ABSTRACT
Thin liquid films supported on horizontal solid surfaces suddenly rupture due to formation of holes when their thicknesses are reduced to several hundred micrometers. The mechanism and conditions of the film breakup are explored by calculating the free energy change produced by the formation of a hole. The hole profile is evaluated from the Young—Laplace equation of capillarity. The free energy change becomes maximum at a certain “transitional” film thickness, which signals a possible onset of film instability due to expansion of a metastable hole. However, creation of a transitional hole requires a relatively large amount of external energy, and is, therefore, not probable in quiescent environments. The free energy change continues to decline with further reduction in the film thickness and eventually, a “critical” thickness is reached where the free energy after formation of the hole equals the free energy of the initial state of the unbroken film (ΔF=0). This is a sufficient condition of the film instability because the formation of a critical hole is more probable due to a smaller energy requirement. In general, the region of instability is confined between the critical thickness and the transitional thickness. Both the transitional thickness and the critical thickness increase nonlinearly with increased surface nonwettability (i.e., the equilibrium contact angle) and increased hole radius. A finite “limiting” film thickness is, however, reached for infinitely large holes and films thicker than the limiting thickness cannot be destroyed. The model predictions for the film thickness at the instant of rupture are in qualitative and quantitative agreement with the available data on several solid—liquid systems. © 1990 Academic Press, inc.
