Researchers at the University of Amsterdam, Unilever Vlaardingen and EPFL Lausanne have found that there are two different fracture processes with different types of debris.
When a glass tumbler falls to the floor and breaks, the shards will vary in size from large to very small. But the broken glass of the bus stop will be more or less even. Scientists have tried to explain why this is happening.
Authors Stéphane Cooy, Gerard van Dalen, Jean-François Molinari and Daniel Bonn investigated the fragmentation process and found that there are two very different ways of destruction. Glass at bus stops breaks down differently, because it was processed in a special way, because of this there is an internal tension in it: it ultimately leads to the similarity of the fragments.
To study the process that occurs with the glass of a bus stop, the researchers studied a similar type of glass called the Prince Rupert drop or Dutch tear. They are made by dropping molten glass into cold water. Since the glass first solidifies on the outside and only then on the inside, large stresses arise inside the droplet, comparable to those that occur in the glass of a bus stop.
On the Internet, you can find a large number of videos showing the special properties of such drops: they can withstand a hammer blow, but break into pieces if they pinch the tail.
Researchers have studied various fragmentation processes using the example of 22 thousand types of droplets. They compared Prince Rupert’s droplet shattering process with other objects and found that there are two types of fragmentation processes, which they called hierarchical and random.
When you drop a regular glass on the floor, a hierarchical process takes place. The energy that is present during the movement of glass is much more than is necessary for a single destruction. To get rid of all the kinetic energy, more and more cracks appear inside the glass. The process is hierarchical – it goes from large cracks to ever smaller ones.
Most of the situations in which objects break are of a hierarchical type. An exception occurs when the energy to destroy an object does not come from the outside, but is the result of internal stresses, as in the case of bus stop glass and Prince Rupert’s drops. In this case, the formation of cracks does not occur from large to small, but in a completely random way. As a result, the fragments have a certain size, determined by the magnitude of the internal stress in the material.