Roll-off angle

The roll-off angle is the angle of inclination of a surface at which a drop rolls off it. 

Background

As a rule, it is used to characterize ultrahydrophobic surfaces with very high contact angles (>>90°) where the drop is approximately spherical. With smaller contact angles, although a drop can also move from the surface, it is usually initially deformed and then slides over the surface.

The roll-off angle is an empirical variable which is highly dependent on the particular measuring conditions, such as drop size and tilt speed.