by Utkur Mirsaidov, Haimei Zheng, Dipanjan Bhattacharya, Yosune Casana, Paul Matsudaira
Proc Natl Acad Sci USA 109(19), 7187-7190 (2012)
Dynamics of the first few nanometers of water at the interface are encountered in a wide range of physical, chemical, and biological phenomena. A simple but critical question is whether interfacial forces at these nanoscale dimensions affect an externally induced movement of a water droplet on a surface.
At the bulk-scale water droplets spread on a hydrophilic surface and slip on a nonwetting, hydrophobic surface. Here we report the experimental description of the electron beam-induced dynamics of nanoscale water droplets by direct imaging the translocation of 10- to 80-nm-diameter water nanodroplets by transmission electron microscopy. These nanodroplets move on a hydrophilic surface not by a smooth flow but by a series of stick-slip steps.
We observe that each step is preceded by a unique characteristic deformation of the nanodroplet into a toroidal shape induced by the electron beam. We propose that this beam-induced change in shape increases the surface free energy of the nanodroplet that drives its transition from stick to slip state.
Pictured is a snapshot of 50nm-diameter water droplet formed by the electron beam of a TEM. When irradiated with electrons, the droplet moves by a stick-slip mechanism in which the dome-shape droplet deforms into a high-energy toroidal shape and then translocates by extending a thin lamella from one edge.
Read online: PNAS.