You know when you pour milk into a bowl of breakfast cereal and the floating cereal tends to clump together? In fluid mechanics, that phenomenon is known as the ‘Cheerios effect,’ and it’s inspired the development of a unique but more efficient water-harvesting system that could be particularly useful in arid regions.
Researchers from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, coated a surface with a lubricant film and found that it caused water droplets to combine faster, akin to the Cheerios effect.
“We are interested in designing surfaces that can promote condensation of water, which has important heat-transfer and water-harvesting applications,” said Marcus Lin, who led the research. “Think of water condensing on a cold soda can. The droplets can only move once they grow big enough for gravity to pull them down.”
The researchers thought adding a thin oil film would lubricate a surface, causing water droplets to move faster, freeing up space for further droplet condensation and thereby boosting condensation rates. It worked. But how the water droplets reacted surprised them.
They observed the condensate droplets, ranging in size from tens of microns to several millimeters, engage in a spontaneous and complex collective ‘dance.' The droplets moved in a snake-like fashion until the lubricant was depleted, at which point they switched to a circular motion. As the moving droplets continually redistributed the lubricant across the surface, they resumed their twisty dance.
“They initially moved in a serpentine manner before transitioning into circular motions and then back again,” Lin said. “These motions occurred across scales ranging from micrometers to several centimeters, and they lasted for hours.”
Like the Cheerios in milk, the condensate droplets in oil were drawn towards neighboring droplets. The larger droplets’ self-propulsion was driven by energy released as they merged with smaller droplets in their path.
The researchers say that devices that can efficiently capture water from the air through simple condensation, with no energy input, are widely prized as pressure on freshwater sources grows. This is of particular importance in arid areas such as Saudi Arabia.
“By optimizing the collective motion of condensing droplets, we can greatly increase condensation rates and hence design more efficient water-harvesting systems,” said Lin.
The researchers plan to explore further what drives the droplets’ complex dance, particularly the shift from serpentine to circular movement.
The study was published in the journal Physical Review Letters.
Source: KAUST
