Inner surface of Nepenthes slippery zone: ratchet effect of lunate cells causes anisotropic superhydrophobicity

Abstract

Inner surface of Nepenthes slippery zone shows anisotropic superhydrophobic wettability. Here, we investigate what factors cause the anisotropy via sliding angle measurement, morphology/structure observation and model analysis. Static contact angle of ultrapure-water droplet exhibits the value of 154.80°–156.83°, and sliding angle towards pitcher bottom and up is 2.82 ± 0.45° and 5.22 ± 0.28°, respectively. The slippery zone under investigation is covered by plenty of lunate cells with both ends bending downward, and a dense layer of wax coverings without directional difference in morphology/structure. Results indicate that the slippery zone has a considerable anisotropy in superhydrophobic wettability that is most likely caused by the lunate cells. A model was proposed to quantitatively analyse how the structure characteristics of lunate cells affect the anisotropic superhydrophobicity, and found that the slope/precipice structure of lunate cells forms a ratchet effect to cause ultrapure-water droplet to roll towards pitcher bottom/up in different order of difficulty. Our investigation firstly reveals the mechanism of anisotropic superhydrophobic wettability of Nepenthes slippery zone, and inspires the bionic design of superhydrophobic surfaces with anisotropic properties.

Recent development and emerging applications of robust biomimetic superhydrophobic wood

“…For example, many surfaces of plants and animals exhibit superhydrophobic properties, such as lotus leaves, mussels, butterfly wings, water strider legs, and nepenthes. [1][2][3][4][5] These superhydrophobic phenomena have been thoroughly proven that the micron-sized papillary structures on the surface and waxy hydrophobic substances of the surfaces such as lotus leaf would cause the water droplets falling off the lotus leaf accompanied by carrying away dust or other substances, resulting in a self-cleaning effect. Based on the extensive exploration and analysis of this specific functional phenomenon from nature, German scientists, Barthlott et al first proposed the "lotus effect" in 1997, which was then considered as a special wetting property.…”

Theoretical investigations on the superhydrophobicity of intrinsic hydrophilic surfaces with overhang microstructures

“…Here it is worth noting that some experimental studies suggested that such a transition was plausible. 33,34 For example, for hydrophilic materials with an intrinsic contact angle of 70 , a transition from noncomposite to composite states could occur if the depth of surface topography pores (a similar parameter to the pillar height) was very large. This happens perhaps because the difference in free energy and barrier between the noncomposite to composite states was so small due to adequate roughness that the transition between the two states could be easily realized under the experimental conditions where external resources may be available.…”

Recent innovations in surface topography

“…A biological example of this has been observed on pitcher plants 47 , where crescent shaped cells provide a similar topography which makes escape for its victims even more difficult than usual. Butterflies too have been observed to sometimes have a directionality of wetting 48 .…”