Viscoelastic Phase Separation and Transient Formation of Spongelike Patterns

It has so far been believed that phase separation in isotropic condensed matter can be classified into either solid or fluid models, including some modification due to elastic effects. Contrary to this common belief, we recently found unusual phase separation, which can be described by neither of the above models, in a mixture whose components have a large difference in their dynamics. Thus, we proposed that a new model of phase separation, namely, “viscoelastic model”, is necessary to describe phase separation in such dynamically asymmetric mixtures. This model is likely a general model that can describe all types of isotropic phase separation including solid and fluid model as special cases. Viscoelastic phase separation in dynamically asymmetric mixtures can be characterized by the order-parameter switching phenomena: The primary order parameter switches from the composition to the deformation tensor as in gel, 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 by the above general nature of a viscoelastic model. It is remarkable that orderparameter switching can occur even during an ordering process driven by a ‘single’ thermodynamic driving force. We argue that spongelike patterns observed in phase separation of many materials may be produced by a common physical mechanism of viscoelastic phase separation. For example, spongelike patterns formed during polymerization-induced phase separation can also be explained by our viscoelastic model.