Critical contact angles for liquid drops on inclined surfaces

The contact angles of liquid drops on inclined plane surfaces have been investigated at critical conditions of sliding. Drops of two liquids were tested on eight surfaces covering a wide range of advancing and receding contact angles. Contact angles and contact line dimensions were observed for various drop sizes and surface inclination angles. The contact-angle variation along the circumference of the drop was best fit by a third-degree polynomial of the azimuthal angle. The findings lead to the development of a general relation between the advancing contact angle, the receding contact angle, and the maximum Bond number for a liquid-surface combination. This relation is well supported by data from the literature. The new relation allows the characterization of a surface by its advancing contact angle for a given liquid, and prediction of the corresponding receding angle and maximum drop size.