Water Vapor Absorption and The Surface Law

Many of nature's crucial processes occur at surfaces. Nutrients, water vapor and other material are exchanged by cells at the surface of each cell. Liquid water evaporates into the air (as water vapor) from the surface of a body of water. Sunlight is absorbed at the surface of the leaf. 


Water vapor absorption and adsorption also requires surfaces. Thus, ignoring efficiency considerations for the moment, a larger amount of water vapor will be absorbed if a larger a surface area is available that is capable of absorbing water vapor.


The Surface Law

A cube with sides of length 's'
Source: tutornext.com

The ability to increase surface area is controlled by the Surface Law that states: As volume of a physical object increases, the object's relative surface area decreases. For example, consider the cube on the right:

• A cube 1 unit in size (s=1), has volume equal to1 and surface area equal to 6. Its ratio of surface area/volume is 6
• A cube 2 units in size (s=2), has volume equal to 8 and a surface area equal to 24. Its ratio of surface area/volume is 3
• A cube 4 units in size (s=4), has a volume equal to 64 and surface area equal to 96. Its ratio of surface area/volume is 1.5 

In the above example, the relative surface area reduces as the volume increases. From a biological perspective, this implies that it is relatively more difficult to supply nutrients to the cells at the center of each cube as the volume of cube increases. As expected, Nature has come up with interesting structures that increase surface area without increasing mass


Tracheal System

source: users.rcn.com. 

This is a grouping of tubes, air sacs and various geometries (such as loops or helices) that continuously bring air to tissues so that the tissues can directly exchange nutrients (oxygen, water vapor, carbon dioxide, etc) from the air.


Pores
Nature invented pores to increase surface area. 

source: the poultrysite.com

The typical chicken egg is perforated by about 10,000 pores (approx 1.5 pores per square millimeter of shell surface) that permit passage of respiratory gases and water vapor through the hard shell.


The pore size (17 micro-mm) has been optimized to provide the large amount of oxygen required just prior to hatching.


The Human Respiratory System

Source: Goldiesroom.org

As body size increases, the complexity of providing enough surface area to all the tissues and cells in the body increases.

• Oxygen is extracted from the air we breathe at the surfaces of miniature air sacs (alveoli) in the lungs
• This oxygen diffuses from the blood into a cell at the surface of a capillary

The human system to move oxygen into the body and remove carbon dioxide from the human body increases in complexity by creating new surface areas with specialized functionality that feeds nutrients to cells and extracts waste for disposal.


A Gram with Surface Area of 2 Basketball Courts

Source: physorg.com

MOF-74 is a porous crystalline powder developed at the University of California in Los Angeles that resembles a series of tightly packed straws comprised of mostly carbon atoms (white balls) with columns of zinc ions (blue balls) running down the walls.


The green balls are hydrogen molecules that this design is optimized for.


A gram of MOF-74 has the surface area of 2 basketball courts


Many many more examples exist of structures created by nature and mankind for supplying cells with nutrients and removing waste. So what would a structure look like that presents enough surface area to withdraw water vapor from unsaturated air?

Nature's Creations Absorb Water Vapor From Air

98% of the water in the atmosphere exists in vapor form. While most living creatures must forage for liquid water on a regular basis, Nature has equipped some creatures with specialized structures (unique body creations) that enable them to absorb water vapor directly out of unsaturated air. 


Ixodes Ricinus - Ticks

Source:database.portal.modwest.com

Ticks usually get their allowance of needed water from the blood of the creatures they feed on. However, under drought-like conditions, the Ixodes Ricinus tick absorb water vapor from the atmosphere. Ixodes Ricinus tick is unique in that they can satisfy their water need by drinking liquid water and absorbing water vapor

The Desert Cockroach
Source: bugguide.net 

The Desert Cockroach, Arenivaga Investigata

This cockroach absorbs water vapor from the unsaturated atmosphere. It has two bladder-like extensions in its mouth-parts that are assumed to accomplish vapor absorption and vapor condensation to liquid form - a form that the cockroach needs for survival.

Firebrat, Thermobia Domestica

Firebrat
Source: www.forestryimages.org

The greatest known concentration of mitochondria is found in the cells of what is believed to be the functionally mature water vapour-absorbing epithelium of the posterior rectal (anal) sacs of the firebrat, Thermobia domestica. Water Vapor Absorption from sub-saturated air down to as low as 43% relative humidity (RH) is essential for the growth, development and survival of the Firebrat in hot, dry environments where liquid drinking water is lacking. (source: Journal of Insect Physiology)


The Mealworm Tenebrio molitor 

mealworm Tenebrio molitor
Source: http://www.ozanimals.com/Insect 

Water gain in several xeric insects, like the Mealworm, is accomplished through absorption of water vapor from the atmosphere. This absorption is predominant at the larvae stage of beetle development and is less predominant in the mature beetle.




