From ADRIAN BOWYER
Imagine a small sphere of water on the flat surface of a cup of water.
In order for the two to coalesce, both need to increase their surface area
slightly to form a neck where they will join. So surface tension means that
they are in a local energy minimum (albeit a not very stable one). After
a second or two a tiny ripple disturbs them and they do indeed coalesce.
You can do the trick with icebox distilled water containing a tiny amount
of washing up liquid. Drip drops on the surface, and some will skid about
before merging. As always when water is dropped onto water, you will make
some air bubbles in the main body of water which will quickly float to the
surface. Interestingly, you will also make a water bubble in water. This
is the exact analogue of a soap bubble in air: it is a sphere of water with
a thin film of air round it, surrounded by more water. These float upwards
much more slowly. They exist for the same reason as the coffee and orange
globs – to merge with the main body of water they need to increase their
surface area, which requires a tiny input of energy.
Adrian Bowyer University of Bath
