  In this paper we use molecular dynamics (MD) to answer a classical question:
how does the surface tension on a liquid/gas interface appear? After defining
surface tension from the first principles and performing several consistency
checks, we perform a dynamic experiment with a single simple liquid
nanodroplet. At time zero, we remove all molecules of the interfacial layer,
creating a fresh bare interface with the bulk arrangement of molecules. After
that the system evolves towards equilibrium, and the expected surface tension
is re-established. We found that the system relaxation consists of three
distinct stages. First, the mechanical balance is quickly re-established.
During this process the notion of surface tension is meaningless. In the second
stage, the surface tension equilibrates, and the density profile broadens to a
value which we call "intrinsic" interfacial width. During the third stage, the
density profile continues to broaden due to capillary wave excitations, which
does not however affect the surface tension. We have observed this scenario for
monatomic Lennard-Jones (LJ) liquid as well as for binary LJ mixtures at
different temperatures, monitoring a wide range of physical observables.
