These researchers at VA Tech are creating an improved fog harp.

 These researchers at VA Tech are creating an improved fog harp.

Particularly during protracted droughts, arid coastal areas that are also prone to fog make ideal sites for fog-harvesting systems as a source of water. However, clogging is a common problem with traditional technology. According to a recent study in the Journal of Materials Chemistry A, Virginia Tech researchers have developed an enhanced version of their previous "fog harp" alternative design to solve that problem.
The practice of fog harvesting, also known as dew gathering, dates back to the Incas, who collected condensation by setting buckets beneath trees. Insects that live on the water that condenses on their wings, such as Namib desert beetles, also engage in this behavior. To improve condensation, the wings feature alternating hydrophilic and hydrophobic areas. Mesh nets attached to frames with a trough or basin underneath are the most common type of fog harvesters used today. The mesh filaments are chemically treated to be either hydrophilic or hydrophobic, similar to the beetle's wings.
Reducing the size of the mesh pores and filaments is essential to these water harvesters' effectiveness. Co-author James Kaindu, a student in researcher Jonathan Boreyko's Virginia Tech lab, told Ars that "you wouldn't harvest much water if the holes were too big because the microscopic droplets would pass through it." Smaller filaments and holes are more likely to clog, which is the trade-off. "If it was too small, the droplets would coalesce and create a water film on it," explained Kaindu. "It would impede the flow and act as a barrier that would dramatically affect its capture efficiency.
For this reason, in 2018, Boreyko's team invented the fog harp. The fog harp, as its name suggests, is made up of a frame that holds vertical wires rather than horizontal ones that cross each other. It was modeled after California redwoods, which rely on fog dropping from the parallel rows of needle-like leaves for up to 35 percent of their yearly water intake.

During scale model testing in the lab, the design collected two to seven times as much water as mesh nets, demonstrating excellent performance. However, they later used Monterey Bay to test their plan. Larger versions, roughly one meter by one meter (three by three feet), were needed for that real-world deployment. The vertical threads begin to cluster together at that bigger scale due to surface tension, "like your hair when it gets clumped up with water," Kaindu explained. Additionally, harvesting efficiencies have decreased once again.

Best Of Both World:
The team therefore chose a happy medium, fusing aspects of the fog harp with traditional mesh to produce a hybrid version that is basically a stack of tiny model fog harps on top of one another, with the occasional cross-support wire between them. Boreyko compares the structure to a guitar neck with distinct fret lines. In order to achieve the required water collecting efficiency, this resolved the clumping issue.
According to Kaindu, the team experimented with different iterations of the idea in the lab, starting with the original fog harp on one extreme, which had no interconnects, and ending with standard mesh nets on the other end, which had numerous interconnects. Versions with 100 interconnects, 10 interconnects, 5 interconnects, and 3 interconnects existed in between. Ultrasonic humidifiers shot fog at them, suspending them all in midair.
To determine the overall efficiency—the proportion of all the fog water in the air compared to how much was captured—for each modification, they next measured the mass of collected water after a predetermined amount of time. Depending on other factors, the enhanced hybrid fog harp's fog harvesting efficiency was between two and eight times higher than that of the original fog harp and pure mesh fog harvesters. The most efficient designs turned out to be those with three or five interconnects.
We're trying to use clever geometric designs in place of chemistry," Boreyko told Ars, referring to conventional fog collecting devices. "Almost everyone was utilizing nets when I initially started working in this subject, but they were only attempting to create increasingly complex chemical coatings to apply to the nets in an effort to lessen clogging. We discovered that switching from a net to a harp—without the use of any chemicals or coatings—improved the clogging issue significantly.

Boreyko's group 3D printed their harp "strings" from a weakly hydrophobic plastic for their lab scale prototypes. "But in general, the harp works fantastic with uncoated stainless steel wires and definitely doesn't require any kind of fancy coating," Boreyko stated. Additionally, like traditional nets, the hybrid harp is relatively easy to scale up. To achieve the required size, a number of harps of varying heights must be strung together, meter by meter. "There is no limit to how big this thing could be," he stated.

The next logical step is to scale up the model and test larger prototypes outside. Additionally, Boreyko wants to test an electric hybrid fog harp. "If you apply a voltage, it turns out you can catch even more water," he stated. You can have the best of both worlds with our hybrid as it doesn't clog. In real-world systems, you can apply an electric field to increase harvesting while also avoiding clogging.
Boreyko sees other, less evident potential uses for high-efficiency fog harvesters, such as roads, highways, or airport landing strips that are susceptible to fog that could present safety risks, even though the hybrid fog harp is ideally suited for collecting water in any coastal area that experiences a lot of fog. "There's even industrial chemical supply manufacturers creating things like pressurized nitrogen gas," he stated. By cooling the surrounding air, the technique creates an ice fog that can spread over streets and cause chaos on entire city blocks.

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