Exploring superhydrophilia mechanisms to produce self-cleaning and antifouling coatings

Self-cleaning and antifouling coatings based on superhydrophilic surfaces offer potential advantages respect to superhydrophobic, being effective against non-polar and polar contaminants. Their reported action mechanism involves the formation of a water layer that decreases interaction with contaminants, though this does not consider the cleaning in dry conditions. Adhesion to the surface is largely dependent on the interfacial interactions between all phases involved (surface-contaminant-cleaning media). Hence, a comprehensive analysis of interfacial tensions should be able to predict the self-cleaning performance on different scenarios.

A theoretical approach was proposed to estimate the work of adhesion and its variation respect to the coating surface energy components for different contaminants-cleaning configurations: (i) polar powder removal by water; (ii) soot removal with air; (iii) Oil-contaminated powder removal by water and (iv) bacterial adhesion. The methodology is based on the balance of interfacial tensions in the surface-contaminant-media system, using Owens-Wendt-Rabel-Kaelble and Good's models. A TiO₂-ormosil coating, combining photoactivated superhydrophilic surface with hydrophobicity inside the pores was applied on cement mortar for experimental validation. Superhydrophobic surfaces and hydrophobic surfaces (w/o TiO₂) were also evaluated to discern the effects of roughness.

The calculations correctly predicted a markedly lower adhesion of the non-polar and biological contaminants to the superhydrophilic surfaces, as confirmed by the experimental results. In addition, we prove and explain for the first time the removal of soot in dry conditions. A satisfactory performance was confirmed on polar contaminants, explained by the similar calculated adhesion respect to the superhydrophobic surface and decreased water absorption and micro-roughness.