### It's the Humidity

August 22, 2016

Everyone has heard the saying, "It's not the heat, it's the humidity." Most mornings at my home in Northern New Jersey, I'm reminded that there's a lot of water in the air. On cool summer mornings, my automobile is covered with a thick layer of water, condensed as dew from the atmosphere. On some winter mornings, the dew appears as a thick layer of ice. In an earlier article (Fog Water Harvesting, December 2, 2010), I did a "spherical cow calculation" of how effective my automobile is at collecting dew.

The idea of a spherical cow makes fun of the tendency of physicists to simplify their problems.

(Portion of an illustration by Ingrid Kallick, via Wikimedia Commons.)

From observation, I estimated that a heavy dew covered my car with about a liter of water. Approximating the geometry of the car as two rectangular parallelepipeds, one atop the other, having about 220 square feet exposed to the air (about 20 square meters) when the undercarriage is ignored. This results in a collection efficiency of about 50 milliliters per square meter.

The world is trending towards a water crisis because of pollution of potable water, and the competing human and industrial needs for water. Humans need about forty liters of water per day. Trees are natural fog water harvesters, as demonstrated by the trees planted by Charles Darwin on Ascension Island, a dry volcanic island. After two decades, the leaves of these trees were harvesting enough water to grow crops to feed hundreds of British troops.

Trees with needle-like leaves are the best harvesters, especially the ones that have leaves oriented vertically, rather than horizontally. The California Redwood (Sequoia sempervirens) is so adept at harvesting water from the air that wet microclimates are created at its base.[1]

Fog water harvesting has been demonstrated using large plastic nets, 32 square meters in area, in Lima, Peru.[1] During the cooler months of May through November, such nets have captured as much as 590 liters in a single day. A more elaborate, double-net system produced an average of 300 liters per day throughout the year, with 2650 liters being produced in a single day.[1] While automobile metal and glass act as fog water harvesters, such plastic netting is more effective, since air can flow though it.

Scientists at Technion-Israel Institute of Technology (Haifa, Israel) have gone beyond such passive fog water harvesting methods by developing an active type of atmospheric moisture harvester. While such active systems require energy, the Technion system separates the water vapor from the air before cooling and condensation as an energy-saving method.[2-3]

The simplest active fog harvesting system would use electrical refrigeration to condense water vapor through cooling, an energy-intensive process. The Technion system uses a liquid desiccant that separates the water vapor from air, so that just the water vapor needs to be cooled. Using computer modeling, they found a 5-65% percent energy savings using their technique, depending on climate conditions and the use of supplemental solar energy.[2]

In this image, LDS is "liquid-desiccant vapor separation," and AMH is "atmospheric moisture harvesting."

(American Chemical Society image.)

In a recent article (Humidity Sensing, June 13, 2016), I reviewed humidity-sensing technologies, some of which are based on changes in the the mechanical properties of some materials upon exposure to humidity. For example, fibers such as horsehair and human hair will change length or twist when braided as they absorb water from the air.

Another example of a mechanical humidity sensor has an action similar to that of a bimetallic thermometer. In such thermometers, the differential thermal expansion of two bonded metal strips causes the bimetallic strip to bend. A salt-impregnated paper glued to a coil of metal or another elastic material that doesn't absorb water will similarly bend with changes in humidity.

Portion of fig. 1 of US Patent No. 3,284,002, "Control Apparatus," by Walter E. Edelman and David J. Sutton, October 26, 1964.

This is the iconic Honeywell T87 Round Thermostat containing a bimetallic element (11) that moves a mercury switch (13).

(Via Google Patents.)

Bimetallic strips can do a little work in response to a temperature change, and the same is true for bimaterial humidity sensing strips. low power wireless sensors that harvest energy from a variety of environmental sources, such as light, atmospheric pressure change, and vibration, can harvest environmental energy, also, from humidity change. In most environments, the diurnal variation in humidity is large.

Scientists and engineers at the RIKEN Center for Emergent Matter Science (CEMS, Hirosawa and Sayo-cho, Japan), and the University of Tokyo (Tokyo, Japan) have used the water vapor absorptive property of an anisotropic polymer to form an actuator that responds to even small changes in humidity.[4-6] The polymer, produced as a thin film of carbon nitride by physical vapor deposition from guanidinium carbonate, is lightweight, but durable.[4] The film straightens and curls upon exposure to very small changes in ambient humidity; and, exposure to ultraviolet light affects its water absorption ability.[5]

As happens often in science, the humidity sensitivity of the material was unexpected, and it was discovered by accident. Says Daigo Miyajima of CEMS,
"Our study began from a serendipitous finding. When we placed a compound called guanidinium carbonate into a high-temperature oven, we found that it formed not only a powdery substance - as is usual in similar processes—but also a yellowish film that stuck to the surface of the substrate. The film was a carbon nitrite polymer composed of stacked polymers of heptazine oriented parallel to the surface of the substrate."[5]

When the research team started to analyze this unusual film, first removing it from its substrate using warm water, they noticed its tendency to bend and straighten, seemingly at random intervals.[5] They found that this behavior arose from extremely small changes in the ambient humidity.[5] Moving a drop of water into the vicinity of the film would cause it to straighten. A weight difference was detected between the straight and curled states, the weight of the curled state being just 68 nanograms per square millimeter greater than the curled state.[5] This film was seen to respond to quantities of water as small as a few hundred nanograms per 10 square millimeters.[4]

The bending of a film of carbon nitride polymer in response to humidity. (Left image and right image from RIKEN.)

It's proposed that water molecules create mechanical stress as they bond to the polymer, and this changes the film shape. The change caused by exposure to ultraviolet light is extremely rapid, of the order of 50 milliseconds, and this action could be repeated more than 10,000 times without deterioration of the film.[4-5]

Considerable power was evident in the curling process. A film on a flat surface could jump to a one centimeter height upon ultraviolet light induced curling, a distance ten thousand times the film thickness.[4-5] Such films have potential for environmental energy harvesting.[5-6]

### References:

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