The plant world has evolved numerous ingenious ways to distribute seeds. But one of the least known and least celebrated is raindrop dispersal.

Just a couple of dozen plants are known to exploit the kinetic energy of rain drops to disperse their seeds. And they do it remarkably well. These plants tend to be small, none of them grow taller than 35 cm and their average height is just 12cm. And yet their dispersal mechanisms jettison seeds over distances of up to a metre.

The question we look at today is how. The answer comes courtesy of Guillermo Amador and buddies at the Georgia Institute of Technology in Atlanta. These guys have used a 3D printer to recreate the cup-shaped flowers of these plants. They then filled the cups with water and seeds, and then filmed what happens as drops fall into the cups.

They’ve discovered that when a drop lands in the centre of the cup, water splashes out in all directions at about the same speed as the drop.

But when the drop lands off-centre, the water tends to splash out of the opposite side of the cup. What’s more, the shape of the cup directs and accelerates the water so that its exit speed can be up to five times the velocity of the incoming drop.

Finally, the angle of the cup sides needs to be about 45 degrees to maximise the distance the jettisoned seeds can travel.

Simple really!

All this is captured beautifully in a video for this year’s Gallery of Fluid Motion, organised by the fluid dynamics division of the American Physical Society.

Ref: arxiv.org/abs/1110.3993: How Flowers Catch Raindrops
Hi-res video
Lo-res video