DEWETTING DYNAMICS FOR LARGE EQUILIBRIUM CONTACT ANGLES

The dynamics of dewetting were investigated for thin films of poly(ethylenepropylene) (PEP), on polystyrene (PS) and poly(methyl methacrylate) (PMMA) substrates. Dewetting was induced by immersing the films in aliphatic alcohols, thereby changing the equilibrium contact angle of the PEP film from zero to some very high value. Contact angles of 180 degrees were obtained with each of the alcohols whenever PMMA was used as the substrate, whereas contact angles from 75 degrees to 180 degrees were obtained for the PS substrates. Dewetting dynamics were quantified by analyzing the growth rate of circular dry patches during the early stages of the dewetting process. These patches grew at a constant velocity, which, for theta(e) = 180 degrees, varied from 0.25 gamma/eta to 0.76 gamma/eta, where gamma is the PEP/alcohol interfacial tension and eta is the viscosity of alcohol-saturated PEP. Much lower velocities were obtained for the lower contact angles. All results are in good agreement with a simple theory of dewetting, where it is assumed that the geometry of the rim surrounding the dry patch is fixed by the dynamic contact angle, which is in turn related to the equilibrium contact angle and the dewetting velocity.