Drainage of horizontal films

Film thickness is determined by interferometry, which is based on comparison between the intensities of the light falling on the film and that reflected from it [4]. 

The drainage time T is determined and compared with the theoretical value for a flat film calculated from the Reynolds Eq. (8.5), 

For horizontal, fairly thick films ( 100 nm), Scheludko [3] derived an expression for the thinning between two disc surfaces under the influence of a uniform external pressure. The change in film thickness with drainage time, Vn-, is given by 

6) applies for the following conditions (1) The liquid flows between parallel plane surfaces (2) film surfaces are tangentially immobile and (3) the rate of thinning due to evaporation is negligible compared with the thinning due to drainage. 

Experimental results obtained by Scheludko and co-workers [2-4] with comparatively thick rigid films, produced from dilute solutions of sodium oleate and isoamyl alcohol, gave reasonably good agreement with the drainage equation. However, deviation from the Reynolds equation was observed in many cases due to tangential surface mobility. Surface viscosity can also slow down the drainage. 

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