https://www.economist.com/science-and-technology/2021/05/26/a-better-fog-trap

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[20210529_stp002_0]

Innovation
A better fog-trap

A clever upgrade of a humble but useful technology

Science & technologyMay 29th 2021 edition
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May 26th 2021

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EVERY NIGHT, air cooled and moistened by the Humboldt current blows
over Chile's northern coast and across the Atacama desert. The
billowing banks of fog thus created might look insubstantial, but
there is water here to be captured--and in this, the driest place on
Earth, capture it people do. Fog-harvesting will never be big
business, for it needs particular conditions to operate well. But in
zones like the Atacama, where moisture-laden breezes bring fog but no
rain, the invention in the 1960s of traps which can pluck that
moisture from the air has helped sustain settlements otherwise on the
brink of drought.

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Fog traps are polymer-mesh screens mounted in metal frames (see
picture). As misty zephyrs blow through them, droplets of water
adhere to the mesh. These absorb others until the result breaks free
and runs down the screen, as a raindrop runs down a window pane, into
a collector. A typical trap, with a 40-square-metre collecting area,
yields about 200 litres a day. That is enough to supply around 60
people with drinking water. Such a collector costs $1,000 or so, and
will last a decade.

A simple idea, then. But even simple technology can be upgraded. And
that is exactly what Urszula Stachewicz of the AGH University of
Science and Technology, in Krakow, Poland, proposes to do.

Her upgrade relies on giving the mesh a slight electrical charge.
Overall, the molecules in it are electrically neutral. But polymer
molecules are large, and their electrons can move about within them.
Uneven distribution of these electrons means some parts of a molecule
are positive and some negative. That fact might be exploited to give
the outside of each thread in a mesh one electrical polarity (either
positive or negative) while burying the other polarity in the
thread's interior. Just as static electricity will attract a balloon
to a wall, a surface charge created in this way will attract water
droplets from the air.

Previous attempts to do this have coated the threads with metal. But
that is expensive. As they describe in ACS Nano, Dr Stachewicz and
her student Daniel Ura have done it by changing the way the threads
are spun, which is cheap.

At the moment, those threads are created by a process called
melt-spinning. This involves melting the polymer and pushing the
resulting liquid through a hollow needle, out of a hole in the
needle's tip and onto a spinning drum, where it cools and solidifies.
Dr Stachewicz and Mr Ura propose instead to use a technique called
electrospinning. This also involves a hollow needle. But instead of
being melted, the polymer is dissolved in a solvent, and instead of
being propelled from the needle's tip by pressure from behind, it is
attracted to the drum by the creation of an electrical potential
difference between the drum and the needle. As the stream of
dissolved polymer travels through the air, the solvent evaporates,
leaving behind a solid thread.

One consequence of electrospinning is that the voltage attracting the
thread from needle to drum also attracts or repels, according to
their polarity, the charges on the molecules within. This reorients
those molecules, aligning them (and thus their charges) in a way that
does not happen in a melt-spun thread. That can create the sort of
polarity difference between the interior and the exterior of a thread
the two researchers were looking for.

Dr Stachewicz and Mr Ura experimented with different potential
differences between needle and drum and eventually found one that
resulted in meshes able to collect 50% more water than commercial
versions, at no extra cost of production. At the moment these are
just prototypes, but they expect soon to have a practical version
which can be made available for sale. And that will certainly improve
the lives of people who rely for their survival on drawing water not
from a well, but from thin air. #

A version of this article was published online on May 26th, 2021

This article appeared in the Science & technology section of the
print edition under the headline "A better fog trap"

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