# Talk of wettability

#### RPI_Quantum

In interfacial studies, there is always talk of wettability. I am wondering if anyone can explain to me the significance of this concept, beyond the magnitude of the contact angle? What is the difference between interfaces with >90 deg contact angle and <90 deg contact angle that merits this distinction between wetting and non-wetting?

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#### brewnog

Gold Member
This answer may be completely misleading, and perhaps even wrong. Bear with me. I also apologise for my non existant latex skills, I'll try and do something about that.

If you imagine a liquid droplet on a nice flat solid surface, you can imagine that in some cases, surface tension is such that the droplet holds its shape (other than being flattened on the contact zone obviously). If this surface tension is decreased, the contact area will spread and the droplet will lose some of its shape.

For 'perfect wetting', the liquid will be spread as a thin film across the surface. In this case, the wetting angle (theta) is said to be zero.

For 'complete non-wetting', the exact opposite happens. The liquid holds itself into a spherical droplet, the contact area is minimal and the wetting angle is said to be 180 degrees.

I've got a nice little diagram here, but have no scanner so it's for my eyes only. If you imagine a semi-wetting droplet on a flat horizontal surface, there is a gamma s,l component parallel to the surface originating at the point at which the droplet boundary touches the solid surface, and a gamma l,v component tangental to the droplet at the same point. The wetting angle is that between these components. These components balance a third gamma s,v component acting the opposite direction to the first, giving rise to the Young Equation:

(gamma s,v) = (gamma s,l) + (gamma l,v)cos(theta)

I hope I've not wrecked this too much, I might try to tidy it up later.

#### RPI_Quantum

Thanks. I've used and worked with Young's equation a lot in many different courses, but I guess I needed someone else to clarify the details. It makes a lot more sense to me now conceptually.

Thanks again!

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