HONG
KONG, July 2, 2024 /PRNewswire/ -- The more we
discover about the natural world, the more we find that nature is
the greatest engineer. Past research believed that liquids can only
be transported in fixed direction on species with specific liquid
communication properties and cannot switch the transport direction.
Recently, The Hong Kong Polytechnic University (PolyU) researchers
have shown that an African plant controls water movement in a
previously unknown way – and this could inspire breakthroughs in a
range of technologies in fluid dynamics and nature-inspired
materials, including applications that require multistep and
repeated reactions, such as microassays, medical diagnosis and
solar desalination etc. The study has been recently published in
the international academic journal Science.
Liquid transport is an unsung miracle of nature. Tall trees, for
example, have to lift huge amounts of water every day from their
roots to their highest leaves, which they accomplish in perfect
silence. Some lizards and plants channel water through capillaries.
In the desert, where making the most of scarce moisture is vital,
some beetles can capture fog-borne water and direct it along their
backs using a chemical gradient.
Scientists have long sought to hone humankind's ability to move
liquids directionally. Applications as diverse as microfluidics,
water harvesting, and heat transfer depend on the efficient
directional transport of water, or other fluids, at small or large
scales. While the above species provide nature-based inspiration,
they are limited to moving liquids in a single direction. A
research team led by Prof. WANG Liqiu, Otto Poon Charitable
Foundation Professor in Smart and Sustainable Energy, Chair
Professor of Thermal-Fluid and Energy Engineering, Department of
Mechanical Engineering of PolyU, has discovered that the
succulent plant Crassula muscosa, native to Namibia and South Africa, can transport
liquid in selected directions.
Together with colleagues from the University of Hong Kong and Shandong University, the PolyU researchers
noticed that when two separate shoots of the plant were infused
with the same liquids, the liquids were transported in opposite
directions. In one case, the liquid travelled exclusively towards
the tip, whereas the other shoot directed the flow straight to the
plant root. Given the arid but foggy conditions in which C.
muscosa lives, the ability to trap water and transport it in
selected directions is a lifeline for the plant.
As the shoots were held horizontally, gravity can be ruled out
as the cause of the selective direction of transport. Instead, the
plant's special properties stem from the tiny leaves packed onto
its shoots. Also known as "fins", they have a unique profile, with
a swept-back body (resembling a shark's fin) tapering to a narrow
ending that points to the tip of the plant. The asymmetry of this
shape is the secret to C. muscosa's selective directional
liquid transport. It all has to do with manipulating the meniscus –
the curved surface on top of a liquid.
Specifically, the key lies in subtle differences between the fin
shapes on different shoots. When the rows of fins bend sharply
towards the tip, the liquid on the shoot also flows in that
direction. However, on a shoot whose fins – although still pointing
at the tip – have a more upward profile, the direction of movement
is instead to the root. The flow direction depends on the angles
between the shoot body and the two sides of the fin, as these
control the forces exerted on droplets by the meniscus – blocking
flow in one direction and sending it in the other.
Armed with this understanding of how the plant directs
liquid flow, the team created an artificial mimic. Dubbed CMIAs,
for 'C. muscosa-inspired arrays', these 3D-printed fins act
like the tilted leaves of C. muscosa, controlling the
orientation of liquid flow. Cleverly, while the fins on a natural
plant shoot are immobile, the use of a magnetic material for
artificial CMIAs allows them to be reoriented at will. Simply by
applying a magnetic field, the liquid flow through a CMIA can be
reversed. This opens up the possibility of liquid transport along
dynamically changing paths in industrial and laboratory settings.
Alternatively, flow could be redirected by changing the spacing
between fins.
Numerous areas of technology stand to benefit from CMIAs.
Prof. Wang said, "There are foresee applications of
real-time directional control of fluid flow in microfluidics,
chemical synthesis, and biomedical diagnostics. The
biology-mimicking CMIA design could also be used not just for
transporting liquids but for mixing them, for example in a T-shaped
valve. The method is suited to a range of chemicals and overcomes
the heating problem found in some other microfluidic
technologies."
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SOURCE The Hong Kong Polytechnic University