Soil-Dwelling Anthropods

SnowBug
Source: http://soils.usda.gov 

Arthropods balance their water budget by actively absorbing water from highly unsaturated atmospheres. This animal maintains its body fluids hyperosmotic to its surroundings so that net water uptake occurs by passive diffusion along the gradient of water potential. The animal's basically manges glucose and myoinositol to effect large increases in osmotic pressure. This allows it to stay active in the same ranges of drought intensity like plants are capable of surviving. 



The above creatures are but a handful of nature's creations that live off water vapor in the atmosphere using  a specialized "condensation sac" and/or managing naturally occurring sugars (glucose) and materials synthesized from sugar e.g. myoinositol.


Do the above creations of nature hold the keys that we can use to guide our development of artificial machines that extract water vapor from the atmosphere? 

Harvesting Liquid Water in Fog - Part II

Mankind has made significant strides towards harvesting from the 2% of water in the air that exists in liquid form. This progress comes from artificially replicating nature's inventions like the back of the African Stenocara beetle. Other moisture capturing systems are found in nature's designs for the Texas Thorny Devil Lizard, the structure of Cactus thorns and the structure of the leaves of plants like the Welwitschia Mirabilis.

Bio-mimicry & Species Adaptation

Stenocara Beetle
(Image: www.rain-barrel.net)

The Stenocara beetle lives in the Namib Desert in Africa which has endured arid and semi-arid condition for over 55 million years, has less than 5mm of rain annually and is practically barren of vegetation. 

The only other sources of water are underground rivers and morning fog that moves in over the desert near the coast from the ocean. This fog travels at gale speeds a few times a month.

Microscopic Detail
(photo: Oxford University)

An inhabitant of the Namib Desert, the Stenocara Beetle has evolved to harvest the liquid water in the fog through a combination of hydrophilic bumps/ridges and hydrophobic troughs/valleys on its back. 

To survive in the desert, Stenocara evolution has adapted it to catch the liquid water in the fog through a back of unique design . As described in http://www.biomimicryeuropa.org, "The peak of each bump is smooth and attracts water. Tiny rounded nodules, no wider than a human hair, cover the slopes of each bump and the troughs in between. The nodules are coated in a wax-like material, making them hydrophobic. When the fog rolls in, the beetle tilts its body into the wind. The water droplets from the fog are repelled from the nodules but stick to the peaks of the bumps. The droplets grow until they are large enough to roll down from the top of the peaks and are channeled to a spot on the beetle's back that leads straight to its mouth."

Design patent No. PCT/GB02/00067

Zoologists, led by Andrew Parker at Oxford University, discovered this hydrophilic-hydrophobic design and have obtained a design patent on it. Now the pace has quickened to find a commercial way to gain from nature's design of the Stenocara beetle's back.

The MIT Project

Chemical engineer Robert E. Cohen and materials scientist Michael F. Rubner have perfected a film with hydrophilic peaks and hydrophobic runways that extract liquid water from air and possibly go a step further in coalescing water vapor into liquid water. The next frontier is commercialization of this film for common use.

The University of Sydney Project

Australia, a country with few inland water resources, has nearly 90% of its population living in coastal cities. At the University of Sydney, a small team has a large goal - develop a film that can be laid on a large surface (like the side of a building or the roof of a house) that is similar in design to nature's design for the Stenocara's back with the very same water-harvesting functionality that nature has adapted for the survival of Stenocara and the propagation of its species

All this effort for 2% of the water contained in the air! 

Harvesting Liquid Water in Fog - Part I

The water in the air, at any one time, is more than the water in all the rivers combined. Of this water in the air, 98 percent is in vapor form and just 2 percent is condensed into liquid droplets. These liquid droplets eventually form a cloud and through rain get deposited on the ground. Fog is a cloud which has its cloud base on the ground or just a few feet above ground.


Nature has always been a fog-catcher ... and people took advantage of it

California Redwood Trees (source: travels.com)

The California Redwood tree is highly adapted to harvest water from fog and satisfies the vast majority of its water needs this way. The trees simply stand like a barrier in the way of the fog that intercepts and precipitates the water droplets in the fog.


Water droplets from the fog collect on leaves and branches. grow larger and eventually drip down to the ground and nourish and plants and even create streams that animals and people can use.


On El Hierro, the smallest island of the Canary islands, people collected their only freshwater for thousands of years (till about a 100 years ago) from the leaves of their local trees - The roman author, Pliny The Elder, mentioned the Holy Fountain Tree growing on El Hierro.


In Oman, to capture water from the fog, there is a long tradition of farmers and breeders putting cisterns under agave, olive, laurel and juniper trees to collect  water dripping off their leaves and branches.

Fog Nets in Bellavista (Source: National Geographic.com)

In the hillside village called Bellavista, outside Lima city, the locals have put up 26-foot long plastic netting that capture fine droplets of water in the air, that eventually clump into large enough drops that fall to the ground and through pipes are collected in two underground tanks. The water that is not collected nourishes 700 trees that have been planted to reforest the landscape that logging wiped out years ago.


The residents of Bellavista are hopeful they will be as successful as Charles Darwin was when he planted seedlings, brought from botanic gardens in London, to make Ascension Island habitable, in the 1800s, for British troops stationed there.