Schematic representation of the compression of a lipid monolayer by a mobile barrier on the surface of a water solution.

Horizontal violet arrow represents the surface pressure that produces area changes by the compression. Vertical red arrow represents the water outflux from the interphase upon compession. The increase in surface concentration of lipids at the surface of the left-hand compartment consequently decreases the chemical potential of water. Vertical violet arrow corresponds to the water influx to the interphase due to the difference in the chemical potential of water between bulk and the interphase. Horizontal red arrow is the expansion (area increase) produced by the water influx. If expansion is produced mechanically the area increased induces the water influx.