YSM Issue 93.2

FEATURE
Bioengineering
KEEPING DRY UNDERWATER
LEARNING SUPERHYDROPHOBICITY FROM PLANTS
BY YU JUN SHEN
ART BY ANMEI LITTLE
Hydrophobic materials have many
applications, yet many are easily
disrupted by the environment,
losing their dryness. To find the key
to the next generation of highly waterrepellent
materials, scientists have
turned to Mother Nature for inspiration,
studying species that thrive in water,
like lotus plants and ferns.
Recently, Xiang Yaolei and his team
at Peking University investigated the
hydrophobic leaves of Salvinia molesta,
a sturdy fern species best known for
being highly invasive. The researchers
discovered that the Salvinia leaf
had evolved surface patterns ideal
for generating a smooth layer of air
underwater. They then replicated this
design on a 3D printed specimen.
This work paves the way for improved
underwater applications, such as reduced
drag on underwater vehicle and improved
protection against corrosion in pipes.
In a laboratory study of carefully
degassed underwater Salvinia leaves, the
researchers observed that an air layer
forms spontaneously on the leaf surface.
For accuracy purposes, the researchers
applied a high water pressure to remove
any trapped air on underwater Salvinia
leaves first, then injected new air via a
small syringe. “It is different from a lotus
leaf. Here, a whole layer of air forms, while
for the lotus only a few individual bubbles
appear. The Salvinia mechanism is an
active replenishment of air,” Xiang said.
Scientists and engineers sought to
discover the underlying design behind
the Salvinia leaf ’s
active replenishment
mechanism. Using
a scanning electron
microscope, the team
found three key
features of the Salvinia
leaf ’s hydrophobicity:
i n t e r c o n n e c t e d
w e d g e - s h a p e d
microgrooves on the
leaf ’s surface, long hair
stems, and egg-beater
shaped heads. The wedgeshaped
microgrooves
enabled stable air pockets to
form and expand by capillary
action, going against the flow
of gravity. The interconnected and
widespread microgrooves allow air to
spread efficiently and spontaneously
across the leaf surface.
The researchers found that, once
formed, the air layer then rises along
the frame provided by the hairy stems.
Moreover, the eggbeater-shaped head,
which caps the microgrooves and hair
stems, stabilizes the entire air layer by
surface tension. As the stems of the plant
are irregular, the air layer arrived at the
top of short stems will pin and “wait” for
the air layer arrived to the top of high
stems. These three features combine
to actively replenish the air layer, even
in the presence of water flow. Hence,
compared to passive hydrophobic
materials, the Salvinia design is more
reliable in sustaining an air layer.
Xiang is enthusiastic about the
industrial applications of Salviniainspired
hydrophobic materials. “As it is
an efficient way to protect the air mattress
in different environmental conditions, it
will expand the applications of superhydrophobic
surfaces, especially in
extreme environments,” Xiang said.
An application of particular interest
is the anti-corrosive coating on ship
keels. Corrosion below the waterline
weakens and damages the ship. A passive
solution is anti-corrosion chemical
paint, though it degrades over time and
leaves environmental residues. Current
active systems inject bubbles to stick
to the surface, but this must be done
continuously and while fully submerged.
26 Yale Scientific Magazine September 2020 www.yalescientific.org