Lasers etch an efficient way to address global water crisis

Date:
July 13, 2020
Source:
University of Rochester
Summary:
Researchers use sunlight and a laser-etched metal surface to
evaporate and purify water for safe drinking at greater than 100
percent efficiency. The method could help relieve water shortages
in drought- stricken areas and be helpful in water desalinization
projects.



FULL STORY ========================================================================== Lasers etch a simple way to address global water crisis

==========================================================================
Amid the coronavirus pandemic, people in developed countries are assured
of ample supplies of clean water to wash their hands as often as needed
to protect themselves from the disease. And yet, nearly a third of the
world's population is not even assured of clean water for drinking.

University of Rochester researchers have now found a way to address this problem by using sunlight -- a resource that everyone can access -- to evaporate and purify contaminated water with greater than 100 percent efficiency.

How is this possible? In a paper in Nature Sustainability, researchers
in the laboratory of Chunlei Guo, professor of optics, demonstrate
how a burst of femtosecond laser pulses etch the surface of a normal
sheet of aluminum into a super wicking (water attracting), super energy absorbing material.

When placed in water at an angle facing the sun, the surface:
* Draws a thin film of water upwards over the metal's surface *
Retains nearly 100 percent of the energy it absorbs from the sun to
quickly heat the water
* Simultaneously, changes the inter-molecular bonds of the water,
significantly increasing the efficiency of the evaporation process
even further.

"These three things together enable the technology to operate better
than an ideal device at 100 percent efficiency," says Guo, who is
also affiliated with the University's Physics and Materials Science
programs. "This is a simple, durable, inexpensive way to address the
global water crisis, especially in developing nations."


========================================================================== Experiments by the lab show that the method reduces the presence of
all common contaminants, such as detergent, dyes, urine, heavy metals,
and glycerin, to safe levels for drinking.

The technology could also be useful in developed countries for relieving
water shortages in drought-stricken areas, and for water desalinization projects, Guo says.

Easy to clean, easy to aim Using sunlight to boil has long been recognized
as a way to eliminate microbial pathogens and reduce deaths from diarrheal infections. But boiling water does not eliminate heavy metals and other contaminants.

Solar-based water purification, however, can greatly reduce these
contaminants because nearly all the impurities are left behind when the evaporating water becomes gaseous and then condenses and gets collected.



==========================================================================
The most common method of solar-based water evaporation is volume heating,
in which a large volume of water is heated but only the top layer can evaporate.

This is obviously inefficient, Guo says, because only a small fraction
of the heating energy gets used.

A more efficient approach, called interfacial heating, places floating,
multi- layered absorbing and wicking materials on top of the water, so
that only water near the surface needs to be heated. But the available materials all have to float horizontally on top of the water and cannot
face the sun directly, Guo says. Thus, the approach is less energy
efficient. Furthermore, the available wicking materials become quickly
clogged with contaminants left behind after evaporation, requiring
frequent replacement of the materials.

The panel developed by the Guo lab avoids these inefficiencies by pulling
a thin layer of water out of the reservoir and directly onto the solar
absorber surface for heating and evaporation. "Moreover, because we use
an open-grooved surface, it is very easy to clean by simply spraying it,"
Guo says.

"The biggest advantage," he adds, "is that the angle of the panels can be continuously adjusted to directly face the sun as it rises, and then moves across the sky before setting" -- maximizing energy absorption. "There
was simply nothing else resembling what we can do here," Guo says.

Latest in series of applications The project was supported by funding from
the Bill and Melinda Gates Foundation, the National Science Foundation,
and the US Army Research Office.

"The Army and its warfighters run on water, so there is particular
interest in basic materials research that could lead to advanced
technologies for generating drinking water," said Evan Runnerstrom,
program manager, Army Research Office, an element of the U.S. Army
Combat Capabilities Development Command's Army Research Laboratory. "The superwicking and light-absorbing properties of these aluminum surfaces
may enable passive or low-power water purification to better sustain the warfighter in the field." In addition to using femto-second laser etching technology to create superhydrophobic (water repellent), superhydrophilic (water-attracting), and super energy absorbing metals, the Guo lab has
created metallic structures that do not sink no matter how often they
are forced into water or how much it is damaged or punctured.

Prior to creating the water attracting and repellent metals, Guo and his assistant, Anatoliy Vorobyev, demonstrated the use of femto-second laser
pulses to turn almost any metal pitch black. The surface structures
created on the metal were incredibly effective at capturing incoming
radiation, such as light.

But they also captured light over a broad range of wavelengths.

Subsequently, his team used a similar process to change the color of a
range of metals to various colors, such as blue, gold, and gray. The applications could include making color filters and optical spectral
devices, using a single laser in a car factory to produce cars of
different colors; or proposing with a gold engagement ring that matches
the color of your fiancee's blue eyes.

The lab also used the initial black and colored metal technique to create
a unique array of nano- and micro-scale structures on the surface of a
regular tungsten filament, enabling a light bulb to glow more brightly
at the same energy usage.


========================================================================== Story Source: Materials provided by University_of_Rochester. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Subhash C. Singh, Mohamed ElKabbash, Zilong Li, Xiaohan Li, Bhabesh
Regmi, Matthew Madsen, Sohail A. Jalil, Zhibing Zhan, Jihua Zhang,
Chunlei Guo. Solar-trackable super-wicking black metal panel for
photothermal water sanitation. Nature Sustainability, 2020; DOI:
10.1038/ s41893-020-0566-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200713154943.htm

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