Communications: Evidence for the role of fluctuations in the thermodynamics of nanoscale drops and the implications in computations of the surface tension.

Test-area deformations are used to analyze vapor-liquid interfaces of Lennard-Jones particles by molecular dynamics simulation. For planar vapor-liquid interfaces the change in free energy is captured by the average of the corresponding change in energy, the leading-order contribution. This is consistent with the commonly used mechanical (pressure-tensor) route for the surface tension. By contrast for liquid drops, one finds a large second-order contribution associated with fluctuations in energy. Both the first- and second-order terms make comparable contributions, invalidating the mechanical relation for the surface tension of small drops. The latter is seen to increase above the planar value for drop radii of approximately 8 particle diameters, followed by an apparent weak maximum and slow decay to the planar limit, consistent with a small negative Tolman length.