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Allow a drop of liquid to fall onto the surface of a bowl of the same liquid and you know intuitively what to expect. The two should coalesce, right?

The process is actually a little more complicated. Back in 1899, the English physicist, Lord Rayleigh, discovered that droplets can bounce off each other. 

With the invention of high speed photographic techniques, physicists  discovered that before coalescing, a droplet that has fallen onto a surface of the same liquid, first bounces and then settles before finally coalescing. All in the blink of an eye.

Then in 2005, Yves Couder, at the Universite Paris 7 Denis Diderot, and a few pals, discovered how to make droplets sit on a liquid surface indefinitely. The trick is to vibrate the surface.

Today, Pablo Cabrera-Garcia and Roberto Zenit at the Universidad Nacional Autonoma de Mexico in Mexico publish a fascinating video showing some of their experiments with this effect (hi-res mpg download 40MB, low-res mpg download or YouTube).

These guys first show how droplets of tap water mixed with a little soap first bounce on an ordinary non-vibrating  surface and then coalesce. They then show how to keep the droplets on the surface indefinitely by placing their bowl of water on a vibrating loudspeaker.

They find some interesting effects with clusters of droplets of the surface, saying they can pack more droplets onto the surface when the vibrations induce nonilinear standing waves called Faraday waves.

Cabrera-Garcia and Zenit offer no explanation for what they find. However, Couder says that when a droplet falls onto the surface, it traps a thin layer of air beneath it and this stops it coalescing immediately. 

The droplet coalesces with the surface only when this air escapes. But vibrating the surface allows this layer to stay in place indefinitely.

What would be interesting, of course, is to see whether it is possible to control the movement of the droplets on the surface. That ought to be possible using interference patterns, like these here, which can generate all kinds of shapes on the surface of water. 

Steerable droplets could have considerable application in chemistry and microfluidics. 

This video is an entrant into the Gallery of Fluid Motion competition, an annual event run by  the Fluid Dynamics Division of the American Physical Society. We’ll look at the best ones as they appear on the arXiv in the coming weeks.

Ref: http://arxiv.org/abs/1210.3538: Droplets Bouncing Over A Vibrating Fluid layer

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