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Granular matter on the boil

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15.02.2008

When grains are shaken fiercely, they show behaviour that can be compared to water on the boil. Convection takes place, with the typical rolling movement that can also be seen in water.

For the first time, researcher Peter Eshuis of the University of Twente in The Netherlands shows this phenomenon in granular matter using a high speed camera. His research gains a better understanding of the behaviour of these materials that are often used and produced in industry.

Convection can be noticed in water when it nears the boiling point. Rolling movements then occur, to get rid of excess heat: heated fluid rises and cooler water falls, causing a roll. A similar and beautiful effect is seen in little balls shaken hard: starting with an eruption of rising fast balls that go down again, clusters are formed and a rotating movement starts. Just like in fluid, there are balls with lower energy clustering and with higher energy, moving fast. Analogous to the temperature of the boiling plate heating fluids, the shaking energy gives rise to phase transitions.

Before convection starts, at lower shaking intensities, the balls already show behaviour typical to fluids: in fluids this is called the Leidenfrost effect, when a droplet is ‘floating’ on a thin layer of gas. The same happens with vertically shaken balls: a packed cluster of balls ‘floats’ on a layer of fast moving balls. This layer is therefore called ‘granular gas’. Eshuis describes the transition from the Leidenfrost condition towards convection. This is not just a matter of rising the level of energy, he found out: there has to be an instability that causes the onset of convection. This instability causes some balls to cluster and others to free themselves.

Stagnation

Granular matter like grain, sugar, sand and pills, often give rise to unexpected effects during transport, processing or storage. This often causes stagnation in industrial processes or excessive energy consumption. Better understanding of the behaviour of the materials, like Eshuis presents in his thesis, helps to prevent these effect. He also proves that many phenomena like clustering of grains can be explained by treating and describing the materials like fluids.

Peter Eshuis (1980) studied Applied Physics at the University of Twente and did his PhD-research within the Physics of Fluids research group of prof. Detlef Lohse, part of the Institute of Mechanics, Processes and Control (IMPACT) of the University of Twente.

Wiebe van der Veen | Source: alphagalileo
Further information: www.utwente.nl/en

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