Nanoscale Contact Line Pinning Boosted by Ångström-Scale Surface Heterogeneity

The pinning effect plays an important role in many fluidic systems but remains poorly understood, especially at the nanoscale. In this study, we measured the contact angles of glycerol nanodroplets on three different substrates using atomic force microscopy. By comparison of the shapes of the three-dimensional images of droplets, we found that a possible origin of the long-discussed deviation of the contact angles of nanodroplets from the macroscopic value is the pinning force induced by ångström-scale surface heterogeneity. It was also revealed that the pinning forces acting on glycerol nanodroplets on a silicon dioxide surface are up to twice as large as those acting on macroscale droplets. On a substrate where the effect of pinning was strong, an unexpected irreversible change from an irregularly shaped droplet to an atomically flat liquid film occurred. This was explained by the transition of the dominant force from liquid/gas interfacial tension to an adsorption force.