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 (Y. Zheng et al. Nature 463, 640–643; 2010).

Credit: C. VARNDELL/NATUREPL.COM

To uncover the water-collection mechanism, Zheng et al. exposed silk fibres of the cribellate spider Uloborus walckenaerius to humid air, and monitored them with a scanning electron microscope. They observed that initial contact with water causes the hydrophilic fibres to re-structure, so that spindle-knots form periodically along the thread axis, separated by elongated joints roughly fourfold thinner.

Small water droplets then condense randomly onto the spindle-knots and joints, and begin to grow as they accumulate moisture from the air. When they reach a critical size, droplets that are attached to joints move towards the nearest spindle-knots, where they coalesce to form larger water drops. This leaves the joints free to start a new cycle of water condensation and collection.

Zheng et al. found that the structure of the silk fibres is crucial for water collection. They observed that the spindle-knots are made up of randomly arranged nanofibrils and have rough surfaces, whereas the joints consist of aligned nanofibrils and have smooth surfaces. The difference in roughness causes a surface-energy gradient that drives water towards the spindle-knots. This effect is boosted by the different shapes of the knots and the joints, which cause the drops to deform differently, thereby generating surface-tension forces that also drive water drops towards the spindle-knots. A final helping hand is provided by the different orientations of the nanofibrils in the fibres: water drops move easily along joint regions where the nanofibrils are aligned, but stick to the spindle-knots where the nanofibrils are randomly orientated.

The authors went on to make artificial fibres that mimicked the structure of spider silk, and found that these successfully reproduced the water-collection properties of the natural material. They speculate that their work will help in the design of fibres that could be used in devices that collect water from the atmosphere, or that remove liquid aerosols in manufacturing processes.