Fig. 1.

Two parameters that are commonly measured to characterize the interaction of water with leaf surfaces are the static contact angle of a sessile drop (A, B) and the drop retention angle of a sliding drop (C, D). The drop retention angle is the angle of a tilting stage at which a drop begins to slide or roll off. It is the more meaningful parameter in the context of water shedding, as can be seen by comparing static contact angles (ca) and drop retention angles (dra) on oak (Quercus robur, A, C) and soapwort (Saponaria officinalis, B, D) leaves. While the static contact angles on both leaves are very similar, the drop retention angles differ drastically.