Viscoelasticity of vesicle dispersions

Linear viscoelastic properties of dispersions of vesicles and microemulsions are investigated in detail. Each droplet is considered to be coated by a film of amphiphilic molecules (fluid membrane) whose shape is mainly governed by the bending rigidity rather than the surface tension. In addition to this elastic property, we also take into account the membrane compressibility which plays an important role in the macroscopic viscoelastic behavior of the system. For dilute emulsions, we calculated the effective complex viscosity by generalizing Taylorʹs old calculation. The mechanical force balance condition at the membrane can be expressed by using the generalized Laplaceʹs formula derived for compressible fluid membranes. On the basis of the cell model, we also calculated the effective complex viscosity for nondilute emulsions. Comparing various characteristic time scales both for vesicles and microemulsions, we discuss the detectable relaxation modes in the viscoelastic experiments.