Book Excerpt: The Physics of Superheroes

James Kakalios is a comic book aficionado. As a professor in the School of Physics and Astronomy at the University of Minnesota, he's been teaching the very popular course "Everything I Needed to Know About Physics I Learned From Reading Comic Books" since 1988. Today we're excited to publish this excerpt from the new second edition of his book, The Physics of Superheroes. Enjoy!

Fresh Air Underwater?

The most striking ability of Aquaman, as well as that of Marvel Comics Prince Namor, the Sub-Mariner, and all the other denizens of comic books' many distinct underwater cities of Atlantis, is the ability to extract oxygen directly underwater. Without this superpower, there doesn't seem to be much point to being a water-based superhero. It turns out that this is the one special power that requires the smallest miracle exception from the laws of nature. Why shouldn't Aquaman breath through water—after all, we do!

Everyone knows that drowning results when the lungs fill up with water. What is less commonly recognized is that normal breathing would be impossible without a small amount of water in the lungs. Fresh air comes in through the nose, and travels down the bronchial tube, where it is warmed to the body's temperature and pre moistened. In fact, the air has to be at 100 percent relative humidity as it moves down the ever more finely branching tubes on its way to the alveoli—small little spherical buds where the exchange of oxygen and carbon dioxide occurs. These pockets are roughly 0.1 to 0.3 mm in diameter, smaller than the period at the end of this sentence. On the other side of the walls of the alveolar bud are the capillaries—very narrow blood vessels in which plasma and red blood cells flow to drop off carbon dioxide molecules and pick up oxygen molecules on their way to the heart. The capillaries are narrow for the same reason that the alveolar spheres are so small—to maximize the ratio of surface area to volume. Since the gas exchange takes place only through the walls of the alveoli and the capillaries, the more surface area there is, the more regions there are for possible gas diffusion to occur.

But even this very thin water layer in the alveoli should be physically capable of causing asphyxiation. The same physics responsible for glistening dewdrops should produce acute shortness of breath, or worse. The magnitude of surface tension in the water layer is sufficient to cause the small alveolar buds to close up entirely, so that even deep breaths would not be enough to provide the necessary pressure to drive the oxygen molecules into the bloodstream. What saves us from choking on an amount of water that could not fully fill a thimble? Soap!

Surface tension is the name given to the pulling force that results from the attraction of molecules in the fluid (let's say water) to each other. Such an attractive force must of course exist—or else the atoms or molecules in the liquid would fly away from each other as they return to the vapor state. For most liquids, this force is a relatively weak electrostatic cling (called the van der Waals attraction) that arises from fluctuating charge distributions in the molecule. The force can't be too strong, for the water molecules must be able to move past each other and flow through hoses or fill up the volume of a container in exactly the manner that a solid doesn't. We'll discuss van der Waals later on, when we consider the physics that enables gecko lizards and Spider-Man to climb up walls and across ceilings.

This tendency of water to curve is less charming when it forces the walls of our alveoli to constrict, requiring extreme pressures to keep the air buds open. When faced with the problem of decreasing the surface tension in alveolian water in the development of our physiology, natural selection chose the same solution we employ when washing our clothing. The cells in the alveolar walls generate a substance known as "pulmonary surfactant." The first term just refers to the lungs, while a "surfactant" is a long, skinny molecule with different chemical groups at either end. Electrostatic interactions result in one end of this molecule being attracted to the charge distributions in water molecules, while the other end is repelled by those same charges. If the long skinny molecule is fairly rigid, like a spine, then a large collection of such molecules will orient themselves so that all of the regions that are repelled by water are pointing in one direction (typically where there is a low concentration of water), while those ends that are attracted to water will extend into the fluid. The region where the surfactant molecules can satisfy both ends at the same time is at the water-air interface, with the water-attracting end inserted into the water and the water-avoiding end protruding out into the air. In such a configuration, the surfactant interferes with the water-water bonding at the surface of the water layer. This reduces the cohesive force between water molecules that was the source of the surface tension. Without pulmonary surfactants, the alveoli—essentially air bubbles in water—are unable to effectively facilitate gas exchange with the bloodstream. These crucial surfactants do not develop in the fetus until late in its gestation, which is why premature babies may suffer from respiratory distress syndrome, an often-fatal condition prior to the development of effective artificial surfactants.

A moment ago I referred to the reason why surface tension arising from even a thin layer of water in the lungs does not kill us as "soap." While not technically correct, in that pulmonary surfactants are not soaps, the converse is true, in that soaps are surfactants, with water-attracting and water-repelling chemical groups at either end of long skinny, chain-like molecules. Soap helps one clean up by reducing the surface tension of water, so that it can make direct contact with the dirt. That is, surfactants make water wetter, and help us breathe easy as well.