RELATIONSHIP OF THIN-FILM STABILITY AND MORPHOLOGY TO MACROSCOPIC PARAMETERS OF WETTING IN THE APOLAR AND POLAR SYSTEMS

The total free energy of a thin fluid film, sandwiched between a substrate and a fluid phase, is derived from the apolar (Lifshitz-van der Waals) and the polar (acid-base) interactions. While the instability and rupture of the thin film engendered by the apolar forces have been extensively studied, the polar interactions cannot be neglected unless the film fluid and at least one of the bounding media are completely apolar. For polar systems (e.g., water films), the total free energy of the film and its second derivative (force per unit volume) are shown to be related to the macroscopic parameters of wetting (i.e., the apolar and polar spreading coefficients and contact angle of macrodrops). Threshold of the film instability is derived from the Young-Laplace equation modified by intermolecular pressure, and the growth rate of instability is obtained from Navier-Stokes equations. Regions of the film stability/instability are determined from characterization of all possible variations of intermolecular forces with the film thickness, with components of spreading coefficients and macroscopic contact angle. For systems that are completely apolar, completely polar, or when components of spreading coefficients are of the same sign, the film rupture is guaranteed whenever the macroscopic drop displays a finite contact angle. Interesting possibilities occur when the apolar and polar spreading coefficients are of opposite signs: (a) the thin film may be unstable even though the corresponding macroscopic drop shows complete wetting; (b) the film may be stable despite a finite contact angle. Conditions under which growth of instability leads to the film breakup, or to spatially nonhomogeneous films, are discussed. Implications of result in rupture of macroscopic films, heterogeneous nucleation, and the equilibrium film pressure are indicated.