Meeting Abstract
P1.91 Friday, Jan. 4 Slippery when wet: the roles of ridge morphology and surface energy in the pitcher plant trapping zone ROBINSON, A P*; LABONTE, D; MUIR, M J; FEDERLE, W; University of Cambridge ar433@cam.ac.uk
Nepenthes pitcher plants employ the peristome, a specialised, microstructured surface at the pitcher rim as a prey capture device which becomes slippery when wet. The peristome’s wettability appears to be aided by radial ridge structures that have two different length scales: first-order ridges on the 100µm length scale, superimposed on which are ca. 10 second-order ridges on the 10µm length scale. In order to study the function of the first- and second-order ridges, we measured single-pad friction forces of Nauphoeta cinerea cockroaches on dry and wetted peristome replicas and artificial surfaces patterned with ridges of each single length scale. Replicas were produced in transparent epoxy using silicone moulds of Nepenthes maxima peristomes; artificial ridge structures were made from the same material after taking moulds of ridge substrates fabricated by photolithography. Friction forces were found to be significantly lower on wetted substrates in all cases; however, the effect was approximately three times stronger for surfaces where the second-order ridges were present. Dynamic contact angle measurements were performed on peristome specimens with both water and diiodomethane. Whilst both fluids rapidly wetted the peristome in the direction parallel to the ridges, movement of fluid was limited across the first-order ridges by high apparent contact angles (around 90º for water and 120º for diiodomethane). We therefore propose that first-order ridges act to confine the fluid wetting to channels running into the pitcher, facilitating prey capture, whilst the second-order ridges are essential to stabilise the water films on which insects aquaplane. By varying the surface energy of peristome replicas and smooth epoxy substrates, we demonstrate that the peristome’s ridge topography facilitates full wetting even for moderately hydrophilic materials.
