Wetting Films on Locally Heterogeneous Surfaces Hydrophilic Surface with Hydrophobic Inclusions

6 WETTING FILMS ON LOCALLY HETEROGENEOUS SURFACES HYDROPHILIC SURFACE WITH HYDROPHOBIC INCLUSIONS 

FIGURE 2.23 Experimental data on average film thickness of tetradecane on steel, obtained by continuous thinning of the film (open symbols) and snbseqnent thickening (closed symbols). Solid line according to the disjoining pressnre isotherm IT =A/h [12]. 

Complete wetting is assumed for the hydrophilic part and partial wetting for the hydrophobic or less hydrophilic spot. The equilibrium profiles should be calculated according to the spot size and the value 6 of the equilibrium contact angle of the hydrophobic spot. It is shown in the following text that the critical width of a hydrophobic spot decreases by an order of magnitude with an increase in the contact angle of the more hydrophobic spot from 10° to 90°. 

As a reference system, a state was chosen in which each part of the surface is covered by a corresponding equilibrium film and an interaction between the films is absent. The equilibrium thickness of each of the films, and is determined by the corresponding disjoining pressure isotherms, n(//) and % h), respectively  

FIGURE 2.24 Schematic representation of a wetting film on a solid surface containing a hydrophobic spot at-L x L. (a) Reference system stepwise film profile, equilibrium film thickness and h, on the corresponding hydrophilic and hydrophobic parts, if each part is unbounded, (b) Depression formation over a hydrophobic spot when the width L of a hydrophobic part is smaller than critical value L. (c) Rupture of a wetting film if the width of the hydrophobic part L and formation of a thin film on the hydrophobic part. 

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