Contact angle and (super)hydrophobicity

An isolated liquid droplet is spherical, due to surface tension. But when in contact to a surface, interaction between liquid and solid will change the matter.

In contact to a surface, the drop will remain spherical in the (unusual) case where the liquid has absolutely no affinity with the substrate (case a) [1]. If there is a strong affinity between the liquid and the solid, the liquid spreads (case c), forming a spherical cap or a rather flat lens, with a well defined angle at the edge. This angle is called the contact angle, and is a measure of the interaction between the liquid and the solid.

For water, for a low contact angle (signaling strong interaction), we talk about a hydrophilic surface. For a large contact angle (weak interaction), we talk about a hydrophobic surface.

We can decrease the affinity between the liquid and the substrate by patterning. When the liquid sits only on top of the textures (case b), interaction is reduced, and the contact angle increases : this is the principle of superhydrophobic surfaces.

Notes

[1This case can be emulated if a thin gaz layer can be trapped between the liquid and the solid, the so called Leidenfrost effect.

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