The Tiniest Spring

By bouncing lasers off a microscopic bead as it bobbed up and down, a team of Danish scientists has measured the stiffness of the shortest spring ever measured. The team reports that a wisp of plastic just 50 millionths of a millimeter long behaved very much like a microscopic Slinky. Experts say the elegantly simple technique, described in the 22 December issue of Physical Review Letters, could help uncover the properties of other polymers--molecular chains that have widespread applications as plastics of all sorts and many other materials.

The researchers--physicist Giovanni Zocchi and graduate student Henriette Jensenius of the Niels Bohr Institute in Copenhagen, Denmark--were measuring the interactions between a glass surface and microscopic plastic beads floating in water. One day they observed a curious phenomenon: Sometimes a bead became "tethered" to the bottom of the dish, apparently by a single strand of plastic that had frayed from the bead. Jensenius recalls that other researchers were annoyed by this stickiness, but she and Zocchi decided to investigate.

They had already been making precise measurements of the heights of the beads above the surface by scattering a laser off them. By tracing the tethered beads as they bobbed up and down, the pair realized, they could determine the strength of the forces tugging on them. After accounting for the electrostatic repulsion and other forces, they found that the remaining force on the beads followed the force law for an ideal spring--whose force is proportional to how far it is extended--to very high accuracy. The tension on the polymer chain comes from water molecules, which buffet it into an irregular shape.

Previous experiments have measured the stretchiness of longer polymer strands, but only under high tension. This new, gentler way to study a polymer's resistance to unfurling is "terribly important," says physicist David Grier of the University of Chicago. The experiment's simplicity has also won admiration. Says Grier: "I think it's lovely."