“…In dry samples, the low contact angle values observed can be attributed to an inherent hydrophilicity of dry materials. The inorganic component of tooth structure (i.e., HAP) has a strong affinity to water [21], so during first wetting, the hydrophilicity might be due to hydrogen bonds between HAP and absorbed water [22]. Following Aronov [9,12], human tooth material surfaces and HAP surfaces should present a certain amount of electric charge; this electric charge is produced by polarization of the surface's atoms.…”