Viscoelastic phase separation in complex fluids

Abstract

Phase separation in isotropic condensed matter has so far been believed to be classified into solid and fluid models. When there is a large difference in the characteristic rheological time between the components of a mixture, however, we need a model of phase separation, which we call “viscoelastic model”. This model is likely a general model that can describe all types of isotropic phase separation including solid and fluid model as special cases. We point out that this dynamic asymmetry between the components is quite common in complex fluids, one of whose components has large internal degrees of freedom. We also demonstrate that viscoelastic phase separation in such dynamically asymmetric mixtures can be characterized by the order-parameter switching phenomena. The primary order parameter switches from the composition to the deformation tensor, and back to the composition again, reflecting viscoelastic relaxation between a characteristic deformation time of phase separation and the slowest rheological time of the system. This unusual behavior can be explained as follows: the viscoelastic model contains various model of phase separation as special cases and the switching between them is induced by viscoelastic relaxational phenomena. Since the deformation tensor intrinsically has a geometrical nature, the pattern in the elastic regime is essentially different from that of usual phase separation in fluid mixtures; and, thus, there is no self-similarity in the pattern evolution.