liquid handling
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Enhancing liquid delivery on nonwetting surfaces
In diverse processes such as fertilisation, insecticide treatment and cooling, liquid delivery is compromised by the super-repellency of the receiving surfaces, causing droplets of liquid to ‘bounce’ off these surfaces like a golf ball bouncing along a fairway. This results in issues such as pesticide overuse, soil contamination and water waste.
N
the water whirls and flows, consuming the kinetic
Another potential application of the new
energy of the water balloon. In this way, the balloon
technology is in spray cooling. The researchers
will not rebound easily.”
estimate that the technology can enhance the
Currently, special polymers are added into the
cooling rate of spray cooling fourfold, meaning
ow, mechanical engineering
droplet to modify the liquid’s interfacial properties.
only one-fourth of the amount of water or time is
researchers at The University of Hong Kong
Such polymers act like superglue to pin the droplet
required to obtain the same cooling effect. Water
(HKU) have developed a strategy to enhance
onto the surface; however, they will also mix with
droplets overlaid with liquids of a higher boiling
liquid disposition by overlaying impacting
the content and change the nature of the droplet.
point will be less easy to vaporise, and hence more
droplets with a tiny amount of lubricant (less
The new method allows a thin layer of lubricant to
readily able to come into direct contact with the
than 0.1 vol% of the droplet), to modify their
be formed on the droplet, while leaving its content
overheated surfaces — whereas in conventional
interfacial properties for easy manipulation. As
intact. The choices for the overlayers can be of a
water spraying, vaporised water will form a barrier
published in the journal Nature Communications,
wide diversity, depending on the liquid type.
inter-facially to prevent more water from reaching
the overlayers suppress the out-of-plane
“The footprint of our method can be made to
rebounds by slowing the departing droplets
be minimal, circumventing surface contamination
through viscous dissipation and sustaining
and toxification,” Dr Han said.
the heated surfaces to substantially retard the cooling process. “Imagine cooling a hot pot by spraying water,”
the droplets’ in-plane mobility through self-
“Our method enables multifunctional and
Dr Han said. “Using the technology, the water is
lubrication — a preferential state for scenarios
dynamic control of droplets that impact different
firmly deposited onto the pot, allowing fast cooling.”
such as shedding of liquid in spray cooling and
types of non-wetting surfaces. The droplet’s
“This is especially important for application
repositioning of droplets in printing.
post-deposition state can be switched between
in major catastrophic events involving high
“Analogically, the ultrathin liquid overlayer
immobilised and sliding on the surface by tuning
temperatures such as fire or overheating at power
turns the elastic golf ball into a balloon filled with
the overlayer volume, enabling rich fluid controls
plants, and in other industrial applications,”
water,” said Dr Xing Han, first author on the
on repellent surfaces, including superhydrophobic,
concluded Liqiu Wang, Chair Professor of
paper, from the HKU Department of Mechanical
superomniphobic and superheated types, similar
Thermal-Fluid Sciences and Engineering, from the
Engineering. “As the water balloon hits the ground,
to depositing golf balls on all kinds of terrain.”
Department of Mechanical Engineering.
16 | LAB+LIFE SCIENTIST - Feb/Mar 2022
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