A magnetic system that dynamically adjusts the surface properties of a material used for solar-driven water purification has been created by researchers in China. Developed by Liangti Qu at Beijing’s Tsinghua University and colleagues, the system achieved higher evaporation rates when compared to static surfaces.
Clean water is in short supply in many parts of the world. Purification and desalination processes can be energy intensive. Developing ways of using solar energy to purify water by evaporation has been the subject of extensive research. However, it is far from being in widespread use. Even though this approach mainly utilises the sun’s energy to separate water from contaminants, it is still too slow for many practical applications.
Interfacial solar vapour generation offers a way to increase evaporation efficiency. It achieves this by concentrating the energy of the Sun’s rays only on the surface of the water. Still, static systems have little control over water flow and its vaporisation. The harsh chemical environment of untreated water leaves such systems prone to rapid deterioration.
Qu and colleagues have created a dynamic magnetically responsive system with controlled porosity and a shifting surface that achieves much higher evaporation rates than its static counterparts.
Transport spikes support water purification
At first glance, the glistening spiky fluid does not look like a water-purification system. Created by the team, it is a slurry of graphene-wrapped iron oxide nanoparticles mixed with the water to be purified. The special graphene coating prevents the nanoparticles from aggregating together. It allows them to reconfigure under an external magnetic field. Alternatively, they can be disassembled simply by washing the slurry with a stream of water. Crucially, the material accelerates the diffusion of water from the bulk to the system’s surface by two orders of magnitude compared to the uncoated nanoparticles.
When the slurry is exposed to an external magnetic field, arrays of cones are formed in a manner characteristic of ferrofluids. The high surface-area cones move, deform and spin along with the motion or change of the applied magnetic field. Thanks to the spiky surface and the concentration gradient, any salt precipitation left behind when the pure water evaporates only occurs at the tips. This allows the sunlight to get through unblocked to the water in the slurry.
The spikes are not the only structures of interest. On a smaller scale, a network of pores with diameters between hundreds of nanometres and tens of millimetres allows the rapid transport of water and fast disassembly of the structure when needed.
Spinning structures
Generally, porous structures are known to have enhanced performance regarding water transport. However, other porous materials’ purification systems rely on the passive flow of liquid water and vapour. As a result, slow water diffusion causes the accumulation of water vapour at interfaces, limiting the evaporation rate.
This problem can be solved by agitating the air around the system. This disrupts the water vapour and speeds up the evaporation process. The ferrofluid does this agitation. The conical arrays rotate in response to a dynamic external magnetic field. This macroscopic motion is also accompanied by the reconfiguration of the magnetic nanoparticles on the microscopic scale to a disordered state while maintaining the conical shape on a macroscopic level. This rearrangement aids salt, heat, and water vapour circulation in the system, enhancing vapour diffusion. The rotating systems show a 23 per cent increase in the evaporation rate compared to the static methods above 100 rpm.
Hierarchy of cones in water purification
Movement is not the only way to increase the performance of magnetic systems. The team also created more complex hierarchal 3D structures that broke the record for the static evaporation rate and exceeded the theoretical limit for evaporation when used dynamically. These proof-of-concept structures were constructed through the synergetic design of the magnetic forces between macroscopic magnets and the magnetic nanoparticles.
The well-distributed conical arrays supported on stalks that extend the space for vapour diffusion. Areas with higher evaporation rates lose energy to the atmosphere faster. At the same time, the surface temperature was used to monitor the evaporation rate. Real-time infrared thermal imaging revealed cooler temperature distributions for the dynamic structures compared to static structures.
While the systems are still in their preliminary research phase, they offer exciting insights into the innovative future possibilities in water management and purification.
The research is described in Nature Communications.
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