  By using a limit analysis for the motion equations of viscous fluid endowed
with internal capillarity, we are able to propose a dynamical expression for
the surface tension of moving liquid-vapour interfaces without any
phenomenological assumption. The proposed relation extends the static case,
yields the Laplace formula in cases of mass transfer across interfacial layers
and allows to take the second coefficient of viscosity of compressible fluids
into account. We generalize the Maxwell rule in dynamics and directly explain
the Marangoni effect.
