Impact of Negative Line Tension on the Shape of Nanometer-Size Sessile Droplets

The sign and value of the line tension has been measured from the size dependence of the contact angle of nanometer-size sessile fullerene (${\mathrm{C}}_{60}$) droplets on the planar ${\mathrm{SiO}}_{2}$ interface, measured with atomic force microscopy (AFM). Analysis according to the modified Young's equation indicates a negative line tension, with a magnitude between $\ensuremath{-}{10}^{\ensuremath{-}11}$ and $\ensuremath{-}{10}^{\ensuremath{-}10}\text{ }\text{ }\mathrm{N}/\mathrm{m}$, in good agreement with theoretical predictions. The experiments also indicate that droplets with contact area radii below 10 nm are in fact two-dimensional round terraces.