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 viscoelastic measurements, but now also observed in steady shear experiments. The effect of the main acyl-chain phase transition, taking place at 23 degrees 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 tau(y) and a frequency independent storage modulus G' develop. In this concentration regime the rheological behavior is determined by the interplay between vesicle deformation and the intervesicle interaction, and so far, there is no indication which phenomenon is dominant. A comparison with recently reported measurement suggests that G' is proportional to alpha(-3), where alpha is the vesicle radius. Furthermore, we show that tau(y) approximate to gamma(c)G' which is in agreement with theory. Here tau(y) is the dynamic yield stress and gamma(c) 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 nad temperature as variables.