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Tuesday, 23 September 2014
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Raw materials & technologies, Technologies, Functional coatings

Anti-slip coatings copied from insect-eating plants

Friday, 22 June 2012

Dead by anti-slip surface: It rains and the ant reaches the protective lid of a carnivore. One more drop falls onto the surface – and the insect slides down without any escape. 

(A) <emphasize>N. gracilis</emphasize> pitcher with visiting ant, showing the epicuticular wax crystal surfaces on the inner pitcher wall and on the underside of the pitcher lid. (B) The horizontal orientation directly above the pitcher opening puts the lower lid surface in an ideal position for prey capture.

Source: University Cambridge
(A) N. gracilis pitcher with visiting ant, showing the epicuticular wax crystal surfaces on the inner pitcher wall and on th...

The plant provides numerous trapping mechanisms , such as specialised slippery surfaces on the peristome  and the inner pitcher wall as well as viscoelastic pitcher fluids. One more good example for functional traits from mnature

Different trapping surfaces under different weather conditions

Ulrike Bauer and her colleagues from University of Cambridge, UK investigated and published the contribution of each Nepenthes gracilis surface (inner wall, lower lid surface, peristome) under different experimental conditions (before/during/after simulated rain).

Especially the difference between the dry surface, which was absolutely unslippery and the change after 2-3 min simulated rain to a highly slippery peristome is an interesting outcome for the coatings industry.

The researchers found a highly significant dependence of the surfaces' trapping efficiency on the experimental conditions. Ants fell from the lower lid surface only under the impact of simulated ‘rain’ drops, in which case up to 57 % of the visitors were captured.

The (weather-independent) wax crystal layer on the inner pitcher wall provided a low but more or less constant baseline trapping efficiency (ca 7 %).

Experimental set-up for testing the slippery

The researchers labeled thirty N. gracilis pitchers (each on a different plant) in the field and randomly assigned to an experimental or a control group.

Using a fine paint brush, a thin layer of a non-toxic, transparent and odourless PDMS polymer ("Sylgard 184”, Dow Corning) was applied to the lower lid surface of the pitchers in the experimental group.

This product has been shown to have no measurable effect on insect attraction but provide a hydrophobic, non-slippery surface for insects.

All prey was removed from the pitchers and the fluid was filtered through a Nuclepore track edge membrane filter (25 mm diameter, 12 µm pore size, Dow Corning).

A small polyurethane cone (cut from a commercial ear plug) was inserted into the tapered bottom end of the pitcher to prevent the loss of prey.

Prey was sampled every third day for a total of 19 days by sucking out the pitcher fluid using a 10 mL syringe with an attached silicone tube, transferring the fluid to a petri dish, and removing all prey manually with a pair of fine spring steel tweezers.

Evolving different structural and functional wax crystal surfaces

Scanning electron micrographs of the inner pitcher wall and underside of the pitcher lid revealed that both wax crystal surfaces are radically different in structure. The inner wall surface was similar in morphology to wax crystal surfaces studied in other Nepenthes species, with a continuous, 3.05 ± 0.36 µm thick layer of leaf-like wax platelets connected to an underlying matrix of shorter wax crystals.

In contrast, the lower lid surface was covered with discrete, pillar-like wax structures, 1.78 ± 0.36 µm in height and 1.57 µm in diameter. The individual micropillars were unevenly distributed across the surface and sometimes densely clustered so that they appeared merged into solid blocks. The cuticular surface in between the micropillars was perfectly smooth and free of any crystal structures. The largest gaps between (clusters of) micropillars were typically 2.34±0.62 µm wide.

 
Test-preparation to compare the  wax crystal structure on the lid and the inner pitcher wall

Five N gracilis upper pitchers were collected in airtight plastic bags and transported in a cool box to the laboratory where they were quench-frozen (3–4 hours after collection) in liquid propane cooled in liquid nitrogen. Approximately 1 cm2 pieces of the pitcher lid and inner pitcher wall were cut with a razor blade, freeze-dried, mounted on SEM stubs and sputter-coated with a 20 nm layer of gold. Alternatively, samples were freeze-fractured before freeze-drying. The microstructure of the wax crystal layers on the lower lid surface and inner wall surface was examined using a Philips FEI XL30-FEG SEM with an accelerating voltage of 5.0 kV.

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