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Water-repellent legs of water striders

Water striders (Gerris remigis) have
remarkable non-wetting legs that
enable them to stand effortlessly and
move quickly on water, a feature believed to
be due to a surface-tension effect caused by
secreted wax1–3.We show here, however, that
it is the special hierarchical structure of the
legs, which are covered by large numbers of
oriented tiny hairs (microsetae) with fine
nanogrooves, that is more important in
inducing this water resistance.
We used a high-sensitivity balance system
to construct force–displacement curves for
a water strider’s legs when pressing on the
water surface (for methods, see supplementary
information). Surprisingly, the leg does
not pierce the water surface until a dimple of
4.380.02 mm depth is formed (Fig. 1a).
The maximal supporting force of a single leg
is 152 dynes (see supplementary information),
or about 15 times the total body weight
of the insect. The corresponding volume of
water ejected is roughly 300 times that of
the leg itself, indicating that its surface is
strikingly water repellent.
For comparison,we made a hydrophobic
‘leg’ from a smooth quartz fibre that was
similar in shape and size to a strider’s leg. Its
surface was modified by a self-assembling
monolayer of low-surface-energy heptadecafluorodecyltrimethoxysilane
(FAS-17),
which makes a contact angle of 109° with a
water droplet on a flat surface4. Water supports
the artificial leg with a maximal force of
only 19.05 dynes (see supplementary information),
which is enough to support the
strider at rest but not to enable it to glide or
dart around rapidly on the surface.
This finding suggests that the force exerted
by the strider’s legs could be due to a ‘superhydrophobic’
effect (that is, the contact angle
with water would be greater than 150°).We
verified that this was indeed the case by sessile
water-drop measurements, which showed
that that the contact angle of the insect’s legs
with water was 167.64.4° (Fig.1a,inset).
The contact angle of water made with the
cuticle wax secreted on a strider’s leg is about
105° (ref. 5), which is not enough to account
for its marked water repellence.Knowing that
microstructures on an object with low surface
energy can enhance its hydrophobicity6, we
investigated the physical features of the legs.
Scanning electronic micrographs revealed
numerous oriented setae on the legs.These are
needle-shaped, with diameters ranging from
3 micrometres down to several hundred
nanometres (Fig. 1b).Most setae are roughly
50 m in length and arranged at an inclined
angle of about 20° from the surface of leg.
Many elaborate, nanoscale grooves are evident
on each microseta, and these form a
unique hierarchical structure (Fig.1c).
According to Cassie’s law for surface wettability,
such microstructures can be regarded
as heterogeneous surfaces composed of
solid and air7. The apparent contact angle
l
of the legs is described by cos
l
f1cos wf2,
where f1 is the area fraction of microsetae
with nanogrooves, f2 is the area fraction of air
on the leg surface and w is the contact angle
of the secreted wax. Using measured values
of
l and w, we deduce from the equation
that the air fraction between the leg and the
water surface corresponds to f2
96.86%.
Available air is trapped in spaces in the
microsetae and nanogrooves to form a cushion
at the leg–water interface that prevents
the legs from being wetted.
This unique hierarchical micro- and
nanostructuring on the leg’s surface therefore
seems to be responsible for its water
resistance and the strong supporting force.
This clever arrangement allows water striders
to survive on water even if they are being
bombarded by raindrops, when they bounce
to avoid being drowned. Our discovery may
be helpful in the design of miniature aquatic
devices and non-wetting materials.