Wetting, paper surface roughness

As has been pointed out several times, the surface of paper is far from being a smooth ideal surface. This has great consequences on the wetting behaviour that is observed. For random rough surfaces, the influence of surface roughness can be described by using Wenzel s equation ... 

The SEM was used to demonstrate and confirm these theoretical projections [306,309]. In the early study [306], molten drops of PE and PMMA were allowed to spread and solidify on a paper substrate prior to standard preparation for SEM. Later, poly(phenyl ether) (PPE) vacuum pump oil (Santovac 5-Monsanto Chemicals), which is not volatile, was used for dynamic studies. The PPE was fed through a hole in the sample stub, from outside the specimen chamber, and the wetting experiment was video tape recorded [309]. Mori et al. [310] used freshly cleaved mica surfaces as steps 60 nm in height inhibit wetting. Surface roughness has a major effect on the local contact angle between the liquid and the substrate of interest. 

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