Anyone who has seen a spider web after the early morning dew will have noticed water droplets strung along its fine threads. 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's structure has been studied extensively for its incredible strength and flexibility. But Jiang, a chemist at the the Institute of Chemistry, Chinese Academy of Sciences, is more interested in its wettability — its ability to maintain contact with liquids. “Large water drops can hang stably on the thin spider silk,” he says. “My colleagues and I thought it likely that a microstructural mechanism was responsible for the water collection.”

To look for this mechanism, Jiang enlisted the help of three postdocs and a graduate student, and between them they collected several hundred webs, each about 10–20 centimetres in diameter, made by a local species of spider called Uloborus walckenaerius. The researchers carefully separated silk threads from the webs and then examined them using an environmental scanning electron microscope, which allows samples to be observed at high relative humidity in low-pressure gaseous environments.

Spider silk is made up of hydrophilic, or 'water-loving', nanofibrils. When dry, these form a string of loose 'puffs' linked by joints. When Jiang and his team humidified the sample chamber, tiny water droplets started to collect on the joints and puffs. As a result, the nanofibrils turned from puffs into larger and denser 'knots'. The water droplets then began to be transported from the joints towards these knots, where they coalesced into sticky and adhesive drops.

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 (see page 640). “This is quite different from other reported surfaces, on which drops are driven just by individual forces,” says Jiang.

Having elucidated the web's mechanism of water collection, Jiang and his team set to work designing a material with similar capabilities. The project was a success, and Jiang says that the most exciting part of the experiment, which he plans to spend the next decade on, will be finding applications for his fibres. One will be to design a large, artificial web that can collect drinking water from fog. “It is very important to develop materials for water capture that can be used to supply drinking water in places where water is scarce,” he says. Another application, he adds, is in “designing materials for smart catalysis” — materials that bring components of a chemical reaction together with the catalyst, promoting faster and more efficient reactions.

Although it is obvious why such applications would be useful to humans, it is not as clear how the spider benefits from water collection. Jiang believes that the structural changes that occur when the web gets wet may serve to 'refresh' the web's structure, making it stronger and more sticky for catching prey. “Many scientists now ask me why spiders collect water,” says Jiang. “This is one possible answer.”