Sliding water droplets surprise scientists –

(a) Droplets sliding down (l-r) a single fibre, a bundle of two fibres and a bundle of three fibres. A liquid film appears behind each droplet. (b) Horizontal cuts of each system, showing dry fibres in front of the droplet (orange), the droplet cross section (light blue), and the liquid film of thickness ? left behind after the passage of the droplet (dark blue). (Courtesy: Physical Review Fluids (2023). DOI: 10.1103/PhysRevFluids.8.103601)

Water droplets move faster down bundles of fibres than down individual fibres - even when the total perimeter of the two structures is identical.

This discovery, from researchers at the University of Liege in Belgium, challenges the expected notion that a droplet would move at the same speed along these different fibres. It also highlights the importance of the underlying structure of the fibres, such as the grooves on a bundle, for the movement of water. While apparently simple, the movement of a droplet sliding down a vertical fibre or thread is difficult to measure experimentally.

This is because the droplet loses volume and speed as it slides, and it may travel several metres before coming to a stop, making its motion hard to monitor. In the new work, researchers coordinated by Matteo Leonard [1] tried an alternative approach. "We approached the problem from a different angle," Leonard tells Physics World. "What if instead of following the drop's fall, we made the thread rise at a speed equal but opposite to that of the drop?" In this set-up, he adds, the droplet remains stationary relative to the camera being used to observe it, simplifying the experiment.

Initially, Leonard and colleagues studied how droplets slide down fibres of different diameters. Theory predicts the droplets will move slower when the fibres are thicker, and this was indeed what they observed.

Next, they braided two or more fibres together. In this configuration, which creates a bundle of fibres with grooves, the overall diameter of the bundles increases with the number of fibres braided, and the team observed the same behaviours as in single fibres: the bigger the bundle, the slower the droplets slide, again as predicted. The Liege team [2] then decided to study what happens when both the single fibres and bundles of fibres have the same diameter.

Here, we might expect that droplets will propagate down the fibres and bundles at the same speeds because the contact surface between the liquid and the fibres is the same. But this was not what the team saw. In fact, the speed of the droplet on the bundle of fibres was faster.

It also lost more volume after travelling the same distance. This could be because the water "fills" the grooves in the bundle, thus creating a liquid rail over which the droplet slides more efficiently.

Ubiquitous in nature

Substructures like the team's braided bundles are ubiquitous in nature. Grooves, spines and knots are found in many plant species in arid or semi-arid regions of the world, and similar structures are found on the backs of desert-dwelling animals.

From the web of a spider to the back of a lizard and the leaves at the tops of trees, these grooved structures are everywhere. The question, Leonard says, is why, and the new research suggests an answer. "In the case we studied, it's a matter of survival," he says. "The faster the water flows over a surface, be it a leaf or a strand of fur, the better the chances of it reaching the part of the organism where it can be absorbed (or indeed evacuated)."

Falling water drops power LEDs

[3]

The insights gained from this research, which is detailed in Physical Review Fluids [4], could have applications in the design of atmospheric water collection systems, Leonard adds. "Improving the efficiency of a system like ours could help improve water harvesting techniques, especially in arid environments, where water is a precious commodity," he says.

Moving forward, the researchers say they plan to further explore how fibre substructures affect droplet dynamics. "We're particularly interested in how variations in the design of fibre bundles could optimize water transport," Leonard says. "We also aim to test the practicality of our findings by integrating them into prototype water harvesting devices."

References

^ Matteo Leonard (www.sciences.uliege.be)
^ Liege team (www.sciences.uliege.be)
^ Falling water drops power LEDs (physicsworld.com)
^ Physical Review Fluids (dx.doi.org)