The shape change of bubbles and drops to spherical is driven by the hidden subject of this portion, a force that scientists call surface tension, and that is what links the shot, bubbles and rain drops, which are also fairly spherical. For a while, rain drops are held aloft by updrafts in the cloud, eventually, they fall from the cloud and plummet down. They aren’t teardrop-shaped, and they aren’t spherical, but they are like flattened spheres.
And when the rain drops land on a waxed marble table, they
are also somewhat spherical, or at least hamburger-bun-shaped. All of the
subjects here tend to be spherical, and the reason is the same: surface
tension.
Balloons are a bit
different, because their shape is determined by the shape of their membrane,
but there are parallels with bubbles. All the other spheres are shaped by surface
tension.
To explore this effect, we need to look at something that seems to
float, but doesn’t really float,
either. You can see the surface tension effect with a glass of water and two
paperclips:
Bend one paper
clip into an L shape. Use this clip to lay another paperclip gently on the top
of the water. The surface of the water bends under the weight of the paperclip
like stretched rubber, but doesn’t let the clip through.
How to do the ‘floating’
paperclip trick.
If the clip doesn’t ‘float’, lift the paperclip out,
dry it and rub a tiny bit of grease on the paperclip, and then try again. If
you look closely at the reflections coming off the water in the third photo,
you can see how the water surface is bent. inder the weight of the wire.
To push the water
surface out of shape you must use force. If the paperclip can’t exert enough
force, it can’t stretch the surface enough to let the paperclip slip through.
One drop of detergent in the water though, and the magic goes!
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