In 1660, Prince Rupert of Germany brought five pieces of tadpole-shaped glass to England and presented them to King Charles II. Although there is no concrete evidence of their first invention, there were already confirmed records of their existence as early as 1625. Many believe these glass drops came from Mecklenburg, Germany, where many fine glass blowers can be found.
The glass did not only capture the interest of the English king, but it also has long captivated scientists for the next 400 years. In a recent study, researchers unveiled the mystery of the glass bead popularly known as the Prince Rupert drop.
Incredible Characteristics of Prince Rupert Drop
The Prince Rupert drop is famous for its glass teardrops with elongated skinny tails, looking like a delicate tadpole. One of the distinguishing features of this bead is the seemingly contrasting behavior of the tail and head.
The tail is so fragile that it can be easily broken by a finger, causing the entire droplet to shatter into fine powder instantly. However, the head is so strong that a hammer cannot damage it. Even a bullet can break apart upon hitting the drop; that is why it can be comparable to the strength of steel.
Prince Rupert drop can be easily made by dropping hot blobs of molten glass into water. As the outer layers get in direct contact with water, they cool rapidly and solidify, forming a distinct tadpole shape.
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Decoding the Mystery of the Beads
The earnest examination of the weird properties of Prince Rupert's drop began in the 1650s. It was even suggested that there were known and studied in France. In 1661, they were presented to the Royal Society, where the members investigated it, the most notable being scientist Robert Hooke. Recently, modern technology allowed researchers to analyze the glass drop thoroughly.
A 1994 study by S. Chandrasekar at Purdue University and M. M. Chaudhri at the University of Cambridge involved using high-speed framing photography to observe the process of shattering the glass into pieces. The scientists concluded that its outer layer experiences high compressive stress while the interior possesses high tension forces. However, how these stresses are distributed throughout Prince Rupert's drop remained unclear.
Chandrasekar and Chaudri collaborated with Professor Hillar Aben at Tallinn University of Technology in Estonia to address this question. They used a transmission polariscope, a microscope that can measure the birefringence in a transparent object with axisymmetry.
The experiment involved suspending Prince Rupert drop in a clear liquid and illuminating it with a red LED. Then the research team measured the optical retardation of the light as it passed through the glass bead. The data provided by the polariscope was then used to construct the stress distribution throughout the entire drop.
The result of the study reveals that the heads of the drop contain higher surface compressive stress reaching up to 700 megapascals. The drop also possesses a thin surface compressive layer which is only 10% of the diameter of the head. These measurements provide very high fracture strength to the head of the drop.
This means that a crack entering the interior tension zone must be created to break a droplet successfully. Since the exterior cracks grow parallel to the surface, they cannot go through the tension zone. Because of this, the easiest way to break the bead is to disturb its tail since the disturbance in this region allows access to the tension zone.
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