Interface

An interface is the region that separates two bulk phases. It is therefore sometimes misspelled as "interphase." It is a molecular structure (one of its characteristic lengths is of molecular size). The cost in free energy to create such a structure is the surface tension.

The simplest, most studied, and perhaps most important interfaces is a fluid/fluid interface: the liquid/vapour interface of a one-component system.

Interfaces are subject to thermal noise that cause thermal capillary waves.