The Physics of Superheroes: Spectacular Second Edition (49 page)

Shape-memory materials are not always long chains of molecules, nor are they necessarily composed of carbon. Rather than inducing a phase transition with an electric field and temperature as in the case of liquid crystals, shape-memory materials undergo a transformation from one crystal structure to another under stress or pressure. Flexon or Nitinol are the commercial names of nickel-titanium alloys that exhibit high elasticity and memory aspects, making them useful for eyeglass frames and other applications. Nitinol was discovered accidentally in 1961 (the same year the Fantastic Four burst upon the scene) when a research-group leader at the Naval Ordinance Laboratory passed a bent sample of a newly synthesized alloy around for inspection to the scientists at a laboratory management meeting. One of the researchers decided to warm up the twisted metal using his pipe lighter (if there’s one thing I’ve learned from reading comic books, it’s that all scientists smoke pipes, or at least they did back in the Silver Age). All were surprised to see that upon warming, the bent alloy snapped back to its original configuration, and Nitinol (the “nol” in Nitinol derives from the initials of the Naval Ordinance Lab, while the “Ni” and “ti” standing for nickel and titanium) was launched. Bending the wire changed the alloy’s crystal structure, and the application of thermal energy from the pipe lighter enabled the metal to return to the lower-energy (more stable) crystal structure and regain its original shape. Varying the ratio of nickel and titanium in the alloy can alter the temperature at which the phase transition occurs. Nitinol wires are also employed in certain orthodontic applications: The wire is bent to fit inside a patient’s jaw and clamped in place. When warmed by body temperature, the wire exerts a steady pressure as it tries to return to its original shape, thus aligning the teeth.

Flexible shape-memory polymers have been around longer than their metallic counterparts. A familiar example is shrink-wrap, which undergoes a structural change upon warming. Here, certain chemical groups that cross-link and connect the long-chain molecules can soften upon an external trigger, such as an electric field or temperature, allowing the molecules to reorient themselves into a lower-energy configuration. In 2002, Andreas Lendlein and Robert Langer reported in
Science
their discovery of a biodegradable shape-memory thermoplastic polymer for surgical applications
.
When formed into a loose knot, threads of this material self-tighten when warmed to 104 degrees Fahrenheit, enabling endoscopic surgery with smaller incisions. Shape-memory alloys can be used in stents, catheters, and probes, following the narrow passageways in the body as easily as the Elongated Man would be able to follow these twists and turns.

While the functional jumpsuits and uniforms of the Fantastic Four and Avengers remain confined to the four-color pages of comic books, flexible shape-memory materials have recently been incorporated into real-world clothing. Certain fabrics expand in response to a lowering of temperature, so that when used as the inner layer in a winter jacket, they automatically increase the air gap, thereby improving thermal insulation. Other materials become more porous at higher temperatures, allowing body heat and water vapor to escape. Polymer-based fabrics have been developed that can be stretched to more than twice their normal length, and yet return to their original proportions when warmed. These materials may provide the answer to an age-old mystery: What holds up the Hulk’s pants?

The Hulk was a founding member of the Avengers, though he wore purple shorts during his brief tenure with the team. More typically, he battles the U.S. Army, the Abomination, or the Leader while wearing only a pair of purple pants. When nuclear scientist Robert Bruce Banner was belted by gamma rays, he gained the ability to transform from a puny nuclear scientist into an eight-foot-tall, two-thousand-pound Jade Giant. This metamorphosis rends all of Banner’s clothing—his shirt is shredded, his feet tear his shoes and socks apart, and the cuffs of his pants are frayed following his transformation to the Hulk, but his vast waistband remains intact. Presumably, his lavender Levi’s are composed of Reed Richard’s unstable molecules in the comic-book world, or shape-memory fabrics in ours.

The Hulk first appeared in his own Marvel comic in 1962, following the success of the
Fantastic Four
. Unfortunately, the Hulk might be the strongest there is, but his sales weren’t, and, his comic was canceled after only six issues. You can’t keep the Jade Giant down, and he returned a year and a half later in
Tales to Astonish # 60.
One can only imagine what the sales would have been if the Hulk’s pants had not managed to survive his many transformations. But here ol’ Greenskin was stymied by a force even he could not smash, stronger than gamma radiation: the Comics Code Authority!

Unstable molecules may do the job in the Marvel universe, but what keeps the Justice League of America’s uniforms intact in DC comics? A variety of explanations have been provided for the durability of superhero costumes in the DC universe, all with a slight patina of scientific justification. Back in the Silver Age, it was explained that Superman’s costume was composed of the same fabric that the baby Kal-El was swaddled with, when his birth parents sent him off in a rocket ship to Earth moments before Krypton’s destruction. The fabric’s extraterrestrial origin accounted for its indestructibility. In later years, it was posited that a thin aura of invulnerability extended from the Man of Steel’s body, which is why his suit could survive devastating forces or being in the heart of the sun, but his cape would not escape damage. This same protective aura also accounted for the Flash’s costume being able to resist shredding when the Scarlet Speedster ran at his top velocity.

Batman’s costume would display the wear and tear that one might expect would result from fighting crime in Gotham City. Similarly, Spider-Man, being more of a loner in the Marvel universe (at least when he first appeared in the 1960s) was apparently not on the “unstable-molecule distribution list,” and he would regularly also have to deal with a torn and damaged costume (not having Batman’s advantage of a millionaire alter ego and a live-in butler, this was more of a burden on Peter Parker, living with his aged aunt in Queens, NY).

