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Design of Network Structures for Fog Harvesting and Other Applications
Fog represents a largely unexploited source of potable drinking water. Fog harvesting nets have been used with various degrees of success in more than fifteen countries around the world. In the Atacama Desert in Northern Chile where MIT and Pontifical University of Chile Santiago have an active fog harvesting collaboration, there is a persistent coastal fog, the Camanchaca that represents an unusual target of opportunity. Estimates suggest that capture of just five percent of the Camanchaca could satisfy the water needs of Chile’s four northern provinces where annual rainfall is exceedingly small.
We have examined the influence of surface properties, weave density and length scale on the fog harvesting performance of woven mesh structures. The majority of our observations have been obtained in laboratory experiments that will be discussed in detail. Some data from first-generation harvesters deployed in the Atacama are beginning to be retrieved reliably. We have integrated an existing theoretical analysis of the hydrodynamics near a woven mesh with the surface characteristics of the mesh materials to produce a model that addresses (i) the issue of mesh clogging by coalesced drops and (ii) re-entrainment of collected droplets into the prevailing wind. We cast our findings into a design chart that displays the collection efficiency of a selected mesh in a specified fog environment. Appropriate tuning of the surface properties, reducing the mesh fiber radius, and optimizing the fiber spacing can all be used to improve the efficiency of water capture from fog. In our mild, but carefully controlled, laboratory fog conditions (fog droplet size 3µm ; wind velocity 2m/s ; liquid water content 0.1g/m^3 , RH = 100%) our best surface-modified stainless steel mesh produces liquid water at a rate of about 2 L/m^2 day. The performance of the Raschel polypropylene nets typically used for fog harvesting is inferior by about a factor of five in the same laboratory conditions. We estimate that the higher velocity, larger drop size and larger liquid water content of the Camanchaca will enable our best mesh materials to collect water at rates of about 12 L/m^2 day.
Similar design charts that combine surface properties with feature morphology and spacing have enabled us to produce omniphobic fabrics and to develop some tentative structure-function relationships for the case of feathers of deep diving aquatic birds. Analogous to a phase diagram, boundaries in the design space allow one to separate the flaw-governed metastable non-wetting behavior of woven fabrics from the thermodynamically stable non-wetting characteristics of Cormorant wing feathers.