PRESS
The Cheerio Effect
Bubbles
trapped at
the interface
between a
liquid and
a gas rarely
rest. Over
a timescale
of several
seconds to
minutes,
long-lived
bubbles move
towards one
another and,
when contained,
tend to drift
towards the
exterior
walls. The
sceptical
reader may
readily verify
these claims
by pouring
themselves
a glass of
bubbly in
the family
room, beer
in the bar,
or soap bubbles
in the kitchen
or bathroom
and following
the motion
of those
bubbles at
the surface
- particularly
those at
the periphery
of the glass.
This phenomenon
has even
been affectionately
dubbed the
`Cheerios
effect' after
the observation
that breakfast
cereals floating
in milk often
clump together
or stick
to the walls
of the breakfast
bowl. Despite
being a subject
with enormous
potential
for simple,
reliable
party tricks,
the technological
implications
of the Cheerios
effect are
far from
frivolous.
This is an
effect due
to the surface
tension between
the milk
and the air:
the air-milk
interface
does not
like to be
deformed
(just like
an elastic
sheet resists
deformation)
but at the
same time,
gravity is
pulling on
the individual
Cheerios.
At some deformation
of the interface,
the two effects
cancel so
that we have
a deformed
interface
and the single
object is
in equilibrium.
This balance
can be illustrated
quite dramatically
by floating
a single
drawing pin
on water,
as shown
below.
|
Figure
1:
A
single
drawing
pin
floating
on
water
illustrating
the
balance
between
surface
tension
and
the
weight
of
the
floating
object.
This
picture
was
described
as
the
`phenomenal
anti-intuitively
floating-upside-down
thumbtack'
by
the blog of
the
Annals
of
Improbable
Research. |
Related
Article
- The ‘Cheerios
Effect’,
D. Vella and L. Mahadevan, American Journal of Physics, 73,
817-825, 2005. 
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