Phase (fluid)

A physical substance consisting of one or more chemical components which:

share the same volume (not necessarily continuous, see blow)

share the same boundary (not necessarily continuous, see blow) with other phases

move with the same macroscopic velocity

and existing in chemical, thermal and hydrodynamic equilibrium.

The contact between two phases provides the interface for exchanging of chemical components to reach the chemical equilibrium specific to a given temperature, pressure and the total mass of the components in all phases.

The total mass of chemical components in all adjacent phases remain constant over time but at certain temperature and pressure values some phases may disappear (due to migration of their chemical components to the bordering phases).

Phases are not always continuous. For example, gas bubbles are not connected to each other but may move at the same speed which is different to the hosting liquid speed and they do represent a separate phase with a inter-phase contact surface comprised of a sum of all bubbles' surfaces.

The most popular appearance of phase segregation is water, oil, gas moving at different speeds in porous media or wellbore or pipelines.