Led by Chunlei Guo, a professor in the University of Rochester’s Institute of Optics, a team of scientists has demonstrated how ultrashort femtosecond laser pulses etch the surface of a sheet of metal (in this research, aluminum) into a super energy-absorbing material. The metal surface in the system is a broadband absorber, which increases the efficiency of the water evaporation process, demonstrating how sunlight can purify water of contaminants and make it safe for drinking.
In addition to absorbing light across a broad wavelength range, the surface of the energy-absorbing aluminum surface in the system is superwicking (hydrophilic). When researchers angled it to face a light source, the metal surface drew a thin film of water over itself in an upward direction against gravity. The surface retained nearly 100% of the light energy it absorbed. As it heated the water, evaporation intensified, and the water’s intermolecular bonds changed.
As water evaporates and becomes a gas, it loses many of the potentially harmful contaminants it may contain, such as heavy metals. This makes solar-based purification methods an appealing option for purification; simply boiling water fails to remove heavy metals and other harmful contaminants, as water does not enter a gaseous state.
Unfortunately, solar-based methods are not always efficient in practice. Volume heating, as its name suggests, involves heating a large quantity of water — though only to a point that its top layer evaporates. From start to finish, volume heating uses only a small amount of the generated heating energy.
Another method, interfacial heating, places floating, multilayered absorbing and wicking materials on top of water. Though only water near the surface must be heated, all materials involved in the process must face away from the sun, in a nonvertical position.
As a result, Guo said, durability is vital to the new technology.
“The biggest advantage,” he said, “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. There was simply nothing else resembling what we can do here.”
Laser interactions and black metals
For Guo, work with lasers and the impact of their interactions on the surface properties of metals predates any of his laboratory’s discoveries with water and superwicking surfaces.
From 2005 to 2006, Guo’s research examined how intense femtosecond laser pulses influenced energy deposited over metal surfaces. The individual laser technologies Guo developed resulted in drastic changes to the optical responses of metal surfaces, to a point that in one application, laser pulses turned a shiny metal surface completely black.
As the black surface absorbs light across a broad wavelength range, it is a valuable resource for collecting light when, for example, building a thermal sensor.
Paired with the black surface’s superwicking quality, it is an ideal surface for the current research.
Solar-based water purification is much more effective at reducing contaminants than simply boiling water. There are various methods of solar-based purification, but none has been as efficient as a new method developed in the lab of University of Rochester optics professor Chunlei Guo that uses laser-etched aluminum panels. Illustration courtesy of H.M. Cao/University of Rochester.
The lab has completed experiments showing reduced levels of contaminants in tested water samples. When treated by the system, the quantities of detergent, dye, urine, heavy metals, and glycerin all fell to levels safe for drinking.
“This is a simple, durable, inexpensive way to address the global water crisis, especially in developing nations,” Guo said. The work may also serve to relieve water shortages in areas affected by drought, and aid in water desalinization.
Surface maintenance and cleaning is another advantage of the system. By using a surface with open grooves, the researchers can easily spray the surface clean. After evaporation, contaminants would clog the wicking materials, requiring the researchers to make frequent replacements. By pulling a thin layer of water out of the reservoir and directly onto the solar absorber surface, the new system avoids contamination.
Funding from the Bill and Melinda Gates Foundation, the U.S. Army Research Office, and the National Science Foundation supported the project.
“While it is important that we as scientists do cutting-edge research, it is more important to for us to solve real-world problems and try to save lives. In that regard, our research experience is certainly a very special one,” Guo said.
The research was published in Nature Sustainability (www.doi.org/10.1038/s41893-020-0566-x).