Linear viscoelastic study of lipid vesicle dispersions: Hard-sphere behavior and bilayer surface dynamics

The frequency-dependent complex viscosity of dispersions of various lipid vesicles was measured. Two clearly distinguishable features in these curves could be observed, suggesting two relaxation processes. The first relaxation time is shown to be dependent on the third power of the vesicle radius. This implies an elasticity of entropic nature, which is the result of the translational ordering of the vesicles that are subjected to shear flow and forced out of their equilibrium distribution. A comparison with linear viscoelastic measurements of dispersions of ‘‘real’’ hard-sphere (silica) particles shows remarkable agreement, and trends are extended. Furthermore, the measurements support theoretical work. For the second relaxation process, a linear dependency of the relaxation time on the radius of the vesicle is observed. In addition, the relaxation time and the effective viscosity were found to be linearly proportional. A combination of various similar theories that treat the dynamics of spherical capsules in terms of capsule wall viscoelasticity proved capable of explaining the dependency of the second relaxation time on the product of radius and effective viscosity. The second relaxation process was found to be pertaining to the surface shear viscoelasticity of the vesicle wall. Thus, from the analysis of the data with theory, the surface shear modulus and the surface shear viscosity of the lipid bilayer could be deduced. Furthermore, a small contribution of the curvature modulus was detected in our measurements, for which an estimate could be given. With the aid of literature data for the curvature modulus, the dilatational modulus, and the present value of the surface shear modulus found in this work, Young’s modulus and the Poisson ratio of the investigated bilayer could be established.