Secret of lotus leaf: Importance of double roughness structures for super water-repellency

Abstract

The physical meaning of double-roughness structure (DRS) surfaces on lotus leaves, which are formed by tens nano-sized and micro-sized structures, is discussed based on the bouncing behavior of water droplets on such surfaces. Super water-repellency and the sliding phenomena of raindrops on lotus leaves are well-known as the “lotus effect”, however, the necessity of the DRS surface has been rarely understood because the lotus effect can be observed on single roughness structure (SRS) surfaces as well. To understand the physical meaning of the DRS surface, bouncing behaviors of water droplets on the lotus leaf were investigated. Two model surfaces that had SRSs and DRSs were prepared from diarylethene (DAE) as a photochromic compound. The SRS surface of DAE, which was covered by random-orientated needle-shaped crystals, showed a non-bouncing behavior. In contrast, the DRS surfaces of DAE and lotus leaves showed bouncing behaviors. The difference between bouncing and non-bouncing behaviors can be elucidated by the Laplace pressure generated by surface structures in comparison with the dynamic pressure generated by the impact of falling water droplets onto the surfaces. The random-orientated needle crystals on the SRS surface generated a smaller Laplace pressure. On the other hand, larger Laplace pressure was generated by some randomly orientated small-sized structures on the large-sized structures of the DRS surfaces of DAE and lotus leaves.