Northwestern University

Abstract:

Recent analyses of the breakdown of a thin liquid film on a solid surface have pointed out the analogy to spinodal decomposition, using Cahn-Hillieard theory (Mitlin 1993; Mitlin and Petviashvili 1994) to explain the almost discontinuous jump in number density of "dry" regions containing adsorbed film interspersed among thicker plateau regions. Sharma and co-workers (1993-1999) have further exploited adsorption theory to explain the behavior of wetting and partially-wetting films undergoing breakdown with polar and apolar interactions. All use the long-wave theory of thin liquid films based on the work of Benney (1996), and extended to long-range molecular forces on isothermal surfaces by Williams and Davis (1982), and Burelbach, et al. (1988) and Joo, et al. (1991) for heated surfaces. However, the long-wave evolution equation assumes that the Navier-Strokes equations apply, which is not true for adsorbed films of thickness 0(1) nm. Hence transfer between the "dry" regions and the adjoining"thick" regions occurs by surface diffusion. This is true whether the repulsive force for apolar films on a bare surface is taken to be the London-van der Waals (3-9) potential, or the exponential form for polar liquids using the Born hard-sphere cut-off distance. However, nearly all real solid surfaces are coated with contamination or corrosion products, which modify the form of concept which allows convective, rather than surface diffusion, effects to evaporation. Migration and shrinkage of the rims is then found, with a dominant "dry" region growing at the expense of the remaining dry regions. The near-equilibrium "dry" region, which may be several nanometers thick, is then a strong attractor and remains nearly constant in thickness until nearly all the excess liquid has been removed by convection from the rims and concomitant evaporation from the thin films. At this point the uniform film dries out completely to an adsorbed layer very rapidly.