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A New Scene in the Disappearing Act of an Evaporating Liquid Drop

Research Published in Physical Review Letters (image courtesy of Duncan Lockerby).

New research Lifetime of a Nanodroplet: Kinetic Effects and Regime Transitions has just been published in Physical Review Letters.

Water evaporating into vapour forms part of our daily existence, creating plumes emanating from a boiling kettle and bulging clouds as part of the earth’s water cycle. In this Letter, we have considered the evaporation of a liquid droplet, key to many applications such as combustion and the cooling of electronics, and ask, simply, how long is the drop’s lifetime?  

Drops on a cobweb that gradually evaporate.

This appears simple to answer, as when the drop is large, a classical law dictates that its diameter-squared decreases in proportion to time; however, this period only accounts for a small portion of the drop’s evolution. As the diameter approaches the unobservable micro- and nano-scale, molecular dynamics have to be used as virtual experiments and these show a crossover to a new behaviour, with the diameter now reducing in proportion to time (the nano-scale law).  

We have shown that this behaviour occurs due to complex effects in the vapour flow, as evaporated molecules take significant time to become equilibrated with their neighbours due to a lack of molecular collisions.  By extending classical theories to account for this physics, we are able to quickly predict the drop’s lifetime and thus create a modelling framework that maintains accuracy from typical engineering scales down to cutting-edge nanoscale applications.

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