"Villain of Sand," Input/Output, ME Departments, September 2007

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Villain of Sand

by Jean Thilmany

When Spider-Man 3 swung into movie theaters this spring, James Kakalios, a University of Minnesota physics professor, was waiting. A diehard fan of superhero comics, he took a particular interest in Spidey's foe, Sandman, who can transform himself into living sand.

It doesn't always happen, but in the case of Sandman the writers manage to get much of the science right, Kakalios said. After 10 years researching the physics of sand and other granular materials—not to mention writing the book The Physics of Superheroes—he was delighted to see sand finally getting the billing it deserves.

One characteristic of sand that comes through in the movie is its changeling—and changing—nature.

"Sand can go from light and fluffy in its loose-packed state to hard and rigid when densely packed," Kakalios said. "Think of getting hit with a sandbag."

That's exactly what Sandman has in mind as he battles Spider-Man. When Spidey tries to punch his shifty nemesis in the stomach, his fist sails right through Sandman's powdery midsection. But in the next instant, Sandman packs the sand grains of his fists into dense, rock-hard clubs, and proceeds to pummel the hapless hero.

Shifting sands: Sometimes sand takes a punch easily and other times it packs a wallop in Spider-Man's world as in real life, says a researcher who has spent quite a bit of time studying the subject.

Sandman may not know it, but as he changes the density of his body sand, he is actually altering the stacking pattern of individual grains. Sand is full of spaces between the grains that resist being squashed, Kakalios said. In fact, the spaces expand when pressure is applied from the top.

A person walking on wet beach sand leaves footprints that are temporarily dry. With each step, the pressure causes new spaces and pores to open up through the matrix of grains. Water drains in to fill the expanded voids, leaving the top surface of the prints drier than the surrounding sand.

At first glance, Kakalios admits, his work with the physics of sand and similar materials may seem a little odd. But studies of granular materials are of intense interest to industries such as pharmaceuticals, agriculture, and construction, which together spend about $80 billion a year in powder processing. Take the pills in your medicine cabinet, for example. They're made up of small, individual grains that have been bound.

Nearly 80 percent of everything made or grown in the United States exists at one point as a granular material.

"The segregation of granular materials is a major concern for the pharmaceutical industry where granular systems need to be well mixed and homogenous over length scales of a pill diameter or less," Kakalios said.

In fact, he said, nearly 80 percent of everything manufactured or grown in the United States exists at some point as a granular material, and three percent of all electrical energy in the U.S. goes into grinding metal ores into powders.

The physics of grains also applies to breakfast cereals, boxes of which usually arrive in stores almost half-empty and bearing the apologetic disclaimer: Contents may have settled during shipping. Isaac Newton studied how spherical objects could best be packed in containers, Kakalios said.

The physics behind sand still fascinates Kakalios, although he has switched his research focus to semiconductor materials and neurological systems.

As Hollywood continues to produce superhero movies, viewers should take home a lesson or two when the science is correct, and take the rest with a grain of salt, he said.