Bead Chains Impersonate Polymer Molecules

Tangled like polymers. The rigidity of a pile made of chains of metallic beads depends on the lengths of the chains involved. Each collection was initially held in a cylindrical container, and then the container was removed.

Chains of metallic beads act a lot like polymer molecules, even though real polymers are in constant motion.

Granular materials such as sand often behave like a collection of molecules, flowing like liquids in some conditions or locking together in solid structures in others. Now researchers experimenting with chains of metallic beads have shown a surprising link to the physics of polymers—long, chain-like molecules that form plastic and rubber materials. Both show a marked tendency to grow stiffer in response to any deforming force. The finding could help in the engineering of stronger materials.

Push the tip of a pencil into a chunk of rubber, and the material’s resistance increases as the probe goes deeper. Researchers understand that this stiffness amplification arises because the polymers in the material are linked together. The material acts as a coherent whole and the probe can only penetrate further by breaking contacts at many points where polymers link together.

A similar linking together of separate strands occurs in the coiled rope or cable used to lift an anchor on a ship, says Pascal Damman of the University of Mons in Belgium. The more you increase tension in the rope—which is typically wrapped around a drum known as a capstan—the more the strands bind together, making them further resistant to movement. Damman and his colleagues wondered if this behavior, called self-amplified friction, might be observed for any collection of chain-like objects. They sought to test the idea in experiments using chains of metallic beads like those found in keychains or connected to sink stoppers.

They placed a collection of such chains into a cylindrical container and subjected it to vibration to allow the chains to pack themselves efficiently. They then measured the force needed to push a vertical, rod-shaped probe slowly down into the pile, recording how the force changed as the probe went deeper. The force required to deform this material is strongly influenced by the same interstrand-binding effects that cause self-amplified friction. Experimenting with chain lengths ranging from 2 to 50 beads, the team found a universal behavior in which the force always increases exponentially with the depth of the tip. They also found that the exponent describing this stiffening grows in proportion to the square root of the chain length.

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