Heterogeneous nucleation and wetting of water thin films on a metal surface: A study by optical second harmonic generation

The condensation of water thin films on a hydrophobic metal surface, Ag(111), was examined using optical second harmonic generation. Condensation coefficient and the fraction of metal surface area covered with water during film deposition were measured in the temperature range of 145–175 K. It was found that under isothermal condensation conditions, the condensation coefficient decreases abruptly to zero at a temperature several degrees lower than that predicted by zero-order desorption kinetics. This catastrophic failure in water film deposition at these temperatures can be explained by the occurrence of wetting–dewetting transition as a result of three-dimensional cluster formation, i.e., the critical nucleus size becomes too large at these temperatures to allow the formation of the first layer and subsequent growth of water film. Model calculations based on classical nucleation theory which depicts that heterogeneous nucleation is the initial step of water film deposition can be used to quantitatively characterize the critical nucleus size as about 100 and the nucleation rate to be slower than 10−3 ML s−1 at these temperatures.