Superhydrophobicity of lotus leaves versus birds wings: Different physical mechanisms leading to similar phenomena

Remarkable water repellency of birds' feathers and lotus leaves is discussed. It is demonstrated that physical mechanisms of superhydrophobicity of birds' feathers and lotus leaves are very different. The topography of lotus leaves is a truly hierarchical one, whereas birds' feathers manifest pseudohierarchical relief, where various scales do not interact. The pronounced stability of the Cassie state observed on birds' feathers is due to the high value of critical pressure necessary for their total wetting, which is on the order of magnitude of 100 kPa. This high value allows feathers to withstand large dynamical pressure of rain droplets and remain dry under the rain. The energy barrier separating the Cassie state from the complete wetting situation calculated for a feather is also very high, allowing the increased stability of superhydrophobicity.