Prince Rupert's drops
Background: Dropping a small bit of hot molten glass into cold water creates a teardrop shape with a long, thin tail called a Rupert’s Drop. It is so strong that you can hammer on one end without causing a crack, but watch out for the other end—it breaks easily and causes the whole thing to explode!
Prince Rupert’s drops are stressed glass drops that may provide a good demonstration of stress.
Most physics courses have a materials section. Stress in materials is often shown optically using crossed polarising filters and suitable specimens such as plastic rules and protractors. It is also suggested that the stress patterns of drops are viewed in this way. The Prince Rupert’s drop phenomenon may also be of interest to new technicians and illustrates the need to anneal glass to relieve stress on cooling.
The method of production of Prince Rupert's drops is very "trial and error" based (see historical reference from the Corning link below). Do not expect a 100% success rate.
The following method was found to work more frequently than not.
A 3.5mm diameter soda-glass rod was heated using a standard bunsen on a roaring blue flame. A working length of 25cm was used, however this was a choice of convenience. The rod was held just above horizontal and only the tip was heated. There was no rotation of the rod. The was bunsen held at the base at an angle of about 30 degrees off vertical. The glass was allowed to form a drip and fall through approximately 50cm into a heavy metal container containing 10cm of warm ~60 degree C water. Once the rod is heated several drops were created to maximise efficiency. The process was stopped when the rod length became too short to handle safely.
Protective eyewear should be worn if attempting this. The area for working should be kept reasonably clear and thought given to the storage of heated glass rod for cooling. The cooling rods will possibly shatter as they have not been destressed by annealing.
Successful drops may be pulled out reasonably soon after by the fiber tails.
Excess water is decanted off the "failures" prior to disposal with normal glass waste.
The tadpole shaped glass is surprisingly strong- unless the tail is struck. At this point catastrophic destruction of the entire bead ocurrs.
It is suggested that a safe method of displaying this should be adopted to prevent eye injury. The method used in this video: you tube video link , is put forward. The bead is held by the tail, poked through the lid, in a clear jar. The tail is then flicked and the fragments are contained within the jar. A more certain method of triggering the "explosion" is to crush the tail using long nose pluiers.
If time is available this could even be used as an open day (teacher performed) exhibit.
- Corning Museum of Glass in Corning, New York: Theoretical Treatment of Prince Rupert's drops
(Note that the you tube video link above is provided by Corning Museum of Glass freely for educational use we provide the link to their website as courtesy.)
- Making Glass
--D.B.Ferguson 13:09, 7 August 2008 (UTC)
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