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# Interface

An **interface** is the region that separates two bulk phases. An interface is a molecular
structure, given that one of its characteristic lengths is of molecular size. It is therefore
non-trivial to precisely define the intrinsic surface, the mathematical surface that describes the interface at the
microscopic level. The cost
in terms of the Helmholtz energy to create such a structure is known as the the surface tension.
The simplest, most studied, and perhaps most important interface is the
fluid/fluid interface: the liquid/vapour interface of a one-component system.
Interfaces are subject to
thermal noise, which leads to thermal capillary waves.

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