Dear Dr. Mennatalla Elmanzalawy ,

as a premise, the measurements of the contact angle, while maintaining the same experimental conditions, depend mainly on the chemistry and morphology of the surface layer of the sample.

You can have complete answers to your question at the following links / references:

- https://www.sciencedirect.com/science/article/pii/B9780444637765000127

- https://www.biolinscientific.com/measurements/contact-angle

- Contact Angle and Wetting Properties by Yuehua Yuan and T. Randall Lee from Surface Science Techniques (2013) (Chapter 1 of Book).

Best regards, Pierluigi Traverso

Dear Dr. Mennatalla Elmanzalawy,

I would answer no if the coating thickness is larger than several hundreds of nm. Otherwise, the nature of the substrate could indeed influence the wetting conditions via van der Waals-like forces.

Otherwise, the robustness (in a chemical or mechanical sense) of the hydrophobic coating can be very much dependent on the substrate chemistry. For instance, a lot of coatings are more robust when glass is activated before (with Plasma O2 or in acid bath). This really much depends on the chemical affinity between coating and substrate. Generally, hydrophobic coatings are more robust when applied on activated glass or silicon, then on metals.

It is possible to have coatings that are fairly substrate-independent in terms of wettability. See, for example, the coatings developed in these articles: Article Mussel-Inspired Surface Chemistry for Multifunctional Coatings

In other situations, such as with self-assembled monolayers (SAMs), the substrate will impact the quality of the layer because a chemical bond between adsorbate and the substrate is what anchors the molecules of the layer (e.g., silane vs. alkanethiol chemistry). The substrate conditions, such as roughness or cleanliness, can also influence how the SAM molecules are oriented and what fraction of the surface is covered with a functional group.

Surface roughness by iteslf also affects wettability as demonstrated by the lotus leaf effect, whereby the superhydrophobicity stems from the surface microstructure, and surfaces that are described by the Wenzel or Cassie-Baxter equations. If you have a rough surface, you may want to consider the surface roughness effect on the wettability independently from the surface chemistry effect, particularly with the help of the Wenzel equation.

It does make sense to consider the coating thickness. Thank you all for the great answers.