Contact Angle (Hydrophobicity(

A hydrophobic, self-cleaning surface requires a high static water contact angle, the condition often quoted is θ>160° to indicate the minimum inclination angle necessary for water to bead, pick up dirt particles, and roll off a surface due to its minimum adhesion. This effect is also called the Lotus Effect because lotus leaf has a double-structured surface that can reach a contact angle of 170°.

The contact angle is measured by the time vapour interface meets a solid surface. It quantifies the wettability applying the Young-Laplace equation; figured by a straight line that starts at the base and travels along its outer surface to the break-off point. It provides a way to determine the protection at surface level.

Formations of large, packed droplets demonstrate that the surface is effectively non-absorbing, otherwise known as the “water break test” or ASTM F-22 which are often used to check for cleanliness.