An interface is a surface forming a common boundary between two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume, the more effect the surface phenomena will have. Therefore interfaces will be considered in systems with big area/volume ratios, such as colloids.
Interfaces can be spherical or flat, so they can be considered to be always spherical with finite or infinite radius. For example oil droplets in a salad dressing are spherical but the interface between water and air in a glass of water is mostly flat.
Surface tension is the function which rules interface processes.
One important interface is the gas liquid interface between aerosols and other atmoshperic molecules; research on this portion of atmospheric chemistry is being conducted at the University of Wisconsin-Madison.