Pitcher plant inspired non-stick surface

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References

1. “Biomimétise” http://fr.wikipedia.org/wiki/Biomimétique

2. David Quéré, “Wetting and roughness”, Annu. Rev. Mater. Res. 2008. 38:71-99

3. A. Tuteja, W. Choi. J.M. Mabry, G.H. McKinley and R. E. Cohen, “Robust omniphobic surfaces”, PNAS 2008 105:47 18200-18205

4. U. Bauer and W. Federle, "The insect-trapping rim of Nepenthes pitchers", Plant Signaling & Behavior 2009 4:11 1019-1023

5. “How flesh-eating pitcher plant trap insects?” http://www.youtube.com/watch?v=ya2ndp1OrPQ

Keywords

Biomimetics, wetting, superhydrophobicity, pitcher plant,

Atricle

Biomimetics may be a new buzz-word in science, but the idea of copying nature’s designs to man-made devices, is an old one. Gustave Eiffel, purportedly, was inspired by the structure of the femur to create the Eiffel tower; The velcro was modelled on the seeds of the burdock plant, after a Swiss Electrical Engineer George de Mestral noticed them sticking to the fur of his hunting dog. [1]

Pic1.jpg

Figure 1 Examples of natural superhydrophobic materials, as revealed by SEM. (a) Leaf of Colocasia esculenta, (b) Lotus leaf, (c) Legs of a water strider, (d) Surface of a mosquito (Culex pipiens) eye. Taken from (2)

Similarly, research on superhydrophobic materials have been inspired by examples nature, from the self-cleaning property of lotus leaf to the strong water-repellency of water strider’s legs. [2] In each case, the superhydrophobic property of these naturally occurring materials intimately linked to their micro- and nano-structures. Lotus leaves exhibit bumps at the scale of 10 um and each bump (or papilla) covered by fine nanostructures at the scale of 100 nm; Similarly, the surface of water strider’s leg is covered by bristles at the scale of 10 um. (See figure 1) The explanation for this rather paradoxical behaviour - where surface roughness actually helps water to glide ‘smoothly’ rather than impede it - stems from the creation of an air cushion between the water droplet and the material surface in the Cassie-Baxter case (see figure 2), reminiscent of the leidenfrost effect.

Microstruct superhydrophobic.png

Figure 2Three different wetting cases. 1) Young's model, 2) Wenzel model, 3) Cassie-Baxter model.

Many groups have attempted to replicate these effects by creating artificial microtextures, ranging from regular micro-pillars, fractal micro-patterns to mushroom pattern pillars. In some cases, these artificial structures are able to perform better than their counterparts in nature. A group from MIT led by Robert E. Cohen [3] successfully created omniphobic surfaces that can repel a huge variety of liquids, from heptane, methanol to water. Their structures are also able to repel liquid (pentane) with surface tension as low as 15.7 mN/m (surface tension of water is 72.1 mN/m). This is normally not possible with naturally occurring structures, because the low liquid tension favours the filling up of gaps by the liquid as in the Wenzel case (see picture 2).

Artificial.png

Figure 3 Examples of synthetic microtextured surfaces. Taken from (2)

One big problem with the approaches outlined so far is that they all depend on the exact configurations of artificial micro- to nano-sized structures which are brittle and have little possibility of self-healing. A possible solution lies literally on the rim of the Nepenthes pitchers. Carnivorous pitcher plants of the genus Nepenthes capture prey in a pitfall trap which depends on the slippery surface of the rim. [4] The micro-structures on the rim are able to trap a thin layer of water and cause insects to slip by aquaplaning on the thin water film. See video in [5]. This is reminiscent of how a thin layer of water is able to cause a skater to glide easily. An artificial structure based on this idea has been developed by Aizenberg's group and preliminary results will be published on 22nd September edition of the Journal Nature. There are many advantages to this approach, such as the possibility of self-healing, and the possibility of creating a repellent surface to a huge variety of fluids. More studies are required however to better understand this phenomenon and the possible future applications are enormous.

Acknowledgement

I owe these ideas to the conversations that I have had with Joanna Aizenberg and her graduate student, Sung Kang.