Coating Terms

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Forces which tend to reduce surface area of any liquid. Depending on the type of liquid, all liquids have intermolecular attractions. Molecules deep within a liquid, are attracted equally in all directions by similar neighbouring molecules. Hence, they are in a stage of balanced equilibrium with their surroundings. Nevertheless, a molecule at the surface (the vapour-liquid interface) is pulled only by the molecules under and near and not by those above, due to the fact that vapour phase density is much lower than that of liquid phase. Work should be done against cohesion forces between liquid molecules, to increase the free surface area by taking the molecules to the surface from bulk liquid. Consequently, molar free energy of surface is higher than molar free energy of other parts of liquid. (Note: Another definition of surface tension is the free surface energy per unit surface area of liquid).
There are various methods to determine the surface tension of liquids. Four of them is frequently used in paint industry.
(a) Du Noüy Ring method for liquid paints.
(b) Wilhelmy plate method for liquid paints.
(c) Measuring the contact angle for liquid paints. (Sessile drop method)
(d) Test ink method for cured paint films