February 4, 2010 | David F. Coppedge

Spider Webs Are Precision Dew Collectors

Photographs of dew drops on spider webs are favorite targets for nature photographers, because they resemble strings of pearls on fine jewelry (example 1, example 2).  But did you know the reason dewdrops bead up so well on webs is due to the fine microstructure of the spider silk?  A team of Chinese scientists studied this phenomenon and reported in Nature how it works.1  Their description is almost as dazzling as the photos:

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.

In other words, it is the structural detail – the pattern of alternating random and aligned nanofibrils – that collects the dew and channels it into drops.  The structure creates a gradient that allows small drops to overcome energy barriers and move to collection points.  Fibers without the alternating nodes do not have this ability.  The researchers compared silkwork silk and nylon fibers and found that they did not exhibit the directional water collection of spider silk.  Moreover, the spider web only exhibits this trick when wet.
    Clearly there is more going on in the humble spider’s output than we realized (and that was a lot; see 05/25/2005).  This function is in addition to the well known strength and flexibility of spider silk (04/18/2007).  Imagine living out in the wild and having your water brought to you.  Magdalena Helmer wrote in her review of this paper in Nature,2 “Why did Incy Wincy Spider climb up the water spout?  If he was after a drink, a report by Yongmei Zheng et al. in this issue suggests that he might have missed a trick � spiders don’t need to look for water because the silk fibres that they spin are highly efficient at collecting it from moist air.
    The authors did not describe how the spider spins its web with this structure.  But they mimicked the same effect with artificial fibers and said, “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.”  Now that we understand the principle, we can use the same water-collecting technique in artificial materials that might help those in parched lands extract water out of the air (cf. 11/16/2007).  PhysOrg published a summary of the findings.
Update 02/08/2010: Spider webs have another optimized feature: structural robustness.  PhysOrg reported that physicists are examining how spider webs achieve flexibility and strength even when damaged.  “By better understanding the unique structural properties of spider webs, researchers could apply the information to other areas, such as designing buildings, bridges, and space structures,” the article said.  But how did the lowly spider learn tricks that human engineers have yet to imitate?  “Although the orb web of a spider is a lightweight structure, it seems to be a highly optimized structure, presumably as a result of evolution from the Jurassic period or earlier,” the physicists said.  The article explained, “As the most familiar web form, orb webs have features that are universal to many spider species, suggesting that they have beneficially evolved by natural selection.”

1.  Zheng et al, “Directional water collection on wetted spider silk,” Nature 463, 640-643 (4 February 2010); doi:10.1038/nature08729.
2.  Magdalena Helmer, “Biomaterials: Dew catchers,” Nature 463, 618 (4 February 2010); doi:10.1038/463618a.

Here’s another teachable moment with your child in the garden.  We should never take simple things for granted.  It’s clear that the spider can teach humans their design principles, but who taught them to the spider?  Be sure to explain to the child that storytelling with tautologies is not an acceptable response (10/29/2002).

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