Pitcher plant inspired non-stick surface
“How flesh-eating pitcher plant trap insects?” http://www.youtube.com/watch?v=ya2ndp1OrPQ
David Quéré “Wetting and roughness” Annu. Rev. Mater. Res. 2008. 38:71-99
A. Tuteja, W. Choi. J.M. Mabry, G.H. McKinley and R. E. Cohen, “Robust omniphobic surfaces”,
U. Bauer and W. Federle
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.
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. 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; Similary, the surface of water strider’s leg is covered by regular arrays of nanopillars of 100 nm scale. (See figure)
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), reminiscent of the leidenfrost effect.
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 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 ).
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 with little possibility of self-healing. A possible solution to this is the approach taken by
I owe these ideas to the conversations that I have had with Joanna Aizenberg and her graduate student, Sung Kang.