A water-absorbing, energy-absorbing, laser-etched metal surface, continuously at a direct angle to the sun, provides an economical and efficient way to purify water from sunlight. The technology was developed by Chunei Guo’s laboratory at the University of Rochester. Credit: HM Cao / University of Rochester
By engraving metal with ultra-short laser bursts, the Rochester researchers demonstrate a way to purify water without wasting energy.
Amid the coronavirus pandemic, people in developed countries are assured of an ample supply of clean water to wash their hands often enough to protect themselves from the virus. And yet, almost a third of the world’s population does not even have safe drinking water.
Researchers at the University of Rochester have found a way to tackle this problem by using sunlight, a resource everyone can access, to evaporate and purify contaminated water with efficiency greater than 100 percent.
How is this possible?
On a paper in Sustainability of nature, researchers at the laboratory of Chunlei Guo, professor of optics, demonstrate how a burst of femtosecond laser pulses etches the surface of a normal sheet of aluminum into a super absorbent (water-attracting), energy-absorbing material.
When placed in water at an angle facing the sun, the surface:
- Draw a thin film of water upward on the metal surface.
- Retains almost 100 percent of the energy it absorbs from the sun to quickly heat water
- At the same time, it changes the intermolecular bonds of the water, which further increases the efficiency of the evaporation process.
“These three things together allow the technology to work better than an ideal device with 100 percent efficiency,” says Guo, who is also affiliated with the University’s Physics and Materials Science programs.
The use of sunlight for boiling has long been recognized as a way to eliminate microbial pathogens and reduce deaths from diarrheal infections. But boiling water does not remove heavy metals and other contaminants.
Laboratory experiments show that your new method reduces the presence of all common contaminants, such as detergents, dyes, urine, heavy metals, and glycerin, to safe levels to drink.
The technology could also be useful in developed countries to alleviate water shortages in drought-stricken areas, and for water desalination projects, says Guo.
Solar-based water purification: looking for an efficient method
Solar-based water purification can greatly reduce contaminants because almost all impurities are left behind when the evaporating water becomes gaseous and then condenses and collects.
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. Obviously, this is inefficient, says Guo, because only a small fraction of the heating energy is used.
A more efficient approach, called interfacial heating, places floating, absorbent, and multilayer absorbent materials on top of the water, so that only the water near the surface needs to be heated. But all available materials have to float horizontally on the water and cannot look directly at the sun. Additionally, available absorbent materials quickly clog with contaminants remaining after evaporation, requiring frequent replacement of materials.
The panel developed by the Guo laboratory avoids these inefficiencies by extracting a thin layer of water from the tank and directly on the surface of the solar absorber for heating and evaporation. “Also, because we use a surface with open grooves, it is very easy to clean just by spraying it,” says Guo.
“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 move across the sky before sunset,” maximizing energy absorption.
“There was simply nothing more like what we can do here,” says Guo.
The latest in a series of applications
Guo, who is also affiliated with the University’s materials science and physics programs, has long envisioned a series of humanitarian applications for an efficient purification method based on solar energy. “This is a simple, long-lasting, and inexpensive way to address the global water crisis, especially in developing countries,” he says, noting that it could help alleviate water shortages in drought-affected areas and be useful in water desalination projects, he adds.
“The Army and its warriors run on water, so there is a particular interest in basic materials research that could lead to advanced technologies to generate drinking water,” said Evan Runnerstrom, program manager, Army Research Office, an element of the US Army Combat Capabilities Development Command Army Research Laboratory. “The light-absorbing and super-warping properties of these aluminum surfaces can allow for low-power or passive water purification to better support the warrior in the field.”
In addition to using femto-second laser engraving technology to create superhydrophobic (water-repellent), superhydrophilic (water-attracting), and super energy-absorbing metals, Guo Lab has created metal structures that won’t sink regardless of the frequency with which they are forced to enter. water or how much is damaged or punctured.
Before creating the metal that attracts and repels water, Guo and his assistant, Anatoliy Vorobyev, demonstrated the use of femto-second laser pulses to convert almost any shade of black metal. The surface structures created in the metal were incredibly effective in capturing incoming radiation, such as light. But they also captured light over a wide 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. Applications could include manufacturing color filters and optical spectral devices, using a single laser at an automobile factory to produce cars of different colors; or proposing a gold engagement ring that matches the color of your fiancee’s blue eyes.
The lab also used the initial color and black metal technique to create a unique matrix of nano and microscale structures on the surface of a regular tungsten filament, allowing a light bulb to shine brighter with the same use of energy.
Reference: Subhash C. Singh, Mohamed ElKabbash, Zilong Li, Xiaohan Li, Bhabesh Regmi, Matthew Madsen, Sohail A. Jalil, Zhibing Zhan, Jihua Zhang, and Chunlei. Guo, July 13, 2020, Sustainability of nature.
DOI: 10.1038 / s41893-020-0566-x
In addition to Guo, co-authors include lead author Subhash Singh, Mohamed ElKabbash, Zilong Li, Xiaohan Li, Bhabesh Regmi, Matthew Madsen, Sohail Jalil, Zhibing Zhan, and Jihua Zhang, all from Guo Lab. Among them, four are undergraduate students .
The project was supported by funds from the Bill and Melinda Gates Foundation, the National Science Foundation, and the U.S. Army Research Office.
