They burst
out of toilet bubbles, swim across drinking water, spread through coughs. Tiny
infectious microbes—from the virus that causes COVID-19 to waterborne
bacteria—kill millions of people around the world each year. Now engineers are
studying how zinc oxide surfaces and natural hydrodynamic churning have the
power to kill pathogens first.
"Bacterial
contamination of common surfaces and of drinking water have been traditionally
the main infection routes for transmission of serious diseases, often leading
to mortality," said Abinash Tripathy, a researcher in mechanical and
process engineering at ETH Zurich. "Our goal was to design a surface that
can address both issues."
His group
submerged clean zinc in hot water for 24 hours, which formed a zinc oxide
surface covered in sharp nanoneedles. Then they introduced E. coli bacteria.
The surface
kills almost all bacteria cultured on top of it very efficiently. And the
biggest surprise? When sitting in contaminated water, the surface kills all
waterborne E. coli within three hours—even bacteria it didn't touch.
This water
disinfection at a distance works because the process generates a reactive
oxygen species, which damages the cell walls of bacteria. The group from ETH
Zurich, IIT Ropar India, and Empa, Switzerland, presented their initial
findings at the 73rd Annual Meeting of the American Physical Society's Division
of Fluid Dynamics.
In
Southeast Asian and African countries where clean drinking water is scarce,
current solar water disinfection methods take up to 48 hours and require a
minimum intensity of sunlight. The new zinc oxide surface speeds up the
disinfection process and does not need light.
"This
surface can be used to disinfect water in remote areas at a very low
cost," said Tripathy. "The fabrication technique is environmentally
friendly, simple, and economical."
Surface
and waterborne pathogens aren't the only killers. As the COVID-19 pandemic has
reinforced, airborne viruses and bacteria pose a serious global challenge for
disinfection.
The very
droplets that carry pathogens through the air can play a role in destroying
them. In the microseconds that droplets take to form, their fluids rearrange
rapidly—stressing the microbes within.
"Think
of a bucket with a fish in it. One imagines that if you start churning the
fluid in the bucket too quickly, the fish won't be very happy," said
Oliver McRae, a mechanical engineer. "It's a similar kind of thing—albeit
on a much, much smaller scale—when you have, say, a pathogen in a droplet.
Eventually the fluid's going to agitate too much for that bacteria or virus to
survive."
McRae and
a team from Boston University and the Centers for Disease Control and
Prevention were studying how hydrodynamic agitation works when environmental
bubbles produce droplets. After the onset of the pandemic, they started
modeling droplets similar to those produced by the lungs and respiratory tract.
Using
computational fluid dynamics, the team predicted how agitation works during
aerosol formation. They discovered that stressors are very sensitive to droplet
size. If the droplet shrinks or grows by one order of magnitude, the stressors
change by two-and-a-half orders of magnitude.
The
research could help explain why pathogens survive in some droplets and not
others.
"Our
focus has been on quantifying what the stressors are in these droplets,"
said McRae. "Hopefully this will be used in the future as part of a larger
model to predict aerosol-based disease transmission."