  Condensation and boiling are phase transitions highly relevant to industry,
geology or atmospheric science. These phase transitions are initiated by the
nucleation of a drop in a supersaturated vapor and of a bubble in an
overstretched liquid respectively. The surface tension between both phases,
liquid and vapor, is a key parameter in the development of such nucleation
stage. Whereas the surface tension can be readily measured for a flat
interface, there are technical and conceptual limitations to obtain it for the
curved interface of the nucleus. On the technical side, it is quite difficult
to observe a critical nucleus in experiments. From a conceptual point of view,
the interfacial free energy depends on the choice of the dividing surface,
being the surface of tension the one relevant for nucleation. We bypass the
technical limitation by performing simulations of a Lennard Jones fluid where
we equilibrate critical nuclei (both drops and bubbles). Regarding the
conceptual hurdle, we find the relevant cluster size by searching the radius
that correctly predicts nucleation rates and nucleation free energy barriers
when combined with Classical Nucleation Theory. With such definition of the
cluster size we find the same value of the surface tension for drops and
bubbles of a given radius. Thus, condensation and boiling can be viewed as two
sides of the same coin. Finally, we combine the data coming from drops and
bubbles to obtain, via two different routes, estimates of the Tolman length, a
parameter that allows describing the curvature dependence of the surface
tension in a theoretical framework.
