It’s not just a monkey puzzle, it’s a hydrodynamics puzzle. The spiky leaves of the tree also known as the Chilean pine confound not only the hands of primates, scientists have found, but also the brains of fluid dynamicists.
If you drip water on to the leaves of araucaria trees, the genus that includes the monkey puzzle, then it flows in one direction and one direction only. Drip ethanol on to it, researchers discovered to their amazement, and it goes in the other.
The finding, said Zuankai Wang, from City University of Hong Kong, “was like a miracle”. Despite the ubiquity of the ornamental plants, “this was the first recorded observation in the scientific world”.
In a paper in the journal Science, he and his colleagues have replicated the mechanism on an artificial surface, and they believe that it could have applications throughout industry.
The original discovery came through sheer chance. One of Wang’s colleagues was from mainland China and when he came to Hong Kong decided to visit a water park, where instead of log flumes and tropical aquarium he was drawn to what turned out to be a different kind of watery attraction.
He spotted an araucaria tree and, said Wang, “he found the structure is very beautiful — because it is non-symmetric.”
Each of the leaves, placed together in a lattice, has a curved upper surface tapering to a point, but a smooth lower surface. This intrigued him. “In breaking the symmetry, he realised there’s something there,” said Wang.
He brought the plant to the laboratory and dropped water on it, noting that it only flowed one way, which was interesting enough. But when he looked to see if the same was true for all liquids, Wang’s colleague discovered that some went the other way.
After two years of study, Wang and his colleagues have worked out the mechanism. Liquids flow through the leaves, they found, but how they do so depends on their surface tension.
With a liquid like ethanol with a low surface tension, a drop bursts at the base of the leaf then wicks up, through a process known as capillary action, to its tip, where it meets the next leaf. In this way it travels consistently from base to tip along the branch.
With a liquid like water with a high surface tension, lit instead sits on the tips of the leaves, where it soaks in to wet them from the tip to the base, travelling consistently in the other direction.
Wang said that it was not clear why the plant evolved this property. Although they tested it on a species called the Norfolk Island pine, he believes that it should be common to all members of the genus, which share the characteristic leaf shape. It is possible, he added, that the process helps the plant draw extra moisture from the air. Dew condensing on its leaves would be pulled down towards the trunk and the roots, while some impurities and contamination would go the other way.
Whatever the purpose for the plant, he believes that the process has obvious applications for industry. Separating and steering liquids is important in many areas, including oil refining and textiles. The most promising uses could come in the transport of water itself in applications that rely on wicking effects, such as the cooling of computer chips.
