IMAGING THE CONDENSATION AND EVAPORATION OF MOLECULARLY THIN-FILMS OFWATER WITH NANOMETER RESOLUTION

The polarization force between an electrically charged atomic force mi croscope tip and a substrate has been used to follow the processes of condensation and evaporation of a monolayer of water on mica at room t emperature. Condensation proceeds in two distinct structural phases. U p to about 25 percent humidity, the water film grows by forming two-di mensional clusters of less than a few 1000 angstroms in diameter. Abov e about 25 percent humidity, a second phase grows, forming large two-d imensional islands with geometrical shapes in epitaxial relation with the underlaying mica lattice. The growth of this second water phase is completed when the humidity reaches about 45 percent. The reverse pro cess of evaporation has also been imaged.