When the movie Spider-Man 3 swung into theaters a few weeks ago, it found University physics professor James Kakalios waiting. A diehard fan of superhero comics, he takes a double interest in Spidey foe Sandman, who can transform all or part of his body 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 says. After 10 years researching the physics of sand and other granular materials--not to mention writing the bestselling book "The Physics of Superheroes"--he is delighted to see sand finally getting the billing it deserves.

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

"Sand can go from light and fluffy in its loose-packed state to hard and rigid when densely packed," Kakalios explains. "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.

The powders that be

At first glance, working on the physics of sand and similar materials may seem a little odd. Kakalios, who once wrote a paper titled "Granular physics or nonlinear dynamics in a sandbox," would agree. But such studies are of intense interest to U.S. 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.

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

In fact, he says, 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 United States 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." Even Isaac Newton was aware of the settling problem and studied how spherical objects could best be packed in containers. Perhaps he was influenced, Kakalios speculates, by a desire to wring more profit from the apple orchard he owned.

The times of sand

When we walk on a sandy ocean beach, we're treading on tiny grains of rock that were born on mountaintops lifted up by movements of the Earth's crust. As plants and the elements erode rock into smaller pieces, they find their way into valleys, where water washes the finer particles toward the sea. On average, says Kakalios, a grain of sand takes about 10,000 years to complete each mile of its journey. And there are plenty of journeys.

"There is a lot of sand on the planet-approximately 10 million cubic miles, all told," he says. "That's enough to cover the United States three miles deep."

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 in rather startling ways. Sand, says Kakalios, is full of spaces between the grains that resist being squashed. In fact, the spaces expand when pressure is applied from the top. This explains why 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.

The presence of spaces also explains why sand, and not water, is used in hourglasses. Normal fluids respond to the pressure of their own weight by initially shooting through a hole in the bottom of their container rapidly, but their speed slackens as the chamber empties and the pressure head decreases. Sand's resistance to compression, however, keeps it from packing at the bottom and allows it to flow at an even rate.

Another surprising ability of granular materials becomes evident when grains of different sizes are poured into a narrow space such as an ant farm. For example, if sand and sugar are well mixed and poured together, they will at first form an ordinary-looking pile. But after a while, the pile will display alternate horizontal stripes as the sand and sugar sort themselves out.

Perhaps more astonishing is what happens when grains of different sizes are mixed in a rotating cylinder. If a uniform mixture of rice and dried peas, for example, is put into a horizontal glass cylinder and the cylinder is rotated at the right speed, the peas and rice will sort themselves into separate bands like rings on a finger.

The physics behind these phenomena still fascinates Kakalios, although he has switched his research focus to semiconductor materials and neurological systems. As Hollywood continues to produce superhero movies, he hopes viewers will take home a lesson or two when the science is correct, and take the rest with a grain of salt.

Source: University of Minnesota