A spider web is not just a sticky net but a delicate instrument that responds to the weather. When the air becomes humid, spider silk threads shorten and twist, like a wet towel being wrung from both ends. Previously, it was thought that the thread twists because the proteins inside are arranged in a spiral. But no direct evidence for such an arrangement was found.
Now researchers have proposed a different explanation. Spider silk consists of a soft amorphous matrix and stiff protein regions aligned along the thread. Water is absorbed into the matrix, causing it to shrink. The stiff regions prevent it from shrinking evenly, like the ribs of an umbrella that stop the fabric from folding flat. As a result, the thread doesn't just shorten; it begins to rotate around its axis. This phenomenon is called the Poynting effect: when a material is compressed in one direction, it spontaneously twists.
The scientists built a mathematical model that accounts for water absorption, rearrangement of the soft matrix, and thread plasticity. The model accurately reproduced experimental results: both single humidification and repeated cycles. It turned out that the degree of twisting depends on how strongly the thread was pre-stretched. This explains why webs remain taut even after rain: the threads shorten and twist, compensating for sagging.
Such a mechanism could be useful in technology. If we make artificial fibers modeled on spider silk, with a soft matrix and stiff inclusions, they could work as humidity sensors or even as tiny motors that rotate in response to changes in air humidity.
Spider silk can shrink by 40% of its length when wetted — more than most synthetic materials.
In Frank Herbert's novel "Dune," suits are described that collect body moisture and recycle it; spider silk with its moisture response is a step toward such smart materials.