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.

Structure formation of surfactant membranes under shear flow

“…10 Thus, there are some experimental evidences of a transient state, but its structure is still under debate. An alternative experimental approach to gain insight into the structural changes is to characterize defects observed in the lamellar state for moderate shear rates, both in surfactant membranes 13,17 and in thermotropic liquid crystals. 18,19 It is also worth mentioning that stable cylindrical structures on a tenmicrometer length scale are observed when strong shear flow is applied in the lamellar-sponge coexistence state.…”

Structural rheology of composite onion phase

“…In such defective lamellar phase, the buckling of the layers will be inhibited because the shear flow induces more defects to relieve accumulated stress on the membranes. Actually the increase of the defect density under shear was experimentally observed by Medronho et al [15]. Scaling behavior G' ~ Γ/l 2 with l ~ γ -0.5 also indicates that the shear flow induces the reduction of the defect spacing l, i.e., the increase in the defect density.…”

“…The linear increase of G' at low shear rates will be explained by the development of the oily streak defects. Increase in the defect density has been actually reported as a prerequisite phenomenon for the shear-induced onion phase formation [1,2,15,20,31,33].…”

“…The compression modulus B typical for the surfactant lamellar phase is B ~ 10 5 Pa [38]. Shearing the lamellar phase induces high density of the oily streak networks [15]. In such situation, the effective thickness h of the sample can be replaced by defect spacing l. From our previous observation of the oily streak [20], each value will be e  0.6, a  5 to 10 μm, and l  10 to 30 μm, respectively.…”

“…In their model, the shear-thickening behavior would be attributed to the buckling of membranes due to the effective lateral pressure. On the basis of their theory, the shear-induced onion phase formation has been studied from the viewpoint of structural development by the use of scattering and rheological methods [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Richtering et al found intermediate structure, which can be either coherently buckled lamellar membranes, or multilamellar cylinder structure [9,11].…”