Ever since I was a child, I have heard that breakfast is the most important meal of the day. Another adage I’ve heard states, “Eat like a king at breakfast, a prince at lunch and a pauper at dinner,” suggesting that breakfast is not only important, but should be your biggest meal. Then on the other end of the spectrum, I know many people who start their day with a smaller meal. Whichever way you choose to eat, there are plenty of opportunities to observe thermodynamics in action and increase your infrared IQ.
I’ve made mention before of the fact that my tiny little town has a Starbucks. I love their coffee, and I patronize them when I can. Whenever they slide my drink across the counter, they have this handy little sleeve on the cup. You’ve seen them, even if you don’t go to Starbucks, because almost every coffee joint uses them. They are there to make it more comfortable to hold our coffee, but have you ever considered them from a thermal perspective? How do they work?
You hopefully remember Fourier’s Law of Conduction from our Level I Thermographic Applications course. In case it’s slipping your mind, here it is:
Fourier’s Law describes the relationships between the variables that affect how quickly heat energy moves through a solid material. When I’m holding my four-shot Café Breve (a milk-based espresso drink made using steamed half-and-half) in my hand, the action of Conduction is what allows the heat from inside of the cup to move to the outside of the cup, where I can feel it. This little cardboard sleeve slows the rate of conduction from the inside of the cup’s surface through its thickness out to my hand. The magic of the sleeve is that it does it in more than one way.
If you look at the sleeve lengthwise, you’ll notice it has ridges. Those ridges rest on the outside surface of the cup, leaving the spaces between the ridges mere millimeters away from the cup’s surface. This impacts “A” in Fourier’s Law. The sleeve reduces the surface area contact between the hot cup and our cooler hand, reducing the amount of heat transfer.
Between the ridges is air. That air between the two surfaces (the cup and the sleeve) impacts “K” in Fourier’s Law. The trapped air has a lower conductivity value than the material from which the sleeve is constructed. Then, finally, the sleeve increases the thickness (“L” in Fourier’s Law) of the material between the hot inner surface of the cup and our cooler hand. Of the four variables that impact conductive heat transfer, this little cardboard sleeve impacts three, slowing the rate of heat transfer so we can enjoy our hot drink without the need to don an oven mitt.
Next time we’ll talk about one of the other laws of heat transfer at the most important meal of the day. Until then, Think Thermally®!