Synopsis: Two Nanodrops Zip Together to Form One  

Simulations reveal that nanometer-scale droplets merge via a zipping-like action initiated by molecular-sized waves on their surfaces.
Synopsis figure
S. Perumanath et al., Phys. Rev. Lett. (2019)

Tiny liquid droplets merge to form larger drops in myriad settings from clouds to 3D printers. Despite this ubiquity, scientists don’t fully understand what triggers this process. In simulations of nanodrops, Sreehari Perumanath from the University of Edinburgh, UK, and colleagues now show that a zipping-like motion of waves along the surface of a drop could be responsible. The team says that accounting for this merging process could enable more accurate forecasting of thunderstorms and more precise control of inks in printing technologies, for example.

The team simulated two spherical water droplets with diameters of a few tens of nanometers and observed tiny waves—just one or two molecules in height—ripple across each droplet’s surface. These waves were induced by the thermal motion of the water molecules. Bringing the droplets into close proximity, so that the crests of two opposing waves were close enough to touch, the team found that the droplets started to merge.

The merge initiated when just two waves met, creating a bridge that pulled the droplet surfaces closer together. Other bridges then formed nearby. The merging proceeded with the bridges widening, drawing the two drops together in a manner resembling a zipper closing as the slider is pulled up. This process finished as the bridges widened and coalesced.

Micrometer-sized liquid bridges have been seen in experiments involving larger, partially joined drops. But whether—and how—these formations initiate in nanodrops was unclear, as the molecular waves and tiny bridges are impossible to observe using currently available experiments. The team says that their findings may also explain how merging starts in larger drops.

This research is published in Physical Review Letters.

–Katherine Wright

Katherine Wright is a Senior Editor of Physics.


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