MOLECULAR LAYERING DURING EVAPORATION OF ULTRATHIN LIQUID-FILMS

DYNAMICAL processes in very thin liquid films play a critical role in phenomena such as lubrication, wetting, spreading and emulsions, but these dynamics remain poorly understood at the molecular level. Heslot et al.1-5 observed molecular layering effects in a spreading liquid droplet, which they interpreted in terms of flow anomalies. Here we report that evaporation dynamics can also lead to the formation of distinct molecular layers in a thin liquid film. We use ellipsometry to follow the evolution in time of the thickness profile of a film of tetrakis(2-ethylhexoxy)silane deposited on silicon wafers. From an initially smooth gradient, we observe the formation of molecular steps 10 angstrom in height. The evaporation rate shows an oscillatory dependence on film thickness, leading us to conclude that layering results from faster evaporation of incomplete layers relative to complete layers.