— Prior to the phase change, few pressure chains exist (Image: Robert Behringer / Duke University)
Extra pressure flips the discs into a jammed state creating a dense network of glowing 'pressure chains' (Image: Robert Behringer / Duke University)
Spectacular images that reveal how a granular material like sand can suddenly jam into a rigid mass have been captured by US researchers.
The work could help engineers devise ways to prevent blockages in industrial piping or machinery used to process small particles or lumpy solids.
Physicist Robert Behringer and colleagues at Duke University in North Carolina, US, used partly transparent plastic discs to simulate the behaviour of a granular material in two dimensions.
The discs have changeable light-transmitting properties and allow more light to pass through when squeezed with greater pressure. These were used to cover a flat surface within a special instrument with adjustable sides, with a camera overhead and a light source below.
The team used the camera to capture patterns of light that occur as pressure from two sides gradually increases. These patterns reveal a sudden "phase change" when the grains jam solid the same effect that causes granular materials to become clogged. The overhead camera captured glowing "force chains" that reveal the way force is distributed between each disc during jamming.
Images taken before the phase change (top right) show very few such chains. The addition of a little extra pressure flips the discs into a jammed state (lower right), creating a dense network of glowing chains. "It's as if the grain suddenly wakes up and says 'Oh! I have neighbours'," says Behringer. A video produced by the University shows force chains appearing in the discs (requires Real Player).
Mathematical models dating back to the 19th century have predicted such a change, but Behringer says he expected did not expect it to be so sudden and dramatic in the real world. "Experience would not suggest that there would be this magic point," he says. "These experiments finally prove the math right."
The discovery could ultimately help engineers design systems that can better cope with sudden blockages. "Jamming is actually a common problem," Behringer told New Scientist, "you open a hopper and your material just won't come out."
Further study of the phase transition could perhaps help engineers design hoppers that prevent jams from occurring in the first place. "The generic solution is currently to bang on the side of your hopper with a sledgehammer," he notes.