Abstract
Nepenthes slippery zone presents surface anisotropy depending on its specialized structures. Herein, via macro–micro–nano scaled experiments, we analysed the contributions of lunate cells and wax crystals to this anisotropy. Macroscopic climbing of insects showed large displacements (triple body length within 3 s) and high velocities (6.16–20.47 mm s−1) in the inverted-fixed (towards digestive zone) slippery zone, but failed to climb forward in the normal-fixed (towards peristome) one. Friction force of insect claws sliding across inverted-fixed lunate cells was about 2.4 times of that sliding across the normal-fixed ones, whereas showed unobvious differences (1.06–1.11 times) between the inverted- and normal-fixed wax crystals. Innovative results from atomic force microscope scanning and microstructure examination demonstrated the upper layer of wax crystals causes the cantilever tip to generate rather small differences in friction data (1.92–2.72%), and the beneath layer provides slightly higher differences (4.96–7.91%). The study confirms the anisotropic configuration of lunate cells produces most of the anisotropy, whereas both surface topography and structural features of the wax crystals generate a slight contribution. These results are helpful for understanding the surface anisotropy of Nepenthes slippery zone, and guide the design of bioinspired surface with anisotropic properties.
Highlights
Nepenthes plants have evolved pitchers growing from the tips of their leaves to capture and digest insects, which provide nutrients& 2018 The Authors
Via macro –micro –nano experiments, including climbing behaviour observation, friction force measurement, atomic force microscope (AFM) scanning and scanning electron microscope (SEM) examination, we showed procedure information about climbing behaviours and friction forces, especially the innovative information about friction data provided by an AFM cantilever, to systematically analyse the contributions of lunate cells and wax crystals to the surface anisotropy of slippery zone
The results indicate that the lunate cells present a significant anisotropy, making insect claws generate much higher friction force when scrambling in the inverted-fixed slippery zone
Summary
Nepenthes plants have evolved pitchers growing from the tips of their leaves to capture and digest insects, which provide nutrients& 2018 The Authors. Considering the significant differences in macromorphology, 2 microstructure and their corresponding functions, the pitchers can be typically divided into four parts: a canopy-like lid, a collar-formed peristome, a slippery zone and a digestive zone [5,6]. The collar-formed peristome consists of radial ridges, which extend towards the pitcher inside [2,9,10] These structures make liquid film generate a continuously unidirectional transport [11], having been considered as a bionic prototype to design functional surfaces for the unidirectional liquid transport [12,13,14,15], and to develop super-lubricant interfaces for the controllable motion of liquid droplets [16,17,18,19,20]. The digestive zone has been well explored in digestion of prey, absorption and transport of the derived nutrients [32,33]
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