Science Literature

Spider silk has been an active area for biomimetics research for several years. Spinoff companies have been launched in anticipation of commercial gains. However, despite the enthusiasm and commitment of research staff, the prizes are still elusive. Whilst the main goal is to produce fibres that are as strong and as flexible as spider silk, there are other aspects of the natural material that have attracted the interest of researchers. One of these concerns the ability of webs to be a site for dew collection.

"When Lei Jiang first observed the phenomenon, he was intrigued. "How does that happen?" he wondered. After all, he says, "if you took a human hair, water would not stick to it like that". His initial curiosity led to an almost five-year-long study. The findings could have implications for the design of materials for water collection and for the efficiency of chemical reactions."

Spider silk manipulates water with skill (source here)

Not only do webs attract dew, the droplets are able to hang stably on the silk fibres. This suggests the presence of a microstructural mechanism. All polymeric fibres have a microstructure and spider silk is no exception. SEM images reveal a series of amorphous regions (called puffs) and crystalline regions (called joints). The nanofibrils are highly hydrophilic: enhancing wettability and favourable for condensing dew. The puffs have a very open structure and are semi-transparent in images. However, when water starts to condense, the puffs shrink - first to "opaque bumps" and then to "spindle-knots". As they shrink, tiny water droplets coalesce to form larger drops with movement from joints to spindle-knots.

"Further work revealed that movement of the droplets towards the knots is directed by two forces acting together: the force generated by a gradient of surface energy on the fibrils and the one produced by the spindle shape of the knots. "This is quite different from other reported surfaces, on which drops are driven just by individual forces," says Jiang."

These findings are stimulating human invention. The research paper reports success with nylon filaments that are coated with a hydrophilic material that dries in tiny knots similar to those found in spider silk. The goal now is to produce something of commercial value.

"These observations clearly show that our artificial spider silk not only mimics the structure of wet-rebuilt spider silk but also its directional water collection capability. We therefore anticipate that the design principles uncovered and implemented in this study will aid the development of functional fibres for use in water collection and in liquid aerosols filtering in manufacturing processes."

Why does the spider produce a web with dew-gathering potential? "The researchers are unsure of why the spider has evolved to possess this ability. "It could be for its drinking activities, or it could be to refresh the web structure to make it stronger and stickier for prey," Jiang told physicsworld.com." Magdalena Helmer wrote a short News & Views piece on "Dew catchers", saying: "spiders don't need to look for water because the silk fibres that they spin are highly efficient at collecting it from moist air". However, direct evidence of functionality is lacking. There is considerable scepticism that spiders make any use of dew-gathering.

"But Fritz Vollrath, who studies spider silk at Oxford University in the UK, disagrees with Jiang's theory. He thinks spider silk has to be dry to function. 'If I am correct, then the authors are studying an artefact, which is still interesting, although it has no biological function,' says Vollrath. [. . .] Brent Opell, a spider expert at Virginia Tech in Virginia, US, is equally cautious about the results, although he says the experimental work is sound. 'The implication that [capture] threads have evolved to harvest moisture is not the view of most arachnologists,' he says."

Is there an ID perspective on this? Wherever researchers recognise "design principles" in the natural world, the answer is, of course, 'yes'. The presumption with ID is that design features imply functionality, whether or not we know the details. Dew gathering is a unique and remarkable feature of spider silk simply because other fibres do not display such behaviour. The authors comment:

"We observed such directional water collection behaviour only with wetted silk fibres (that is, wet-rebuilt silk) from the cribellate spider Uloborus walckenaerius; in contrast, silkworm silk and nylon fibres with a uniform structure did not exhibit the directional water collection phenomenon."

Whether evolutionists can explain 'how the spider came to gather dew' is more uncertain. Even with functionality identified, perfecting this highly engineered system makes it most reasonable to infer intelligent, rather than natural, causation.

Directional water collection on wetted spider silk
Yongmei Zheng, Hao Bai, Zhongbing Huang, Xuelin Tian, Fu-Qiang Nie, Yong Zhao, Jin Zhai & Lei Jiang
Nature, 463, 640-643 (4 February 2010) | doi:10.1038/nature08729

First paragraph: Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the 'wet-rebuilt' fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

Making the paper: Lei Jiang
Nature, 463, 586 (4 February 2010) | doi:10.1038/7281586a

Abstract: Spider silk structure holds secret to catching water as well as flies.

See also:

Birch, H. How spider silk soaks up water, Chemistry World (3 February 2010)

Dacey, J. Spider web inspires fibres for industry, physicsworld.com (3 February 2010)

Helmer, M. Dew catchers, Nature, 463, 618 (4 February 2010) | doi:10.1038/463618a