title: Shear-induced defect formation in a nonionic lamellar phase.
abstract: (2)H NMR experiments on a nonionic oriented lamellar phase demonstrate that shear flow induces structural defects in the lamellar structure. These substantial structural changes give rise to a transition from a viscous to a solidlike behavior; the elastic modulus of presheared samples was found to increase, reversibly, with the applied preshear rate. A similar behavior was found when step-cycling the temperature toward the layer-to-multilamellar-vesicle transition and back at constant shear rate. However, while shear rate controls the defect density, the temperature is found to control the defect rigidity. The lamellar phase exhibits a shear-thinning behavior under steady shear conditions, following the power law eta approximately gamma(n), with n approximately -0.4. Both the shear thinning and the elastic behavior are in agreement with the available theoretical models. The observed shear-induced structural defects are reversible and can be regarded as a pretransition prior to the shear-induced formation of multilamellar vesicles.
authors.affiliation: Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal. bfmedronho@portugalmail.pt
authors.forename: B
authors.initials: B
authors.lastname: Medronho
authors.forename: M
authors.initials: M
authors.lastname: Rodrigues
authors.forename: M G
authors.initials: MG
authors.lastname: Miguel
authors.forename: U
authors.initials: U
authors.lastname: Olsson
authors.forename: C
authors.initials: C
authors.lastname: Schmidt