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.
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