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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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