A pitcher plant captures insects using a pitfall trap, which relies on a slippery surface, the peristome. The peristomes are covered by a pattern of macro- and microscale grooves as shown in the light and scanning electron microscopy images. Macroscopic grooves facilitate the spread of water along them but hinder lateral spreading. Microscopic grooves reduce instabilities in the water film.
A pitcher plant captures insects using a pitfall trap, which relies on a slippery surface, the peristome. The peristomes are covered by a pattern of macro- and microscale grooves as shown in the light and scanning electron microscopy images. Macroscopic grooves facilitate the spread of water along them but hinder lateral spreading. Microscopic grooves reduce instabilities in the water film.

Carnivorous pitcher plants Nepenthes capture insects using pitfall-trap-shaped leaves. Insects landing on the wet rim of the trap, called the peristome, aquaplane like an out-of-control vehicle on a wet road into the trap. Scientists from Imperial College London and the Universities of Bristol and Cambridge have shed new light on how the plant creates the right surface conditions to prevent prey escaping from its trap [Labonte et al., Acta Biomaterialia 119 (2021) 225-233, https://doi.org/10.1016/j.actbio.2020.11.005].

Many plant surfaces have remarkable wetting properties enabling them to float on water, self-clean, or move water around. The pitcher plant’s peristome differs from these because it makes water spread out rather than repelling it. This continuous thin film of water makes the peristome so slippery that unsuspecting insect visitors slide straight into the plant’s trap. Like other natural surfaces with extreme wetting properties, the properties of the peristome are determined by a combination of surface chemistry and topography. Understanding the relative importance of these factors could help biologists and engineers mimic similar smart surfaces.

“Research has focused on plant surfaces which repel liquids, but there are several examples of natural surfaces that stabilize liquid films,” explains first author David Labonte. “These surfaces are promising candidates for inspiring ‘omni-repellent’ surfaces to which neither polar liquids, such as water, nor non-polar liquids, such as oil, can stick.”

Using a combination of friction measurements on insect foot pads, photolithography, wetting experiments, and physical modelling, the researchers determined that the unique surface topography of the peristome creates the right conditions for aquaplaning when the surface is wet without the need for highly hydrophilic surface chemistry.

The peristome surface is patterned with radial macro- and microscopic grooves separated by ridges that restrict the lateral spread of water but enhance radial spread to create a continuous slippery surface even when there is very little water. A droplet of water on the peristome rapidly enters the grooves because the dimensions are below the capillary length of water and spontaneously runs down them because surface tension trumps gravity. Even with at least one macroscopic groove along the length of the peristome filled with water, insects could still displace the water film to grab on with their adhesive foot pads. This is where the microscopic ridges come in because they render the film of water between the peristome and insects’ foot pads stable.

“The pitcher plant has already inspired omni-repellent surfaces with outstanding performance, says Labonte. “Our understanding of how pitcher plants manage to trap high-surface tension liquids such as water, could inform further improvements in the design and performance of these surfaces.”