An example for an interface out of equilibrium is the grain boundary in polycrystalline matter.
Surface tension is the physical property which rules interface processes involving liquids.
For a liquid film on flat surfaces, the liquid-vapor interface keeps flat to minimize interfacial area and system free energy.
For a liquid film on rough surfaces, the surface tension tends to keep the meniscus flat, while the disjoining pressure makes the film conformal to the substrate.
Optical lenses serve as an example of a practical application of the interface between glass and air.