I’d heard of this and wondered, “how the hell did anyone ever think of doing this?”
The glass pieces, which are made by dropping blobs of molten glass into water, have been shown to withstand the head of a bullet.
The researchers found that the heads of the drops can withstand nearly 7,000 times atmospheric pressure, and it order to break it, a crack entering the drops interior tension zone must be created.
Prince Rupert’s drops are named after Prince Rupert from Germany, who during the 17th century brought some of the glass drops to England’s King Charles II, who was interested in their unusual properties.
The drop’s head is strong enough to withstand the impact of a hammer, but the tail end is so fragile that bending it with one’s fingers not only breaks the tail – it shatters the entire drop, which disintegrates into a fine powder.
Researchers have for a long time tried to understand what causes these unusual properties, but it wasn’t until recently that new technologies have allowed researchers to investigate them in detail.
In 1994, Dr Srinivasan Chandrasekar, an engineering professor Purdue University, and Dr Munawar Chaudhri, Head of the Materials Group at the University of Cambridge, used high-speed framing photography to observe the drop-shattering process.
They concluded that the surface of these drops experiences highly compressive stresses, while the inside experience high tension forces.
A compressive force is one that squeezes material together, while a tensions force is one that pulls materials apart.
So, the drop is in a state of unstable equilibrium, which can easily be disturbed by breaking the tail.
Despite this finding, a question remained: How are the stresses distributed throughout a Prince Rupert’s drop?
Understanding this would help to more fully explain why the heads of these drops are so strong.
So in a new study, Dr Chandrasekar and Dr Chaudhri collaborated with Dr Hillar Aben, a professor at Tallinn University of Technology in Estonia who specializes in determining residual stresses in transparent three-dimensional objects.
The research team and their co-authors conducted their investigation using a transmission polariscope – a microscope that measures stress distribution in a material experimentally by measuring the angle at which a ray of light reflects at each point inside it.
In the experiments, the researchers suspended a Prince Rupert’s drop in a clear liquid, and then illuminated the drop with a red LED.
Using the polariscope, the researchers measured the optical retardation of the light as it traveled through the glass drop, and then used the data to construct the stress distribution pattern throughout the drop.
They found that the heads of the drops have much higher surface compressive strength than thought.
They can withstand up to 700 megapascals – nearly 7,000 times atmospheric pressure – but this surface compressive layer is also thin, approximately 10 per cent of the diameter of the drop’s head.
The researchers said that this gives the droplet heads very high fracture strength, and to break the droplet, a crack that enters the interior tension zone of the drop must be created.
But since cracks tend to grow parallel to the surface, they can’t enter this tension zone.
So the easiest way to break a drop is to break its tail, since disturbances in this area allow cracks to enter the tension zone.
‘The work has fully explained why the head of a drop is so strong,’ Dr Chaudhri told Phys.org.
‘I believe we have now solved most of the main aspects of this area.
‘However, new questions may emerge unexpectedly.’