The Physics Teacher Magazine features IR article

The Physics Teacher Magazine published by the American Association of Physics Teachers (AAPT) selected our article "Infrared Imaging for Inquiry-Based Learning" as a featured article on the September 2011 issue. A featured article is made free to the public. Each issue chooses three featured articles.

In this paper, we described a series of IR experiments that can be readily used to teach the basic concepts of heat transfer and their applications to engineering.

Students enjoyed Energy3D in Engineering Energy Efficiency Summer School 2011

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This week 11 students of different ages (10-17) participated in our three-day summer school for the Engineering Energy Efficiency project. They were charged with using Energy3D to design their own model houses on a computer first and then construct them using inexpensive materials.

Although Energy3D is still in its alpha phase, it seemed to work remarkably well for these students who used the Mac computers we provided (thanks to Dr. Saeid Nourian, the lead developer of the software). Despite of some glitches, the students easily designed their own computer models. Creating the roof, the hardest part using other programs such as SketchUp, has been greatly simplified in Energy3D.

Interestingly, of the five groups, none used the template houses we provided to help them get started, indicating the fact that the students actually preferred designing their very own houses from scratch.

Strange thermal conductivity of leaves?

One way to tell if a plant is a plastic fake or not is to touch a leaf. If it feels cool, the plant is a real one. Have you ever wondered why a leaf feels cool? (A leaf of an indoor plant always rests at about the room temperature, plastic or real. It is not really cooler before you touch it. You can confirm this by measuring its temperature using a sensitive temperature sensor.)

We know metals feel cold because they conduct heat fast. Within a given amount of time, our fingers lose more thermal energy to a piece of metal than to a piece of wood.

Do leaves also conduct heat fast? On the contrary.

Let's put a fresh leaf on top of a piece of dry paper. The first set of IR images in this post shows what happened after I used two fingers to touch the leaf (on the left) and the paper to warm them up. The result tells that the leaf actually conducted heat more slowly than the paper, which has much lower thermal conductivity than metals.

Source: Wikipedia.

Now, we have a problem. We know leaves feel cooler than paper. But leaves conduct heat more slowly than paper! Our sense of touch honestly tells us that our fingers lose more thermal energy to leaves than to paper. So where does the thermal energy go on a leaf, if it doesn't diffuse to other parts?

My theory is that the thermal energy goes to heat up the water in the spongy layer of the leaf. The spongy layer lies beneath the palisade layer--the waxy surface layer of the leaf. Its cells are irregular in shape and loosely packed--hence the name "the spongy layer." During transpiration, the spongy layer is full of water in the spaces before they exit stoma. The specific heat of water is considerably high--4.18 J/(g*K) and the spongy layer is filled with water.

My theory is backed by the fact that a dry leaf conducts heat as fast as paper (IR images not shown here). This should not surprise you as paper is made of dehydrated wood fibers. 

Now, the question is why the water in the spongy layer doesn't dissipate thermal energy quickly as water in a cup does (I confirmed the energy dissipation in water by IR imaging, which is not shown here). The thermal conductivity of liquid water is about 0.58 W/(m*K), compared with 0.024 W/(m*K) for air, 0.016 W/(m*K) for water vapor, and 0.05 W/(m*K) for paper. Somehow, the water trapped in the spongy layer cannot conduct heat like free water does.

Let's get get a wet (20% of full water absorption capacity) sponge (left) and a dry one (right) and look at their thermal conductivities under an IR camera. Again, I used my fingers to leave a heat mark on each. The second set of IR images shows a surprising result: the wet sponge appeared to conduct heat more slowly than the dry one!

Does this thermal conductivity protect plants' leaves? Have you wondered why some plants are anti-freezing and some are not? Leaves may have very complicated thermal regulation that we don't quite understand.

Updates on 8/14/2012: 

See these YouTube videos of IR imaging:

Fresh leaf vs. dry leaf: http://www.youtube.com/watch?v=5I2eAU6AZ3Y
Wet sponge vs. dry sponge: http://www.youtube.com/watch?v=2LGfriM3O0Y