The DC hero the Atom had a unique solution to the problem of needing a costume that accommodated his shrinking power. As will be explained in detail later, the Atom’s ability to change his size and weight from his normal six feet and 180 pounds down to essentially zero feet and zero pounds stemmed from his mastering miniaturization technology that employed a remnant of a white-dwarf star. This exotic material was fashioned into the Atom’s blue and red costume. The miniaturization mechanism that enables the Atom to fight crime at six inches or six nanometers is built into his costume—consequently, his superhero suit is covered under the same miracle exception that accounts for the Atom’s powers. In fact, the Atom never has to worry about changing out of his street clothes because his costume only appears when he is reduced in size. When he resumes his normal height, his costume stays miniature, and his street clothes grow with the rest of him. Consequently, the Atom is the only superhero who deliberately gives himself a wedgie whenever he returns to his secret identity!
⁸⁰

Now that we have addressed the materials science underlying their uniforms, we come to the two main rules of superhero costumes. Rule One: No capes! Rule Two: Accessorize! We next turn to the physics issues concerning some of the more famous crime-fighting accoutrements.

Two years after the Caped Crusader made his debut in
Detective # 27
in 1939, another millionaire playboy took up a costumed identity, also fighting criminals without the benefit of superpowers and armed only with his wits and a variety of technological weapons of his own design. Green Arrow was a fairly transparent attempt to replicate Batman’s earlier success. Millionaire Bruce Wayne would dress like a giant bat and, together with his teen sidekick Robin, drive from the Batcave in the Batmobile to Gotham City, where he would use the high-tech wonders in his utility belt to fight common criminals and costumed villains. Millionaire Oliver Queen, on the other hand, would dress in a modified Robin Hood costume and, together with his teen sidekick Speedy, drive from the Arrow Cave in the Arrow Car to Star City, where he would use a quiver full of “trick arrows” to fight common criminals and costumed villains. Green Arrow’s technological wonders included a boomerang arrow, an exploding arrow, a handcuff arrow, a boxing-glove arrow, an arrow that projected a net near a target, an acetylene-torch arrow, and, particularly useful for underwater adventures, an aqua-lung arrow. I understand why you might need to bring a small breathing tank along with you on an underwater case, but what possible benefit you would derive from mounting it on the head of an arrow, I cannot begin to imagine.

Making his first appearance in the same issue of
More Fun Comics
that featured the debut of Aquaman, Green Arrow was created by writer Mort Weisinger (who also co-created the Aquatic Ace) and drawn by George Papp. Both Green Arrow and Aquaman would continue to be published through the transition period between the Golden Age of comics and the Silver Age, appearing as backup features in issues of Superboy comics. While Aquaman was one of the original set of heroes tapped for membership in the Justice League of America in 1960’s
The Brave and The Bold # 28
, Green Arrow would not be offered membership until a year later, when he managed to save Wonder Woman, Aquaman, Flash, Green Lantern, and the Martian Manhunter from the “Doom of the Star Diamond” in
Justice League of America # 4.
As shown in fig. 41, the JLA were trapped within a large diamond prison, and were freed only when Green Arrow struck the crystal cage with a diamond-tipped arrow, fired with sufficient force at the only stress point of the diamond prison. The conclusion of Green Arrow’s first adventure with the Justice League demonstrates some real materials science, related to crystalline defects, elastic-strain energy of bows, and the centuries-long history of trick arrows.

Let’s first consider the diamond trap holding the Justice League. Diamonds are hard to break not because they are dense, but because the carbon atoms in the crystal are connected via very stiff covalent bonds. The strength of a material is primarily governed by the nature of the chemical bonds holding the atoms together in the solid phase. The strongest chemical bonds are termed “covalent bonds,” where the individual atoms quantum mechanically share their outermost electrons with their atomic neighbors. In order to break these bonds, one must remove the electrons from all of the bonds connecting an atom to all of its neighbors. How the carbon atoms are configured relative to their neighbors will determine the strength of the bonds. It turns out that the overlap of the electronic matter wavefunctions that occurs when the carbon atoms all lie in a plane results in stronger bonds between the carbon atoms than those in a diamond. That is, the carbon planes that peel off on a sheet of paper when writing with a pencil have stronger bonds than those in a diamond crystal. Flaws or imperfections in a diamond occur at regions in the solid where atomic bonds, through strains or the inclusion of impurities, are broken or weakened. It is at these locations where the covalent bonding network is easiest to break, either by a jeweler cutting a diamond or by Green Arrow with his diamond-tipped arrow.

Fig. 41.
Green Arrow, about to conduct an empirical test of stress fracture in covalently bonded solids, in
Justice League of America # 4
(May, 1961)
.

The construction of an archery bow also involves considerable materials science, as a premium is placed on substances that can be bent without breaking. A bow is essentially a spring, and the stored potential energy in the bow is converted into kinetic energy in the arrow. Recall from Chapter 12, and our discussion of the Principle of Conservation of Energy, that the definition of Work in physics is when an external force is applied across a given distance. Pulling back on the bowstring does not change the length of the string—the force supplied by the archer results in the flexing of the bow. The ideal bow will store the most potential energy per mass—that is, one wants a lightweight bow that is nevertheless strong and elastic. In materials physics, an object is termed “elastic” if, following compression or stretching, it returns to its original configuration. If the force results in a permanent alteration, then this is called a “plastic” deformation.

As it relates to Green Arrow, the larger the force necessary to reach the elastic/plastic transition, the more force he can apply without fear of breaking his bow. The more potential energy that can be stored in the bow, the more kinetic energy can be transferred to the arrow. In turn, the faster the arrow leaves the bowstring, the greater the distance it will fly through the air. With a superior bow, one could stand farther from the target and still score a bull’s-eye effectively.