamellar-phase displayed an unexpected scaling with membrane volume fraction and shear quench experiments revealed relaxation times for the structural recovery of the sponge-phase orders of magnitude slower than the inverse critical shear rate. Finally, a study of instrumental concern was carried out to solve the problem of the asymmetry of the tangential beam for a Couette or Searle-type shear cell used in Rheo-SANS experiments. In addition to considering the geometry of a tangential experiment, a practical method for experimentally accounting for the asymmetry is proposed.