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Drops to Droplet Mechanics
Name: Edward Q.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 2001-2002
Question:
I have noticed that when water drops hit certain
surfaces like stainless steel and porcelain,they sometimes burst into tiny balls
of water that spread out from the origin of the drop.I have looked in
many books and asked all the people I can think of, why this happens,
but no one knows.
Replies:
In detail, this is a very complicated phenomena. However, the rough
outlines are quite straightforward, I believe. When the water hits the
surface, it is broken up and spread in a variety of directions. The main
force keeping the water molecules together is surface tension, which is
more effective for small droplets. The pressure inside a spherical
droplet of water due to surface tension is given by p = 2S/r, where S is
the coefficient of surface tension for water (about 0.022 N/m) and r is
the radius of the droplet. So for a drop 0.1 mm = 0.00001m in radius, the
surface tension pressure is 440 N/m. The pressure due to gravity trying
to collapse the droplet is only 1 N/m2 at the bottom of the droplet, so
the surface tension, which tries to keep the droplet spherical, is
dominant.
Nice question!
Best, Dick Plano
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