Hydrophobicity phenomena are present in nature; in the leaves of plants such as Nelumbo nucifera (lotus), in the wings of butterflies, or in the legs of water striders. It is also a leading surface property for corrosion protection, as hydrophobic surfaces are water and aqueous electrolytes repellant, anti-icing, bio-corrosion and anti-fouling [1]. These properties can be reached by adding functional species to the coating or by modifying the composition, structure or morphology of the surface layer, using techniques such as **dip coating** and **spray coating**.

Lotus effect. Image from [2]

Hydrophobicity is determined by the measurement of the contact angle of a water drop with the surface (WCA). Depending on the angle, three different regimes can be established:

- Hydrophilic: 10º < θ < 90º
- Hydrophobic: 90º < θ < 150º
- Superhydrophobic: θ > 150º

Atoms or molecules that are located at the surface are not able to form so many bonds with neighboring atoms than those that are in the interior, and therefore they have higher energy. This surface energy or surface tension, c, is equal to the work required to create a unit area of the surface at constant pressure and temperature. When a drop is placed in contact with a solid, the equilibrium of the solid and liquid surfaces are established at a certain angle called the static contact angle CA, θ_{0}, calculated by the Young equation [3]:

where γ_{SL}, γ_{SG}, γ_{LG} are the free surface energies by unit of area of, respectively, the solid-liquid, solid-gas and liquid-gas.

They can be calculated as follows:

Where A is the area and G is the Gibbs free energy, obtained by the following expression:

*G = H-TS*

* *where H is the enthalpy, S the entropy and T the absolute temperature.

**Wenzel model for rough surfaces: **since the Young equation cannot be applied to rough surfaces, Wenzel [4][5]**, **developed a model that relates the contact angle on a rough surface, θ’ to that with a flat surface θ0:

where *r *is a roughness factor, non-dimensional, and defined by the quotient between the area of a rough surface, *A**SL*, and the area of its geometric projection, *A**F**.*

**Cassie-Baxter model for rough surfaces: **Wenzel model is applied only for homogeneous interfaces. Therefore, another model was proposed in 1944 by Cassie and Baxter, that takes into account the air bubbles that form between the rough surface and the liquid. It consists on two fractions, each one with a fractional area and a contact angle *f *and θ.

* *

where *f *is the fraction of the fraction of the solid surface that is in contact with the liquid.

[1] Montemor, M.F., Functional and smart coatings for corrosion protection: A review of recent advances. Surface and Coatings Technology, 2014. 258: p. 17-37.

[2] Lotus Image by: Description: *Tropaeolum majus* – Kapuzinerkresse: Lotuseffekt oben Source: selbst fotographiert Date: 5.Sep’05 17h Photographer: Michael Gasperl (Migas) Location: AUT/OÖ/Steyr

[3] Mohamed, A.M.A., A.M. Abdullah, and N.A. Younan, Corrosion behavior of superhydrophobic surfaces: A review. Arabian Journal of Chemistry, 2015. 8(6): p. 749-765.

[4] Wenzel, R.N., Resistance of solid surfaces to wetting by water. Industrial & Engineering Chemistry, 1936. 28(8): p. 988-994.

[5] Wenzel, R.N., *Surface Roughness and Contact Angle. *The Journal of Physical Chemistry, 1949. 53(9): p. 1466-1467.

Adapted from: Redin, Maeztu, Juan Deyo. “Design and study of nanostructured coatings for the improvement of the corrosion resistance of aluminium alloys.” (2016), Universidad Pública de Navarra

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