Transitions from static wetting to steady dewetting and deposition of liquid-film on partially wettable polymer surface

The transitions from static to steadily moving wetting perimeter and further to deposition of a liquid-film on partially wettable surface were studied with the same system under the same conditions. A polyethylene terephthalate (PET) tape was vertically withdrawn at constant velocity from glycerol–water mixture. Elevation L of the three-phase contact line above the liquid level was measured under static, steady, and dynamic wetting. The static receding Θ_R and the apparent dynamic angles Θ_app at different withdrawal velocities U were calculated from the static relationship Θ(L). It was found that the limiting static angle Θ_R,min, at which the wetting perimeter starts moving, depends on withdrawal velocity. Extrapolation of the Θ_R,min/U dependence to U = 0 yields the quasi-static value of this parameter , that coincides with the relaxation static angle Θ_R,rlx achieved after meniscus motion ceases. This conclusion holds also for the wetting mode, where the limiting static advancing angle = Θ_A,rlx. Both the limiting and relaxation angles could be used for calculation of the effective Young's contact angle on non-ideal surface following Adam's suggestion N.K. Adam, Adv. Chem. Ser. 43 (1964) 53.].The critical velocity U_cr anfd apparent dynamic angle Θ_app,cr, at which transition between steady dewetting and dynamic wetting occurs, were determined. The value of Θ_app,cr = 0° ± 5° agrees with our previous results R.V. Sedev, J.G. Petrov, Colloids and Surfaces, 53 (1991) 147] implying a quasi-static shape of the moving meniscus up to U_cr. At U > U_cr, the speed V of the contact line relative to the solid wall is independent of withdrawal velocity and thickness of the deposited film. The present data confirm the earlier findings J.G. Petrov, R.V. Sedev, Colloids and Surfaces, 13 (1985) 317, T.D. Blake, K.J. Ruschak, Nature, 282 (1979) 489] that at U = U_cr, V reaches its maximum value V_max, which is most important parameter of dewetting kinetics.Weak linear decrease of Θ_app with U was found above Ca = 2.4 × 10⁻⁵ up to the critical capillary number Ca_cr = 4.1 × 10⁻⁴. Below Ca = 10⁻⁵ the apparent receding angle depends much stronger on withdrawal velocity. The hydrodynamic (HD), and the simple and more general versions of the molecular-kinetic (MK) and molecular-hydrodynamic (MHD) theories of the wetting dynamics were used for quantitative characterization of the system in the steady dewetting regime. The effective Young's angle was used in the MK and MHD treatment of the experimental data following our previous publication J.G. Petrov, J. Ralston, M. Schneemilch, R. Hayes, J. Phys. Chem B, 107(7) (2003) 1637]. The HD theory only qualitatively satisfies our experimental data giving physically unreasonable value of the hydrodynamic cut-off (slip) length and too small static receding angle at U = 0. The MK theory gives acceptable values of the oscillation frequency K₀ of the molecules at the contact line. Its more general version, including the viscous dissipation in the contact line vicinity, yields higher oscillation frequency. Very large distance λ between adsorption centers on the solid substrate (about five times the diameter of a glycerol molecule) was obtained with both MK and MHD theories. The too small frequency K₀ obtained with the simple MHD theory is removed by the more general version, accounting for contact line and viscous friction in the inner and intermediate zone of the moving meniscus. All theories show discrepancies between theoretically expected and experimentally estimated values of some of the parameters of wetting dynamics.