February 3 - 4:00pm Phys 223
(Coffee at 3:30p.m. in room 242)
Professor Srinivasan Chandrasekar
Title: "Prince Rupert's Drops"
On 4 March 1660-61 "glass bubbles" were first introduced to a meeting of the Royal Society of London. The bubbles were what were later to be called 'Prince Rupert's drops." These objects, glass bubbles with the form of a tear-drop tapering to a fine tail, made by dripping molten glass into cold water, exhibited a paradoxical combination of strength and fragility not without interest to the materials scientist of the present day, and which could not fail to excite the imagination of natural (and not so natural) philosophers of the 17th century. The head of the drop withstands hammering on an anvil without fracture: yet breaking the tail with finger pressure caused the whole to explode into powder. This phenomenon has interested many scientists from the 17th century to date, including Hooke, Brewster and Lord Kelvin. Several hypotheses relying on the tensile residual stresses produced within the drop during cooling have been put forth to explain the peculiar characteristics of these drops. However, none of these explanations appear to be fully satisfactory.
We have carried out high-speed and semi-Schlieren photographic studies of the explosive disintegration of Prince Rupert's drops at framing rates ranging from 6500 frames per second to 1 million frames per second. The high-speed photographic sequences revealed that in a disintegrating drop the crack front having been initiated in the tail, propagated at a high velocity (~ 1450-1900 m/s) within the tensile zone towards the drop's head. Finger-type cracks at the moving crack front were observed, which indicated that these were a consequence of repeating bifurcation events. From the high-speed photographic observations and other measurements, a mechanism based on the repeated bifurcation of fast moving cracks within the tensile zone in the drop has been proposed for explaining its disintegration. The talk will describe these studies as well as how modern toughened glasses evolved from these drops.