A deadly viscoelastic fluid

Via Bora at A Blog Around the Clock, I came across this very interesting paper in PLoS ONE titled A Viscoelastic Deadly Fluid in Carnivorous Pitcher Plants; here is the abstract:

Background

The carnivorous plants of the genus Nepenthes, widely distributed in the Asian tropics, rely mostly on nutrients derived from arthropods trapped in their pitcher-shaped leaves and digested by their enzymatic fluid. The genus exhibits a great diversity of prey and pitcher forms and its mechanism of trapping has long intrigued scientists. The slippery inner surfaces of the pitchers, which can be waxy or highly wettable, have so far been considered as the key trapping devices. However, the occurrence of species lacking such epidermal specializations but still effective at trapping insects suggests the possible implication of other mechanisms.

Methodology/Principal Findings

Using a combination of insect bioassays, high-speed video and rheological measurements, we show that the digestive fluid of Nepenthes rafflesiana is highly viscoelastic and that this physical property is crucial for the retention of insects in its traps. Trapping efficiency is shown to remain strong even when the fluid is highly diluted by water, as long as the elastic relaxation time of the fluid is higher than the typical time scale of insect movements.

Conclusions/Significance

This finding challenges the common classification of Nepenthes pitchers as simple passive traps and is of great adaptive significance for these tropical plants, which are often submitted to high rainfalls and variations in fluid concentration. The viscoelastic trap constitutes a cryptic but potentially widespread adaptation of Nepenthes species and could be a homologous trait shared through common ancestry with the sundew (Drosera) flypaper plants. Such large production of a highly viscoelastic biopolymer fluid in permanent pools is nevertheless unique in the plant kingdom and suggests novel applications for pest control.

The high dilution that they are talking about is of the order of 95%; apparently, if the fluid is diluted with more than 95% of water, then, and only then, the insect capture rate starts falling. The paper also discusses experimental results showing how the surface tension of this deadly fluid is the same as that of the de-ionised water measured under the same conditions, and also, how the surface physical properties of this digestive fluid (like surface tension) may have very little role to play, if any, in capturing the insects.

Deborah number

Apparently, the insect capture rate depends not only on the viscoelastic properties of the fluid (more specifically, on the extensional viscosity), but also on the Deborah number:

The Deborah number is the ratio of the fluid elastic relaxation time to the typical time scale of fluid movement. For small Deborah numbers, the time scale of fluid movement is large compared to the relaxation time of elastic forces: the fluid thus behaves like a simple viscous fluid. For large Deborah numbers, the fluid movement is too fast for elastic forces to relax: in this case the fluid behaves like an elastic material. When insects struggle in the pitcher fluid, insect movements control the time scale of the flow. (…) This suggests that trapping occurs when the elastic forces created by insect movements have no time to relax.

Yes; it is not just the liquid, but the movements of the insect that makes the trap deadly.

Any case, if like me, you are also curious about the name Deborah number (this is the first time I heard that), Wiki has some interesting information:

The Deborah number is a dimensionless number, used in rheology to characterize how “fluid” a material is. Even some apparent solids “flow” if they are observed long enough; the origin of the name, coined by Prof. Markus Reiner, is the line “The mountains flowed before the Lord” in a song by prophetess Deborah recorded in the Bible (Judges 5:5).

Wiki also links to a piece by Reiner on Deborah number in which he describes the history behind the naming of this number and the connection between theology and rheology!

On the whole, a very enjoyable paper that I have read in quite a while. Take a look! As a bonus, there are also plenty of videos to accompany the paper which makes reading the paper a real pleasure. Happy reading!

Tags: carnivorous tropical plants, Deborah number, fluid dynamics, insect capture, Nepenthes rafflesiana, pitcher plants, rheology