Two-order-parameter description of liquids: critical phenomena and phase separation of supercooled liquids

Because of the isotropic and disordered nature of liquids, anisotropy hidden in many body intermolecular interactions is usually neglected. Thus, it has been believed so far that the only order parameter required to describe a simple liquid is the density. Not accepting this common-sense view, we argue that a new additional order parameter, which represents the spontaneous formation of locally favoured structures, is necessary for the physical description of liquids near the lower stability limit. This model explains well two mysterious phenomena of supercooled liquids: (i) the large-scale density fluctuations known as 'Fischer clusters' and (ii) the phase separation of a one-component liquid into two phases.