Energy efficiency in living things often means combining the recycling of material and fuel with the hitching of a free ride. Soaring birds, drifting larvae and even prairie dogs make use of dependable physical forces in order to survive with the least amount of expended energy.
Many of these free rides capitalize on gradients, or differences across distance. Gradients are ubiquitous in living systems, from sodium and potassium gradients in cells to pattern-forming factors in ecosystems. Whatever the scale at which this phenomenon is exhibited, its foundation starts at the very small.
There are several important principles at work here:
1. Nature needs an inside and an outside, a “this” or a “that.” In others words, difference makes things happen, whether it's in information code (much like our 1's and 0's in computer code), or in cell structure, or in energy production.
2. Making things move (change velocity) is the definition of work. Biological life does a lot of its work by establishing these differences, in density or, say, pressure, and then letting physical forces take over. For example, there really is no need to stir the cream in your coffee other than impatience. Once you have poured one into the other, the Second Law of Thermodynamics will dictate that the cream will disperse evenly into the coffee. Diffusion of the molecules will lead to equilibrium.
3. Life can't abide equilibrium, however, because things would come to a rest. No rotors would turn, nor fluids move. Cells would die. Therefore, organisms are constantly working shy of equilibrium and maintaining these gradients.
4. When we say that nature likes to surf for free, we often more precisely are speaking of this phenomenon: Being in a position to take advantage of constant laws, whether through natural selection or individual exertion. Put another way, thermodynamically favored pathways are constantly being used by energy-transforming organisms. It happens at many scales, including the cellular one.
This kind of passive transport can be scaled up to be most impressive. Take a look at any large tree. It raises water far above the 30 feet or so that atmospheric pressure allows mere suction to achieve, sometimes to over 400 feet, all without a dedicated energy expenditure. Instead, it uses a structure evolved over millions of years to take advantage of a gradient.
The molecular properties of water are at the heart of this transport mechanism. Water is a so-called polar molecule, having a negatively charged oxygen end and a positively charged hydrogen end. This polarity accounts for all sorts of characteristics we associate with water, even its liquidness. The questions “Why does water dissolve salt? Why does water bead up in drops? Why does water stick to the edges of your glass?” can all be answered with one word: polarity.
The relatively weak ionic bonds that stick the oppositely charged hydrogen atoms in one molecule of water to the oxygen atoms in another are what cause water to attach to itself (cohesion) and other materials (adhesion). Though the individual bonds are weak, there are a lot of them, and together they are a powerful force called electrostatic attraction. It is the cohesive property that allows water to stay in a column of over 400 feet, without succumbing to gravity. The tension in the water column itself is enough to resist the downward pull of its own weight. Adhesion allows the water to move up the narrow tubes of xylem to replace what is used in the leaves above.
Transpiration, or the loss of water vapor through the pore openings or stomata of the tree's leaves, creates the gradient that starts this capillary action. Water is pulled out of the xylem into the mesophyll cells and their interstices and then through the stomata by the evaporation effects of the sun. The pulling effect extends all the way down the water column in the tree to its rootlets.
Michael Maharbiz of the Electrical Engineering and Computer Sciences (EECS) department at U.C. Berkeley, with Ruba Borno at the University of Michigan and Joe Steinmeyer at MIT, is working on a way to mimick this transpiration gradient to produce small amounts of electricity. They have constructed artificial leaves from glass wafers which contain networks of channels patterned after leaf veins. They have been able to set up a passive water pump similar to a tree's using the draw of evaporation at the edges of their constructed “leaves.”
The passive pump system is then used to harvest electricity from the flowing water. How do they do it? Again, it comes back to the key principle of difference. By entraining air bubbles into the water flow they are able to introduce a material with a different electrical property from water and this is enough to create a tiny charge. To capture the charge they sandwich the water column between two metal plates and these two plates are connected to an external circuit.
When you bracket an insulating material or, more precisely, a dielectric (water), with two conducting materials (metal plates) you make a capacitor. Remember, water itself does a poor job of conducting electricity, it's the “impurities” within water that cause the electrician to avoid standing in it. The changed capacitance of the device as the bubbles float by drive a rectified charge-pump circuit which uses each transition to increase the energy in a storage capacitor.
Granted, the amount of energy produced this way is small, only 2 to 5 microvolts each time a bubble passes, with a energy density of two microwatts per cubic centimeter, but much of the most impressive work of nature is done this way by aggregating tiny efforts within an optimized system.
The scaling of such a device to tree size would be hampered by the use of air bubbles. In our tree example, it is the unbroken chain of water molecule cohesion that allows the 400 foot column to stay intact. When air is introduced it breaks this chain and the water is subject to the now dominant force of gravity: down goes the water. Maharbiz believes that substituting solid particles that spin in place for the air bubbles will solve this problem and that the device is imminently scalable.
This apparatus has been described as an energy scavenging device to be used as a complementary technology; piggybacked unto more robust energy producers like solar panels. It seems to me that its potential is great, however, because it has hitched a ride on a basic principle and is one of many biomimetic techniques that will allow us to surf for free.
Tom McKeag teaches bio-inspired design at the California College of the Arts and University of California, Berkeley. He is the founder and president of BioDreamMachine, a nonprofit educational institute that brings bio-inspired design and science education to K12 schools.
Bird - CC license by law_keven
[Editor's note: This blog post has been revised.]