FORCED ADVANCEMENT AND RETRACTION OF POLAR LIQUIDS ON A LOW-ENERGY SURFACE

Dependences of advancing and receding dynamic contact angles on contact-line velocity during forced spreading and retraction of drops of polar liquids (water and aqueous glycerol solutions) on a low energy polyethylene terephthalate surface are studied. The experimental data for theta(a)/V and theta(r)/V are interpreted on the basis of the molecular-kinetic and hydrodynamic models of contact-line motion. It is shown that the molecular-kinetic approach provides the best description of the experimental data in the velocity range studied (1 . 10(-4) -2 . 10(-1) cm s-1) when two sets of molecular parameters are used for the initial and final branches of the kinetic dependences. Blake (T.D. Blake, 1988 AlChE Int. Symp. on the Mechanics of Thin-Film Coating, Paper 1a: Wetting kinetics - how do wetting lines move?, March 6-10, 1988, New Orleans, LA) assumed that such behaviour is due to heterogeneity of the solid surface. The hydrodynamic model predicts physically unreasonable values of its characteristic parameter, the slip length L(s). It is shown that the plateaux in the kinetic dependences theta(a)/V and theta(r)/V, established in a previous study represent parts of stretched S-shaped curves with horizontal inflection regions. The significance of the initial slopes preceding the plateaux is confirmed again and it is shown that extrapolation of these initial branches to V = 0 leads to values that coincide well with the static hysteresis angles of advancement and retraction.