The race of water droplets

How fast does a droplet flow along a fiber? It depends on the diameter of the fiber... and also on its substructure! These are the findings of a study conducted by researchers from the University of Liège who are interested in microfluidics, especially water harvesting in arid/semi-arid regions of our planet. These results are the subject of a publication highlighted by the editors in the journal Physical Review Fluids.

Similar to moisture farms imagined in science-fiction worlds like that of "Star Wars," many plant species from arid or semi-arid regions of Earth have developed ingenious strategies to capture water from the air, ensuring their survival. Recently, researchers have focused on understanding the fundamental mechanisms of water transport, intending to reproduce and improve them, especially for facilitating the collection of atmospheric moisture in deserts. A recent study led by the GRASP at the University of Liège (ULiège) has sought to better grasp the factors influencing these precious droplets' movement. To do this, scientists have tracked in real-time the characteristics and dynamics of these droplets as they slid along individual fibres or bundles of fibres.

"Following a droplet as it descends along a vertical fibre under the influence of gravity presents a complex experimental challenge: how to track a droplet over several meters of thread? Explains Matteo Léonard, a researcher at GRASP and the study's lead author. To address this problem, researchers devised a clever device in their laboratory. "Instead of following the fall of a droplet, we set the fibre in motion so that its speed is exactly equal and opposite to that of the droplet. This way, the droplet remains 'stationary' in front of the camera." With this challenge overcome, they first used fibres of different diameters. They observed that droplets had a lower speed at a given volume when the fibres were thicker, as predicted by theory. Subsequently, researchers created bundles of fibres by tying the ends of two or more fibres together and applying slight torsion to ensure contact between all the fibres. "This configuration created a bundle of fibres with grooves, similar to the braiding of strands in a rope, which resulted in grooves appearing on the cord," explains Matteo Leonard. In this configuration, researchers observed the same behaviours as with single fibres: as the number of fibres in the bundle increased, the overall diameter of the bundle increased, resulting in lower speed at a given volume. This predictable behaviour, however, concealed a more complex phenomenon...

Indeed, what about the behaviour of the droplet in the case where both configurations (single and bundle) have the same diameter (i.e., a fibre with a diameter of 0.28mm versus two fibres with a diameter of 0.14mm)? Since the hindrance of the phenomenon is dissipation (i.e., friction within the liquid and between the liquid and the fibre), one might expect that both cases would yield identical results because the contact surface between the liquid and the fibre is the same. "Not at all. We observed that over the same distance travelled, the droplet on the bundle of fibres was faster. It also lost the most volume." Researchers believe that in this configuration, the droplet loses volume because it tends to "fill" the grooves with its own volume, thereby creating a liquid rail on which it slides more efficiently and thus faster.

The results of this study make a significant contribution to the field of designing structures for atmospheric water collection. Notably, it can potentially improve the efficiency of cloud nets, which consist of a network of fibres, at a low cost. Furthermore, this research highlights the growing importance of substructures regularly observed on organisms living in desert environments. These substructures, such as micro-grooves or micro-hairs, demonstrate nature's ingenuity in capturing and transporting water, inspiring future technological innovations.

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*A moisture farm is an area of land devoted to the production of water by extracting moisture from dry air.