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A fog fence or fog collector is an apparatus for collecting liquid water from fog. It is made up of a vertically hung mesh net, a frame, and a trough. Water condenses onto the array of parallel wires and collects at the bottom of the net. This requires no external energy and is facilitated naturally through temperature fluctuation, making it attractive for deployment in less developed areas. The term fog fence comes from its long rectangular shape that resemble a fence, but fog collectors are not confined to only this style of structure. The efficiency of the fog collector is based on material of the net, the size of the holes and filament, and chemical coating. Efficient fog collectors can harvest up to 10% of the moisture in the air, and inefficient collectors yield 2% of moisture. An ideal location is a high altitude arid area near cold offshore currents, where fog is common, and therefore, the fog collector can produce the highest yield.

History
Fog collectors were first seen in nature as a technique for collecting water by some insects and foliage. Namib Desert beetle live off water that condenses on their wings due to a pattern of alternating hydrophilic, water attracting, and hydrophobic, water repelling, regions. Redwood forest are able to survive on limited rainfall do the addition of condensation on needles that drips into its root system. The first man made fog collectors have origins stretching back as far as the Inca Empire, where buckets were placed under these trees to take advantage of this natural phenomenon.

Modern fog collectors were first studied in the 1900's. One of the first recordings of fog collection occurred in 1969 in South Africa as a water source for an air force base. The project consisted of two fences each 100m^2. Between the two 11L of water was produced on average per day over the 14th month study, which is .05L of water for every square meter. The next large study to occur was preformed by the National Catholic University of Chile and the International Development Research Centre in Canada in 1987. One hundred 48m^2 fog fences were assembled in northern Italy. The project was able to yield on average .5L of water for every square meter or 33L for each of the 300 villagers on the town each day.

Principle
Fog contains typically from 0.05 to 0.5 grams of water per cubic meter, with droplets from 1 to 40 micrometers in diameter. It settles slowly and is carried by wind. Therefore, an efficient fog fence must be placed facing the prevailing winds. Furthermore, it must be a fine mesh, as wind would flow around a solid wall and take the fog with it. The water droplets in the fog deposit on the mesh. A second mesh rubbing against the first causes the droplets to coalesce and run to the bottom of the meshes, where the water may be collected and led away.

Parts of Fog Collector
The fog collector is made up of three major parts: the frame, the mesh netting, and the trough or basin.

The frame supports the mesh netting and can be made from a wide array of materials from stainless steel poles to bamboo. The frame can vary in shape. Proposed geometries include linear, similar to a fence and cylindrical. Linear frames are rectangles with the vertical endpoints embedded into the ground. They have rope supports connected at the top and staked into the ground to provide stability.

The mesh netting is where the condensation of water droplets appear. It consists of filaments knitted together with small openings, coated with a chemical to increase condensation. Shade Cloth is used for mesh structure because it can be locally sourced in underdeveloped countries. The filaments are coated to be hydrophilic and hydrophobic, which attracts and repels water to increase the condensation. This can retrieve 2% of moisture in the air. Efficiency increases as the size of the filaments and the holes decrease. The most optimal mesh netting is made from stainless steal filaments the size of three to four human hairs and with holes that are twice as big as the filament. The netting is coated in a chemical that decreases water droplet's contact angle hysteria, which allows for more small droplets to form. This type of netting can capture 10% of the moisture in the air.

Below the mesh netting of a fog fence, there is a small trough for the water to be collected in. The water runs from the trough to some type of storage container or irrigation system for use. If the fog collector is circular the water will be deposited into a basin placed at the bottom of the netting.

Advantages
Water can be collected in any environment, including extremely arid environments such as the Atacama Desert, one of the driest places on earth. The harvested water can be safer to drink than ground water. Fog collection is considered low maintenance because it requires no exterior energy and only an occasional brushing of the nets to keep them clean. Parts can sometimes be sourced locally in underdeveloped countries, which allows for the collector to be fixed if broken and to not sit in disrepair. No in-depth training is necessary for repairing the collector. Fog collectors are low cost to implement compared to other water alternatives.

Disadvantages
Fog fences are limited in quantity by the regional climate and topography and cannot produce more water on demand. Their yields are not consistent year round and are effected by local weather and global weather fluctuations (such as El Niño). Their water supply can still be contaminated by windborne dust, birds, and insects.. The moisture collected can promote growth of mold and other possibly toxic microorganisms on the mesh.