Exploding drops produce miniature showers.
In every drop of rain, a shower.Emmanuel Villermaux
A single drop of water can produce a shower of droplets that has all the hallmarks of rainfall, an elegant model by researchers in France suggests. Scientists had previously thought that the pattern of rainfall seen at ground level was created by drops of rain colliding as they fell.
Fluid dynamicist Emmanuel Villermaux of Aix-Marseille University and graduate student Benjamin Bossa filmed small drops of water dripping from a tap to see what happened to the drops as they fell.
The air resistance on an accelerating drop increases until it exceeds the cohesive forces that keep it together — and the drop bursts into a shower of droplets (see video).
But to see that, Villermaux says, “the experiment would need a height of typically 10 metres — that is high for a standard lab”. So he and Bossa used jets of air blowing upwards to increase the air resistance drops experienced as they fell. This meant that the drops fragmented within a shorter distance.
Researchers knew that in still air, a drop initially flattens into a pancake, then deforms into an upturned bowl-shape before bursting into droplets.
Villermaux and Bossa used these shape changes together with their observations to connect the bursting of a single drop to the distribution of raindrop sizes in showers. Their model, published in Nature Physics, shows that the explosion of a drop into droplets is enough to explain the distribution1.
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Villermaux, E. & Bossa, B. Nature Phys. doi:10.1038/NPHYS1340 (2009).