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Why dew drops sit on leaf tip

LONDON - Nobel laureate and poet Rabindranath Tagore once wrote: “Let your life lightly dance on the edges of time like dew on the tip of a leaf.” A new study is finally offering an explanation for why small dew drops, as Tagore said, form on the tips, rather than on the flat surfaces of leaves.

In the study, researcher Martin E.R. Shanahan from the Institute for Materials and Processes, University of Edinburgh, observes that drops of water have a preference for exactly where they collect on leaves as their surfaces cool in the morning and afternoon, reported the journal Langmuir.

Those droplets, which condense from water vapour - moisture - in the air, collect randomly across the surfaces of flat leaves. However, dew drops tend to accumulate at the tips of spindly leaves, even if that means defying gravity by moving upwards, according to a university statement.

He explained that an inherent unwillingness or lack of necessity of water drops to move on a dry surface governs their positioning on flat leaves, causing them to stay where they form.

Dew’s tendency to head to the end of finely pointed leaves, however, sent Shanahan looking for a different explanation. The answer is based on the fundamental principle of free energy, that everything in nature seeks the lowest possible energy state.

Shanahan modelled two types of dew drops on a theoretical (simplified) cone-shaped leaf: a thin, cylindrical sheath of water and a spherical drop centred on the cone’s axis. In both cases, he found that the drop lowered its energy by moving toward the point of the leaf.