From IAN RUSSELL
Mike Petty’s explanation of those mysterious ‘floating’ water drops
is, at best, only partially correct and may be completely wrong (Letters,
2 November).
He quoted an article by C. L. Strong (‘Water Droplets that Float on
Water’, Scientific American, 1976), in which the phenomenon is attributed
to electrostatic repulsion aided by the presence of surfactants. It may
play some part, but a more recent article by Jearl Walker in his discontinued
and much-lamented Amateur Scientist column (Scientific American, June 1978)
casts considerable doubt on this theory as a full explanation.
Walker claims that a more widely accepted view is that such drops ride
the waves like miniature hovercraft, on a thin air cushion flowing radially
out of a lens-shaped body of air trapped below each drop. This is rather
similar to the Leidenfrost effect, in which drops can briefly rest above
a hotplate on a layer of steam. He describes a series of simple, fascinating
experiments showing that the phenomenon depends more on the surface being
vibrated by small ripples than on the presence of surfactants (in fact it
can periodically be seen on a calm lake lashed by a heavy rainstorm). The
ripples, whether in flicked orange juice or splashing coffee, are somehow
responsible for ‘pumping’ air into the space below each drop. It appears
that the precise mechanism remains uncertain.
How good it must be for the scientific education and motivation of those
commendably curious children at Clifton College Preparatory School, that
they have been able to make an observation for which textbooks and New Scientist
readers have offered no comprehensive explanation. There is more to science
education than the passive soaking up of explanations, and everyone thought
they were just messing about with their orange juice!
Advertisement
Ian Russell New Mills, Derbyshire
