From MIKE PETTY
In reply to Ian Anderson (Letters, 5 October and 19 October), I too
have been fascinated for some time by these ‘bubbles’, or globules, forming
on water surfaces.
The globules occur when water (or a suitable liquid, such as coffee)
is allowed to drip onto a liquid surface. Casual observation and simple
experimentation have revealed that these globules form most readily and
to greatest size when surface active molecules (surfactants) are present.
This can be seen to greatest effect with dishwater, black coffee and Indian
tonic water. This is a puzzle as one would normally expect surfactants to
lower the surface tension of the liquid and, hence, reduce its ability to
support such globules.
The answer to this conundrum is to be found in C. L. Strong’s article
‘Water Droplets that Float on Water’ (Scientific American, 1976). Water
is a polar solvent and, consequently, its surface is polarised – in effect,
electrically charged. This means that when two water surfaces are brought
near to each other, such as Ian Anderson’s coffee globules skating on his
morning coffee, there is an electrostatic repulsion between the two. Surfactant
molecules are also polarised and, although they reduce the surface tension
of the liquid, they increase the surface polarisation. This increases the
electrostatic repulsion and prevents the globules merging with the main
body of the liquid. With careful manipulation the globules can be made to
survive for several minutes.
Mike Petty University of Durham
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