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

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.

2. This manuscript concentrate on the anisotropic wettability of Nepenthes slippery zone. Thus, in addition to directions of pitcher up and pitcher bottom, sliding angles of other two directions need to be measured and studied as well. 3. Chen et al. (Chen, Zhang, Zhang, Liu, Jiang, Zhang, Hang, Jiang. Continuous directional water transport on the peristome surface of Nepethes alata. Nat. 532, 85-89.) studied the morphology of the carnivorous plant Nepenthes alata. Different from the sliding droplets observed in the present experiment, Chen et al. found that water would spread directionally on Nepenthes alata. What is the reason causing the different droplet moving behaviors? 4. From the rough morphology structure of the Nepenthes slippery zone as well as the excellent superhydrophobicity, it seems that the droplet on the slippery zone falls in the state I in the Molecular Dynamics study of Guo et al. (Guo, Tang, Kumar. Droplet morphology and mobility on lubricant-impregnated surfaces: a molecular dynamic study. Langmuir, 35, 16377-16387, 2019). More discussions of droplet morphology would deepen this work by comparing with the existing theoretical work.

06-Feb-2020
Dear Dr Wang On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-200066 entitled "Inner surface of Nepenthes slippery zone: Ratchet effect of lunate cells causes anisotropic superhydrophobicity" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. 1. In the results part, only used the droplet of volume 3 μl to test the sliding angle. For the accuracy of the experiment, the author was advised to do more experiment with different volume.
2. It is generally believed that the pitcher plant's surface is a kind of super-slippery surface, the author is recommended to compare the model's difference between traditional super-slippery surface with the established model in this paper. Comments to the Author(s) This manuscript experimentally studied the anisotropic wettability of the Nepenthes slippery zone. It was found that the anisotropic superhydrophobicity was affected by structure characteristics of lunate cells, which was also verified by the derived quantitative model. This work is expected to offer inspirations for bionic design of anisotropic superhydrophobic surfaces. Yet, I have some concerns below: 1. The sliding angle towards pitcher up is around 5.22 ° while the sliding angle towards pitcher bottom is around 2.82 °. Thus, a conclusion that the Nepenthes slippery zone has a remarkable anisotropic superhydrophobicity is claimed. My argument is, such a difference of the sliding angle is large enough to support the conclusion? 2. This manuscript concentrate on the anisotropic wettability of Nepenthes slippery zone. Thus, in addition to directions of pitcher up and pitcher bottom, sliding angles of other two directions need to be measured and studied as well. 3. Chen et al. (Chen, Zhang, Zhang, Liu, Jiang, Zhang, Hang, Jiang. Continuous directional water transport on the peristome surface of Nepethes alata. Nat. 532, 85-89.) studied the morphology of the carnivorous plant Nepenthes alata. Different from the sliding droplets observed in the present experiment, Chen et al. found that water would spread directionally on Nepenthes alata. What is the reason causing the different droplet moving behaviors? 4. From the rough morphology structure of the Nepenthes slippery zone as well as the excellent superhydrophobicity, it seems that the droplet on the slippery zone falls in the state I in the Molecular Dynamics study of Guo et al. (Guo, Tang, Kumar. Droplet morphology and mobility on lubricant-impregnated surfaces: a molecular dynamic study. Langmuir, 35, 16377-16387, 2019). More discussions of droplet morphology would deepen this work by comparing with the existing theoretical work.

26-Feb-2020
Dear Dr Wang, It is a pleasure to accept your manuscript entitled "Inner surface of Nepenthes slippery zone: Ratchet effect of lunate cells causes anisotropic superhydrophobicity" in its current form for publication in Royal Society Open Science. The comments of the reviewer(s) who reviewed your manuscript are included at the foot of this letter.
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Dear editors,
Thanks for your effort to this manuscript, also appreciate the reviewers for their valuable and constructive comments. In the following, we try our best to reply the comments in detail.
Reviewer 1-Comment 1: In the results part, only used the droplet of volume 3μl to test the sliding angle.
For the accuracy of the experiment, the author was advised to do more experiment with different volume.
Reply: Thanks for the suggestion. In fact, in measuring all the sliding angles, considering the evaporation and the syringe accuracy, the volume of ultrapure-water droplet is not strictly limited to 3μl, sometimes more than 3μl, sometimes less than 3μl. In order to express this point accurately, in the revised manuscript, we put a word 'approximate' before 3μl and have highlighted this changed point with red colour.

Reviewer 1-Comment 2:
It is generally believed that the pitcher plant's surface is a kind of super-slippery surface, the author is recommended to compare the model's difference between traditional super-slippery surface with the established model in this paper.
Reply: Exactly, the Nepenthes pitcher consists of lid, peristome, slippery zone and digestive zone.
The well-known 'super-slippery surface' is the inner surface of slippery zone. It shows super-slippage properties to insects (Roy. Soc. Open Sci. 2018, 5, 180766.). The super-slippery surface causes the water-droplet to present a sliding angle of about 3° when sliding toward pitcher bottom, and about 6° when sliding toward pitcher up. In the manuscript, our established model gives an explanation to the difference in the two types of sliding angles. Compared with the previous theories, our model is the first to focus on the difference in sliding angles.
According to the reviewer's suggestion, in the revised manuscript, we analyze the model's difference between the traditional super-slippery surface and our established model, as follows: Previous studies have shown some extremely important theoretical models to characterize the super-slippage properties of the Nepenthes slippery zone, in the aspects of superhydrophobicity [17,24,33], insect attachment ability [15,24,25], and static contact angle [22]. Here, our proposed model firstly explains how the structure characteristics of lunate cells, including the slope/precipice angle and the slope/precipice height, affect the anisotropic superhydrophobic wettability of Nepenthes slippery zone.
The changed point has been highlighted with blue colour in the revised manuscript. In Chen's article (Nature, 2016, 532, 85-89), the authors focused on the peristome, they studied the droplet moving behaviors on the peristome ( Figure R1). In our manuscript, we investigated the sliding behaviors of water droplet on the slippery zone ( Figure R1). Reply: Thanks for the valuable suggestion, and we have read this article. According to this article, we discussed the ultra-water droplet morphology on the slippery zone, as the follows: According to a molecular dynamic investigation of droplet morphology on lubricant-impregnated surface [35], when an ultrapure-water droplet is sliding on the Nepenthes slippery zone towards pitcher up/bottom, the ultrapure-water droplet neither completely infiltrates into nor floats on the micro-nano scaled structures of slippery zone. Instead, the ultrapure-water droplet partly infiltrates into the isotropic wax coverings and anisotropic lunate cells.
And, this changed point has been highlighted with orange color in the revised manuscript.
The above are our reply to the comments. However, we are not sure that the reviewers can completely accept our reply. If not, we will revise this manuscript again, thank you.
Best regards, Lixin Wang