Abstract

THE large values given by Fogg1 for the contact angles of water with leaves suggest that these are apparent rather than true contact angles. Adam2 and Wenzel3 have shown that rough surfaces give an apparent contact angle which is greater than the true contact angle for the smooth material of the surface when the true contact angle is greater than 90°. We have recently extended this theory to porous surfaces4, and to surfaces so rough that much air is entrapped at the interface between the water and the solid; large apparent contact angles are then possible when the true contact angle is even less than 90°. The apparent contact angle is given by where θD is the apparent or observed contact angle, θ is the true angle, f1 is the area of solid – water contact and f2 is the area of air – water contact per unit superficial area of the interface.

References

1. Fogg, Nature, 154, 515 (1944).
2. Adam, “Physics and Chemistry of Surfaces”,186 (3rd ed., Oxford, 1941).
3. Wenzel, Ind. Eng. Chem., 28, 988 (1936).
4. Cassie and Baxter, Trans. Farad. Soc., in the press.