To demonstrate the difference in surface tension between glass, plastics and metals, during the practical training of our adhesive courses a test is performed with (demi)water droplets on the surface of these materials.
According to various publications, the surface tension of plastics is between 18 and 57 [mN/m], water at 72 [mN/m], glass at approximately 290 [mN/m], aluminium 1,200 [mN/m] and iron 2,550 [mN/m].
In practice, however, the behaviour of water droplets on metals turns out to be consistently worse than on glass. To explain this, researchers look for arguments such as oxidation, passivation, contamination and adsorption on/of the metal surface.
New insight shows that the published values for metals refer to the melt and not to the solid material surface. According to current understanding, it is more likely that the surface tension of metals in solid state is rather in the range of 50 - 60 [mN/m], which corresponds well with what is observed in a droplet test.
Further research is currently being conducted so that more accurate values for the solid phase of various metals will soon be available.