How To Make Droplets Levitate on Water
Fluid
dynamicists have developed a trick
to make droplets sit on water
indefinitely.See it in action in this video
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
Aut onoma de M exico 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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