A nuisance becomes a method
By observing the up-and-down motion of microscopic plastic beads, Danish scientists measured the strength of the shortest coil spring ever found. In this way, they found a new method for examining the stretchability of polymers. Physicist Giovanni Zocchi and his doctoral student Henriette Jensenius from the Niels Bohr Institute in Copenhagen are measuring interactions between a glass surface and microscopic plastic beads floating in water. They observed a strange phenomenon: sometimes a pearl settled at the bottom of the glass bowl. The pearl was "frayed" and had attached itself to the glass with the loosened thread. Jensenius says other researchers have been annoyed by this attachment, but she and Zocchi decided to investigate this phenomenon scientifically.
In their previous work, the two scientists had already carried out precise measurements of the height of the pearls above the surface with the help of lasers. By tracking the fixed beads as they moved up and down, they were able to determine the strength of the forces acting on the bodies. After accounting for electrostatic repulsion and other forces, Zocchi and Jensenius found that the residual force is proportional to the polymer's elongation to a very good approximation. This behaves like an ideal (Hooke's) spring. The force that deflects the polymer chain from its resting position comes from water molecules.
In earlier experiments, the elongation of longer polymer strands could be measured - but only under high pressure. A new and more cautious method now exists to study a polymer's resistance to stretching.
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