-
It's midnight and all is still,
-
except for the soft skittering
of a gecko hunting a spider.
-
Geckos seem to defy gravity,
-
scaling vertical surfaces
-
and walking upside down
without claws,
-
adhesive glues or superpowered spiderwebs.
-
Instead, they take advantage
of a simple principle:
-
that positive
and negative charges attract.
-
That attraction binds together
compounds, like table salt,
-
which is made of positively
charged sodium ions
-
stuck to negatively charged chloride ions.
-
But a gecko's feet aren't charged
-
and neither are the surfaces
their walking on.
-
So, what makes them stick?
-
The answer lies in a clever combination
of intermolecular forces
-
and stuctural engineering.
-
All the elements in the Periodic Table
have a different affinity for electrons.
-
Elements like oxygen and fluorine
really, really want electrons,
-
while elements like hydrogen and lithium
don't attract them as strongly.
-
An atom's relative greed for electrons
is called its electronegativity.
-
Electrons are moving around all the time
-
and can easily relocated
to wherever they're wanted most.
-
So when there are atoms with different
electronegativities in the same molecule,
-
the molecules cloud of electrons
-
gets pulled towards
the more electronegative atom.
-
That creates a thin spot
in the electron cloud
-
where positive charge
from the atomic nuclei shines through,
-
as well as negatively charged
lump of electrons somewhere else.
-
So the molecule itself isn't charged,
-
but it does have a positively
and negatively charged patches.
-
These patchy charges can attract
neighboring molecules to each other.
-
They'll line up so that
the positive spots on one
-
are next to the negative
spots on the other.
-
There doesn't have to be a strongly
electronegative atom
-
to create these attractive forces.
-
Electrons are always on the move,
-
and sometimes they pile up
temporarily in one spot.
-
That flicker of charge is enough
to attract molecules to each other.
-
Such interactions between
uncharged molecules
-
are called van der Waals forces.
-
They're not as strong as the interactions
between charged particles,
-
but if you have enough of them,
they can really add up.
-
That's the gecko's secret.
-
Gecko toes are padded
with flexible ridges.
-
Those ridges are covered
in tiny hair-like structures,
-
much thinner than human hair,
called setae.
-
And each of the setae is covered
in even tinier bristles called spatulae.
-
Their tiny spatula-like shape is perfect
for what the gecko needs them to do:
-
stick and release on command.
-
When the gecko unfrills its flexible toes
onto the ceiling,
-
the spatulae hit at the perfect angle
for the van der Waals force to engage.
-
The spatulae flatten,
-
creating lots of surface area
for their positively
-
and negatively charged patches to find
complimentary patches on the ceiling.
-
Each spatula only contributes a minuscule
amount of that van der Waals stickiness.
-
But a gecko has about two billion of them,
-
creating enough combined force
to support its weight.
-
In fact, the whole gecko could dangle
from single one of its toes.
-
That super stickiness
can be broken, though,
-
by changing the angle just a little bit.
-
So, the gecko can peel its foot back off,
-
scurrying towards a meal
or away from a predator.
-
This strategy, using a forest
of specially shaped bristles
-
to maximize the van der Waals forces
between ordinary molecules
-
has inspired man-made materials
-
designed to imitate
the gecko's amazing adhesive ability.
-
Artificial versions aren't as strong
as gecko toes quite yet,
-
but they're good enough to allow
a full-grown man
-
to climb 25 feet up a glass wall.
-
In fact, our gecko's prey is also using
van der Waals forces
-
to stick to the ceiling.
-
So, the gecko peels up its toes
and the chase is back on.