Rheological Behavior of a Dispersion of Small Lipid Bilayer Vesicles

Rheological behavior of a dispersion of small nearly-unilamellar phospholipid bilayer vesicles has been investigated. We conducted steady-state shear experiments and linear viscoelastic experiments. In the dilute and semidilute regime the rheological behavior is similar to that of a hard-sphere dispersion as reported in the literature for viscoelasticmeasurements, butnowalso observed in steady shear experiments. The effect of themain acyl-chain phase transition, taking place at 23 °C, can be described with an increase of the effective volume fraction. As a result, with temperature variation one can obtain effective volume fractions larger than the maximum packing fraction for hard spheres. Near and above the maximum packing fraction a dynamic yield stress τy and a frequency independent storage modulus G′ develop. In this concentrationregimetherheologicalbehavior isdeterminedbythe interplaybetweenvesicledeformation and the intervesicle interaction, and so far, there is no indication which phenomenon is dominant. A comparison with recently reported measurements suggests that G′ is proportional to a-3, where a is the vesicle radius. Furthermore, we show that τy ≈ γcG′ which is in agreement with theory. Here τy is the dynamic yield stress and γc the critical strain which indicates the transition to nonlinear behavior in a viscoelastic experiment. There is a striking resemblance between our high concentration results and those reported in literature for vesicles in the so-called onion phase. To the best of our knowledge this is the first rheological study for concentrated nearly-unilamellar vesicle dispersions with volume fraction and temperature as variables.