Super-hydrophobic: How can a droplet sit on a surface

Most of the real surfaces are rough. There two options for the water droplet.

i) To follow the roughness of the underlying substrate. When the substrate is hydrophilic the surface roughness reduces the water contact angle and when it is hydrophobic it enhances it.

So the parameter r (the ratio between the true surface area over the apparent one) controls θ* contact angle (Wenzel equation):

cosθ*= rcosθ

θ: Young’s contact angle

ii) The droplet does not follow the roughness of the surface

In this case the droplet is like a fakir. It is sitting upon a patchwork of solid and air. The more it is sitting on air, the higher the measured water contact angle.

The contact angle θ* is an average between the angle on the solid (cos θ) and the one on the air (cos180^o = -1):

cosθ^*= -1+f_s(cosθ+1)

f_s: fraction of the liquid that contacts the solid

The fakir state is also called Cassie – Baxter state.

A surface is called super-hydrophobic when water contact angle exceeds 150⁰. In this case usually a water droplet can bounce on the surface and also can roll-off on it with a tilt of less than 5⁰. In other words the surface is water-repellant. This kind of surfaces is also called self-cleaning. On a hydrophilic or on a hydrophobic surface the pollutants adhered to the water droplet, but the water droplet sticks on the substrate. So the water remains and evaporates on the surface and so the pollutants remain on it. Consequently the pollutants do not remove from the substrate. On a super-hydrophobic substrate the water rolls-off and leaves the surface taking the dirt with it. So the water cleans the surface. This is a very useful application especially for glasses of buildings. The self-cleaning mechanism is presented in the next figure:

Click for larger image

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