The twinkling of dew drops on a spider’s web is one of nature’s most beautiful sights. Researchers in Beijing have now discovered how the structure of spider silk encourages water to collect into drops, and have designed artificial fibers to mimic this remarkable water-collecting ability.

Lei Jiang, Yong Zhao and co-workers at the Chinese Academy of Sciences, the National Center for Nanoscience and Technology and Beijing University of Aeronautics and Astronautics studied silk from the spider Uloborus walckenaerius, which uses a comb-like structure called a cribbelum to separate its silk into very thin fibers1. The silk is sensitive to humidity, and has excellent mechanical properties.

“For the past decade, our group has concentrated on the structure–function relationship between liquids (mainly water) and natural solid surfaces such as lotus leaves, rice leaves, water striders’ legs, moth eyes and butterfly wings,” explains Zhao. “So, it is no coincidence that we were attracted by the shining water droplets on spider webs.”

Fig. 1: Dew drops form on spider webs because of nanostructures on the silk.

On examining the silk under a scanning electron microscope, the researchers observed that each fiber consists of two long nanofibrils forming its main axis, and larger, semi-transparent ‘puffs’ of randomly oriented nanofibrils that are evenly spaced along its length. As water condenses on the silk, the puffs shrink into smaller ‘spindle-knots’ separated by joints of aligned nanofibers — a process called structural wet-rebuilding. Droplets then form randomly on both the spindle-knots and the joints in-between. The drops forming on the joints, however, tend to move towards the spindle knots, where they coalesce.

This directional water collection occurs because the bulbous, random structure of the nanofibers in the spindle-knots is more hydrophilic than the flatter, aligned nanofibers in the joints. The phenomenon is not seen in silkworm silk or artificial nylon fibers, which both have uniform structures along their lengths.

Inspired by their findings, the researchers designed their own artificial spider silk. They immersed a nylon fiber in a solvent, and then quickly withdrew it such that the solvent broke up into drops, before drying to form structures similar to the spindle-knots. The fibers display properties similar to those of the natural spider silk.

“We plan to explore applications of our artificial fibers in areas such as collecting water, liquid aerosol filtering, smart catalysis and microfluidics,” says Zhao.