Feb 20 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.