Wetting of a Solid Substrate

If a small drop of liquid is placed on the surface of a solid substrate the shape of the drop is determined by the equilibrium equation 

FIGURE 3.6. Possible shapes of a drop of a liquid on a solid substrate (a) wetting, (b) nonwetting, and (c) complete wetting. 

If we deal with a volatile liquid and F 0, the adsorption of its vapor at a solid surface changes all 7-coefficients so that complete wetting is achieved (F = 0, cos a = 1). For nonvolatile liquid with F 0 such a case is impossible and the drop is spreading. The equilibrium thickness of the film is different for the two cases. For the complete wetting with a volatile liquid the thickness is determined by the Van der Waals forces [35] 

Equations (3.24) and (3.25) are true for both isotropic and nematic phases. However, in the latter case, it is assumed that the orientation of the director L at the free surface of a nematic, and at the interface with a solid substrate, is the same. If this is not the case, the Prank elastic energy must be taken into account [1]. 

Spreading phenomena and the formation of the so-called precursor films, moving ahead of the apparent front of a liquid, were studied both in isotropic liquids [37] and liquid crystals [38]. 

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The ease of wetting of a solid substrate may be determined by means of critical surface tension, this is defined as the highest surface tension liquid which will wet the surface. The critical tensions of some common substrates in dynes/cm are given in Table 7-4. 

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