38/news.2008.915
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How to walk on water
Published online 26 June 2008 | Nature | doi:10.1038/news.2008.915
News
Water snails crawl along under the water surface on ripples of slime.
How
to walk on water
Philip Ball
Published online 26 June 2008 | Nature | doi:10.1038/news.2008.915
Water snails crawl along under the water surface on ripples of slime.
Some water snails have an extraordinary way of getting around: they crawl upside down at the
water's surface. A team of US researchers may now have figured out how they do it.
Philip Ball (/news/author/Philip+Ball/index.html)
Some water snails have an extraordinary way of getting around: they
crawl upside down at the water's surface. A team of US researchers
At face
value,
snails’
motion
seems almost
may
now have
figuredthe
out how
they do
it.
impossible: how can they drag themselves across a
fluid surface that they can’t actually grip? But Eric Lauga of the University of California, San Diego
At
face
value,colleagues
the snails’ motion
seems
almost
canripples in the surface, water snails transform it into
er surface on ripples
of
slime.
and
his
say
that,
by impossible:
creatinghow
little
they drag themselves across a fluid surface that they can’t actually
one that does effectively offer a ‘foothold’.
Gripping stuff: snails grab
getting around: they
m of US researchers
mpossible: how can
they can’t actually
rnia, San Diego and
in the surface, water
grip? But Eric Lauga of the University of California, San Diego and
his colleagues say that, by creating little ripples in the surface, water
snails transform it into one that does effectively offer a ‘foothold’.
ripples of slime to hang
onto the surface.
NHPA/ IMAGE QUEST
3-D
The question of how snails move has been studied for at least a
century, but only relatively recently has an understanding emerged. For land snails, the secret’s in
the slime: the trail of mucus is sticky, but flows like a liquid if stressed beyond a certain level. So as
theGripping
snail's 'foot'
on it, the mucus offers an adhesive grip for some parts of the foot while
stuff:presses
snails grab
ripples of the
slime
to hang of other parts.
lubricating
movement
onto the surface.
offer a ‘foothold’.
NHPA/
IMAGE QUEST
But this
mechanism
needs a solid surface to which the snail can stick. It won’t work in water.
3-D
Nonetheless, some freshwater and marine snails crawl ‘hanging’ from the water surface while
ed for at least a
secreting a trail of mucus. How do they do it?
erstanding emerged. For land snails, the secret’s in
like a liquid if stressed beyond
a certain
Making
waveslevel. So as
The question
of how snails move has been studied for at least a century, but only relatively
n adhesive grip for some parts of the foot while
recently
has anstudied
understanding
emerged.
For land snails,
the
secret’s
in the slime: the trail of
Lauga
and colleagues
the common freshwater
snail Sorbeoconcha
physidae,
which
can
crawl
at a respectable
snail terms)
cm per
second. The snails
stay buoyant
by
mucus
is sticky,speed
but(inflows
like aof 0.2
liquid
if stressed
beyond
a certain
level. So as the snail's 'foot'
ich the snail can stick. Ittrapping
won’t work
in
water.
some
airit,
inside
shells. offers an adhesive grip for some parts of the foot while lubricating the
presses
on
thetheir
mucus
s crawl ‘hanging’ from the water surface while
movement
of
other
parts.
The snail's foot wrinkles into little rippling waves with a wavelength of about a millimetre and this
produces corresponding waves in the mucus layer that it secretes between the foot and the air. But
But this
mechanism
needs
a solid surface
to the
which
can(instick. It won’t work in water.
because
surface
tension constrains
the deformation
of the mucus,
shapethe
of itssnail
top surface
contact
with air) doesn’t
exactly
mirror that of the
bottom
surface
(in contact
with the
foot). In from the water surface while
Nonetheless,
some
freshwater
and
marine
snails
crawl
‘hanging’
hwater snail Sorbeoconcha physidae, which can
effect,
parts of the
mucus
film get squeezed, and parts get stretched, creating a pressure difference
secreting
a trail
0.2 cm per second. The snails
stay buoyant
by of mucus. How do they do it?
that pushes the foot forwards.
Making
waves
The
researchers
think their theory, described in a paper soon to be published in the journal
es with a wavelength of about a millimetre and this
yer that it secretes between the foot and the air. But
Lauga and colleagues studied the common freshwater snail Sorbeoconcha physidae, which can
terms) of 0.2 cm per second. The snails stay buoyant by
tion of the mucus, the shape of its top surface (in
crawl at a respectable speed (in snail
he bottom surface (in contact with the foot). In
trapping
some
air inside their shells.
d parts get stretched, creating
a pressure
difference
The snail's foot wrinkles into little rippling waves with a wavelength of about a millimetre and this
produces corresponding waves in the mucus layer that it secretes between the foot and the air.
But because surface tension constrains the deformation of the mucus, the shape of its top surface
(in contact with air) doesn’t exactly mirror that of the bottom surface (in contact with the foot). In
effect, parts of the mucus film get squeezed, and parts get stretched, creating a pressure
difference that pushes the foot forwards.
n a paper soon to be published in the journal
The researchers think their theory, described in a paper soon to be published in the journal
Physics of Fluids [1], offers only the beginnings of a full answer. It may also depend on the
complex ways in which the mucus flows when squeezed, for example.
Sticky problem
Not all water snails move this way, says Lauga's colleague Anette Hosoi, of the Massachusetts
Institute of Technology in Cambridge, Massachusetts. Many propel themselves using arrays of tiny
hair-like appendages called cilia. Others simply swim underwater, or crawl along the bottom.
The researchers suspect that, as well as elucidating a biological puzzle, the findings might point to
a new method of propulsion. Hosoi and her co-workers have already copied the adhesive/
lubricating propulsive method of land snails to drive a robotic device [2]. Now it might be feasible to
build similar devices that walk on water, although Hosoi says that this will require tricky mastery of
buoyancy to keep them floating right at the water surface.
That earlier ‘artificial snail’ was built for sheer curiosity. But Hosoi says that once she and her
colleagues reported it, "suddenly people were coming out of the woodwork": the researchers were
approached by people in the oil industry and in medical research, among others.
So she suspects that, if they succeed with a synthetic water snail, "we may be hearing from
industry again".
References
(1) Lee, S. et al., Phys. Fluids (in press, 2008).
(2) Chan, B. et al. Phys. Fluids 17, 113101 (2005).