What’s So Special About This Drop Shaped Glass?

We are talking about the Prince Rupert’s Drop. It is essentially a small a water droplet-like shaped glass. Before we understand the mystery behind it let’s take a quick run through the process of making glass which can be made by just using ordinary sand. That’s right, sand which mostly comprises of silicon dioxide is the main ingredient along with recycled glass, soda ash or sodium carbonate and limestone or calcium carbonate.

prince rupert's drop

Now, this mixture is put into a heated furnace. The result is super hot molten glass that can be moulded as desired. Why is soda ash and limestone added you may ask? Simply to conserve energy. The soda brings the heating point of sand down thereby making it more viable to manufacture. But this also makes the resultant glass soluble in water hence the addition of limestone.

Who Was Prince Rupert?

So coming back to Prince Rupert’s Drop. When a tiny bit of the molten glass is dropped into cold water, it cools down and forms a drop of solid glass. But unlike other glass, it has some super cool properties and hence got a special name after the Prince who brought them to England. In the 17th century, Prince Rupert brought some of these glass beads to England’s King Charles II from Germany.

prince rupert's drop

The king was highly intrigued by its unusual properties. What exactly is it? The head of these beads is so strong that it can easily withstand bash with a hammer. But the tail of the bead is so fragile that a slight twist will shatter the entire bead into glass dust. Sounds strange right?

Years Of Research

The Royal Society of London set up a research team on this. They began their journey to decipher the reason behind the properties of these glass beads. But it wasn’t before 1994 when Srinivasan Chandrasekar and M. Munawar Chaudhari observed the shattering of the drop under a high-speed camera that the quest saw any progress. Both material scientists from the Purdue University and the University of Cambridge respectively made use of the technological advancement around them.

prince rupert's drop

They observed at almost 1 million frames per second that the snipping of the tail cracks within the head at over 4000 mph. By studying the exterior of the drop they were able to conclude 2 things. The exterior experiences high compressive stress while the interior experiences high tensile stress. This leaves the drop in an equilibrium state which is disturbed by the snipping of the tail end.

The Recent Development

prince rupert's drop

Some recent tests and experiments have led to the advancement of the previous theory. It states that the Prince Rupert’s Drop’s extraordinary strength is not just because of the strong exterior. When external compressive pressure is added to the head, the cracks that form are parallel to the core of the head. In order for the glass bead to break the cracks need to reach the centre which happens easily when you snap the tail.

Read more in the Applied Physics Letters.