All Solids, Including Teflon, Are Hydrophilic (To Some Extent), But Some Have Roughness Induced Hydrophobic Tendencies

It has recently been argued on the basis of four experiments that Teflon can often be regarded as hydrophilic when considering the interaction between liquid water and the solid surface of Teflon [Gao and McCarthy, Langmuir 2008, 24, 9183-9188]. The authors also recommend that more recognition be given to hydrophilic and hydrophobic as qualitative adjectives and discuss the importance of advancing and receding contact angles. In this work, I use net Surface free energy changes for events consisting of (i) a smooth solid wrapping a droplet of water and (ii) a grain attaching to a droplet, to show that all solids with a Young's law contact angle theta(e) < 180 degrees can be considered absolutely hydrophilic. This terminology is true in the sense that attachment of the solid to the liquid is always preferred even though the relative strength decreases as theta(e) increases. However, I also demonstrate that, within a surface free energy model, solids with theta(e) > 90 degrees can be regarded as possessing hydrophobic tendencies with increasing roughness. The effect of the bending rigidity of the substrate is discussed, and a condition for minimum droplet radius for wrapping to occur is given.