Prince Rupert’s Drops

Again, we present another beloved classroom demonstration in
materials science. This one is a non-intuitive display of surface
tension, residual stress, interior tension, potential energy and
tempered glass.

To create a Prince Rupert drop, molten glass is dropped into cold
water. The glass rapidly forms into teardrop shape with a extended,
fine tail. The material in the exterior of the drop cools and hardens
nearly immediate while the interior material cools slowly. As the
interior material cools, it contracts and sets up powerful compressive
stresses on the surface.

The residual stress
within the drop gives rise to unique properties that every demonstrator
loves: The drop can be hammered on the fat end without breaking, but
disintegrates explosively if the tail end is even slightly damaged.
This illustrates the release of the potential energy contained within
the drop’s amorphous atomic structure.

The process of the “explosion” has been studied closely and shows
that fractures move from the tail through the material at very high
speed. Purdue University’s Srinivasan Chandrasekar used extremely high
speed video to record how the “crack front” propagates in a
disintegrating drop at up to 4,200 miles per hour.

The drops were supposedly discovered around the 1640s by Prince Rupert of the Rhine
(1619–1682). The story is told that Rupert would use the drops as a
practical joke in his court. He would give a drop to someone in his
audience and then surreptitiously break the tail causing a small and
surprising explosion.

We have actually combined two videos. The second part is slightly
repetitious but provides both a slow-mo view of the shattering process
and a lovely image of namesake Rupert.