Factors Affecting Dynamic Wetting Phenomena

The observed effect of surface tension is less obvious. For static or equilibrium wetting, it is usually found that a lower liquid surface tension will improve wetting (i.e., reduce ff ). In the dynamic case, however, the velocity dependence of a, and therefor vi8o, for Uquids of the same viscosity but different surface tensions was in the direction of greater dependence on v with lower s. 

It has been suggested that the role of surface tension forces in the dynamic wetting process may be represented by 

If the reduction in surface tension for the liquid is brought about by the addition of a surfactant, the net affect on 6 will depend on levels and rates of adsorption of the surfactant molecules at the various interfaces involved. Obviously, an analysis of such a complex situation becomes very involved (or impossible), and experiment and experience become the best tools of the trade. 

Beginning with the basic definition of the reversible work of adhesion [Eq. (17.20)] and the Fowkes relationship [eq. (17.31)], one can place the above situation on a more quantitative footing, yielding the relationship 

If this equation is evaluated for the system water-tetradecane-PTFE, a value of Wasw2 = 0.4 mJ m is obtained. For the same system, WaS2W = 99.8 mJ m. Clearly, any wetting process that requires the displacement of an oil from a nonpolar solid surface by an aqueous solution must work against a considerable thermodynamic barrier. 

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