YSM Issue 93.2

Bioengineering
FEATURE
IMAGE COURTESY OF WIKIMEDIA COMMONS
Hydrophobic leaves help aquatic plants thrive in
wet environments.
Xiang believes an industrial translation
of the Salvinia system—microgrooves,
long stems and eggbeater-shaped
heads—could create a self-replenishing
active system that prevents prolonged
wetting along the ship hull. A similar
approach might protect pipelines as
well, reducing maintenance costs and
improving water quality.
Another potential application is drag
reduction to enable faster underwater
vehicles. An object moving in a fluid
experiences a resistance to its motion,
which can be reduced significantly
if the object is enveloped in an air
cocoon. Compared to current methods
of generating this air cocoon, Xiang’s
research may have wider usage. “Methods
like supercavitation work only at high
speeds,” Xiang said. The Salvinia-based
design could achieve drag reduction
“even at lower Reynolds numbers,”
where the object moves slowly in a less
turbulent manner. In the lab, the air
layer is retained at low fluid velocity of
half a meter per second. Further research
would be needed to scale up this speed
for use in ships, where the water flows at
a few meters per second.
To replicate Salvinia’s natural patterns
on artificial surfaces, the Peking
University researchers 3D printed a
specimen with regular microgrooves,
long stems, and eggbeater bulbs. Due
to the extreme precision required, the
overall sample size was a square of four
millimeters in length. The team printed
a Salvinia-inspired design (complete
with microgrooves, stems and eggbeater
heads) as well as a control specimen
(with stems and eggbeater heads only).
Using a confocal microscope, Xiang and
the researchers found that a smooth layer
of air only appeared in the first sample.
In the control, individual air bubbles
formed instead. Hence the Salvinia’s
microgrooves are an essential component
to achieving a smooth air layer, even more
so than the hydrophobic lotus leaf, which
only has long stems and eggbeater bulbs.
The lotus, as represented by the control,
cannot recover its air layer once disrupted.
In a further investigation using the
3D printing apparatus, the researchers
varied the microgroove angles to test the
predictions of their air layer formation
theory. Using a thermodynamic free
energy model, the researchers calculated
the angle requirement for a stable air
layer to form spontaneously. The natural
Salvinia leaf ’s microgroove angle matched
that range, and additional 3D printed
tests verified the range of full, partial,
or no expansion. Again, experiments
showed that the microgroove pattern
is critical for the Salvinia plant’s strong
hydrophobic capability.
Following his lab’s focus on boundary
layer stability research, Xiang and his
team’s research on the Salvinia leaf gives
new understanding of the boundary
layers of a submerged body. “We wanted
to find out what made the air layer on
Salvinia plant so stable. It turns out
the hydrophobicity gives an extremely
strong adaptability to environmental
conditions. This also provides a
theoretical basis for the design of
artificial bionic materials,” Xiang said.
The Salvinia leaf, which actively
replenishes its air layer through a discreet
anatomy, is one of the most effective waterrepellent
surfaces we know of currently. A
humble leaf holds many secrets. ■
Xiang, Y., Huang, S., Huang, T., Dong, A., Cao, D., Li, H., Xue, Y., Lv, P., & Duan, H. (2020). Superrepellency
of underwater hierarchical structures on Salvinia leaf. PNAS, 117(5), 2282-2287.
www.yalescientific.org
September 2020 Yale Scientific Magazine 27