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 Å 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. © 1993 Nature Publishing Group.