Optimizing Fog Harps.

It has recently been demonstrated that harps harvest substantively more fog water than conventional mesh nets, but the optimal design for fog harps remains unknown. Here, we systematically vary key parameters of a scale-model fog harp, the wire material, wire pitch, and wire length, to find the optimal combination. We found stainless steel to not only be the best hydrophilic wire material but also nearly be as effective as Teflon-coated wires. The best choice for the wire pitch was coupled to the wire length, as the smallest pitch collected the most water for short harps but was hampered by tangling for taller harps. Accordingly, we use an elastocapillary wire tangling model to successfully predict the onset of tangling beyond a critical length for any given wire pitch. Combining what we learned, we achieved a water harvesting efficiency of 17% with an optimized stainless-steel harp, over three times higher than that of the current standard of a Raschel mesh. These results suggest that an optimal fog harp should feature high-tension, uncoated wires within a large aspect ratio frame to avoid tangling and promote efficient and reliable fog harvesting.

Harps under Heavy Fog Conditions: Superior to Meshes but Prone to Tangling.

In arid yet foggy regions, fog harvesting is emerging as a promising approach to combat water scarcity. The mesh netting used by current fog harvesters suffers from inefficient drainage, which severely constrains the water collection efficiency. Recently, it was demonstrated that fog harps can significantly enhance water harvesting as the vertical wire array does not obstruct the drainage pathway. However, fabrication limitations resulted in a very low shade coefficient of 18% for the initial fog harp prototype and the field testing was geographically confined to light fog conditions. Here, we use wire-electrical discharge machining (wire-EDM) to machine ultrafine comb arrays; winding the harp wire along a comb-embedded reinforced frame enabled a shade coefficient of 50%. To field test under heavy fog conditions, we placed the harvesters on a closed-circuit test road and inundated them with fog produced by an array of overlying fog towers. On average, the fog harps collected about three times more water than the mesh netting. During fog harvesting, the harp wires were observed to tangle together due to the surface tension of water. We developed a rational model to predict the extent of the tangling problem for any given fog harp design. By designing next-generation fog harps to be anti-tangling, we expect that even larger performance multipliers will be possible compared to the current mesh harvesters.

Biodiversifying bioinspiration.

Bioinspiration-using insights into the function of biological systems for the development of new engineering concepts-is already a successful and rapidly growing field. However, only a small portion of the world's biodiversity has thus far been considered as a potential source for engineering inspiration. This means that vast numbers of biological systems of potentially high value to engineering have likely gone unnoticed. Even more important, insights into form and function that reside in the evolutionary relationships across the tree of life have not yet received attention by engineers. These insights could soon become accessible through recent developments in disparate areas of research; in particular, advancements in digitization of museum specimens, methods to describe and analyze complex biological shapes, quantitative prediction of biological function from form, and analysis of large digital data sets. Taken together, these emerging capabilities should make it possible to mine the world's known biodiversity as a natural resource for knowledge relevant to engineering. This transformation of bioinspiration would be very timely in the development of engineering, because it could yield exactly the kind of insights that are needed to make technology more autonomous, adaptive, and capable of operation in complex environments.

Fog Harvesting with Harps.

Fog harvesting is a useful technique for obtaining fresh water in arid climates. The wire meshes currently utilized for fog harvesting suffer from dual constraints: coarse meshes cannot efficiently capture microscopic fog droplets, whereas fine meshes suffer from clogging issues. Here, we design and fabricate fog harvesters comprising an array of vertical wires, which we call "fog harps". Under controlled laboratory conditions, the fog-harvesting rates for fog harps with three different wire diameters were compared to conventional meshes of equivalent dimensions. As expected for the mesh structures, the mid-sized wires exhibited the largest fog collection rate, with a drop-off in performance for the fine or coarse meshes. In contrast, the fog-harvesting rate continually increased with decreasing wire diameter for the fog harps due to efficient droplet shedding that prevented clogging. This resulted in a 3-fold enhancement in the fog-harvesting rate for the harp design compared to an equivalent mesh.