A study inspired by spiders that live underwater could help industry control toxic or inflammable gases in fluid lines.
Researchers at the PSL Research University in Paris, France, were inspired by the diving bell spider (Argyroneta Aquatica), which traps a layer of air around its body so it can breathe underwater.
“The inspiration came from looking at these animals that use aerophilic structures to capture air – for them it’s a survival strategy. Can we exploit the ideas developed by nature?” says Hélène de Maleprade, lead author of a paper in Applied Physics Letters.
The spiders have hairs on their abdomen that repel water, a property known as hydrophobic – and less commonly as aerophilic, or air attracting. They breathe through their abdomen, and the layer of air they trap allows them to effectively live underwater – hunting, resting, mating and laying eggs without dry land.
The team mimicked a single hair of a spider’s tummy by coating a thread with commercial hydrophobic spray. When they immersed it, they observed that bubbles would attach to the thread, and if the thread were tilted, would be guided upwards along the thread, even at angles as low as ten degrees to the horizontal.
The similar scenario of water droplets running down a thread in air is well understood, but the bubble-in-water situation surprised de Maleprade and her colleagues.
Their analysis of the forces on the bubbles showed that most of the drag came from a skin of water around the bubble, in contrast to the droplet, in which the drag originates from the ends of the contact area between the water and the thread.
“Air is easy to move, but water is not – the friction always happens in the water. It’s harder to move the water around the bubble rather than to move the bubble itself,” de Maleprade says.
By varying the angle of the threads, the size of the bubbles and the viscosity of the fluid, the team was able to build up a complete picture of the physics and work out the interplay between inertia, buoyancy, viscosity and surface tension.
Although they began their study without applications in mind, they have already been contacted by industry interests who can see the potential of the new knowledge.
Trapped gases can be problematic in industries based on liquid flows: the ability to trap and guide gas bubbles out of flow lines could be a major boon in reducing safety risks, especially if the gases are dangerously toxic or flammable.
