Phase separation in soft matter: the concept of dynamic asymmetry

In this article, we review the basic physics of viscoelastic phase separation including fracture phase separation. We show that with an increase in the ratio of the deformation rate of phase separation to the slowest mechanical relaxation rate the type of phase separation changes from fluid phase separation, to viscoelastic phase separation, to fracture phase separation. We point out that there is a physical analogy of this to the transition of the mechanical fracture behaviour of materials under shear from liquid-type, to ductile, to brittle fracture. This allows us to discuss phase separation and shear-induced instability of disordered materials including soft matter, on the same physical ground. Finally it should be noted that what we are going to describe in this article has not necessarily been firmly established and there still remain many open problems to be studied in the future.