  Surface tension is a key parameter for understanding nucleation from the very
initial stage of phase transformation. Although surface tension has been
predicted to vary with the curvature of the liquid-vapor interface,
particularly at the large curvature of, e.g., the subnanometric critical
nucleus, experimental study still remains challenging due to inaccessibility to
such a small cluster. Here, by directly measuring the critical size of a single
capillary-condensed nanomeniscus using atomic force microscopy, we address the
curvature dependence of surface tension of alcohols and observe the surface
tension is doubled for ethanol and n-propanol with the radius-of-curvature of ~
-0.46 nm. We also find that the interface of larger negative (positive)
curvature exhibits the larger (smaller) surface tension, which evidently
governs nucleation at ~ 1 nm scale, indicating more facilitated nucleation than
normally expected. Such well characterized curvature effects contribute to
better understanding and accurate analysis of nucleation occurring in various
fields including material science and atmospheric science.
