# Measuring the solid air surface tension

For hydrophilic and hydrophobic materials,

$Y_{SG}$= Interfacial tension between the solid and gas

$Y_{SL}$ = Interfacial tension between the solid and liquid

$Y_{LG}$ = Interfacial tension between the liquid and gas

If you wanted to measure the effects of putting a layer of hydrophobic material on a glass sheet, how would you do it?

I know you could first put a drop of water on the sheet and check the angle and then compare it to when you added the hydrophobic material on the surface.

But how would you find the surface tension the water droplet has on the hydrophobic material? (By that I am asking what effect does the hydrophobic material have on the surface tension.)

I know $Y_{LG}$ would be the same as before, and it would be interfacial tension between water and air.

But what would $Y_{SL}$ and $Y_{SG}$ be? If I knew one of those I could find $Y_{SL}$ which would be the interfacial tension between the hydrophobic material and the water droplet.

I'm basically trying to find a value that quantifies the huge difference in surface tension between the surrounding water in a hydrophilic material and then putting a hydrophobic material around it which keeps the water inside.

• It is a very common issue that solid-liquid and solid-gas surface tensions are hard to measure, but it is commonly accepted to use the contact angle as a measure for the hydrophobicity of the surface, so you could just use $\theta_c$. Just be careful to measure both advancing and receding angles, because the equilibrium angle of a surface that is not molecularly smooth is hard to determine – Michiel Apr 29 '14 at 5:08