The authors have studied on the theory in relation to wetting-out action of solutions against textile materials, containing wetting-out agent, and some experiments have been carried out with following conclusions:
(1) The essential motive force of wetting-out action is an interfacial expansion, not a tension, which occurs on the interface of a liquid against a solid. The surface tension of a liquid against air, and the interfacial expansion of the same against a solid, have an important relation to wetting. The interfacial expansion coincides. vectorically with the adhesion tension (Haftspannung), which, advanced by Freundlich, is assumed to be the difference between the interfacial tensions of the solid.
(2) Let σ be the surface tension of a liquid against air, ε the interfacial expansion of that against a solid. If a drop of the liquid lies upon the solid surface, two forces act at the point O, as represented in Fig. 1. When the drop remains lying in equilibrium condition upon the solid surface with formation of an angle of contact θ, the cosine of contact angle indicates the ratio of ε to σ, namely:
cosθ=ε/σ or ε=σ.cosθ
If the contact angle is smaller than 90°, as in case a drop of quicksilver lies on the surface of a glass plate, an interfacial tension ε' occurs on the interface of the liquid against the solid, as represented in Fig. 2.
(3) The surface tension which acts, in the case of wetting is dynamic, not statip, and the dynamic tension is always larger than the static. The static tension of an aqueous solution of wetting-out agent is very small but the dynamic of the same is always not so small. The dynamic surface tension, which acts in the case of high rate of wetting, is nearly equal to that of pure water. It is recongnised that the surface tension of a pure liquid, such as water, has, in most cases, the same magnitude either in the static state or in the dynamic.
(4) Although the static surface tension of water is lowered remarkably by the presence of wetting-out agent, the substantial action of the same is to enlarge the contact angle without noticeably depressing the dynamic tension. The wetting power of a solution against textile material is shown by the product of the dynamic surface tension and the cosine of the contact angle.
(5) The wetting-out action is a dynamic phenomenon. Therefore, in testing the wetting power of a solution, the rate of wetting must be measured at its various stages.
In the capillary test, using filter-paper strips for the solutions containing various amount of Nekal BX dry, it was observed, at any time, that the smaller concentration, the higher the capillary rise was. On the other hand, when scoured calico strips were used in stead of filter-paper, the stronger solution of Nekal showed higher rate of the capillary rise at the beginning but, after a while, the rate was overcome by the weaker solution.
In the former case, the contact angle of pure water against the filter-paper was 180° or nearly so. Therefore, the smaller amount of Nekal the solution
contains, the higher the rate of the capillary rise is, owing to the larger dynamic surface tension as the magnitude of the tension is inversely proportional to the amount of Nekal in the solution. In the latter case, the contact angle of pure water against the scoured calico was between 90°_??_180°. Therefore, Nekal in the solution acts mostly to enlarge the contact angle, as the dynamic surface tension of the solutions is nearly equal to that of water at the beginnig owing to high rate of the capillary rise. However, after a while, in consequence of reduced rate of the capillary rise, the dynamic tension of the solution becomes smaller and tends to translate to the static, and the higher the concentration of Nekal, the more this tendency is pronounced.

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