Granular matter

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Sand consists of many individual grains.

Granular matter , also called granular medium or granulate , consists of many small, solid particles such as grains or spheres. Examples of this condition are granular materials such as plastic granulate , powdery materials such as powder or, in large quantities, loose material such as rubble . The terms pile for mechanical and bulk material for logistical aspects are also used.

Basics

The particles of granular matter have a macroscopic size (in material science they are called grain ) and are therefore not noticeably influenced by quantum effects or thermal movement . They only interact via contact forces ( friction force ). It is not a matter of an aggregate state of matter, but a collection of solids . Granular matter sometimes behaves like a single solid - for example stones can lie on sand without sinking in - and sometimes like a liquid : sand adapts to the shape of a vessel and "flows" out of it when it is tilted. When emptying a silo or bunker, the bulk material sometimes behaves like a solid (does not flow), sometimes like a liquid (flows or shoots out).

Granular Matter Physics

The physics of granular matter is based on mechanical and, with sufficiently small grains, electrostatic interactions. Due to the large number of reaction partners, however, a multi-body problem with high complexity arises , which leads to a variety of effects. One example is the Brazil nut effect , in which moving a mixture of grains of different sizes causes the larger grains to drift to the surface. The properties of granular matter change greatly when small amounts of liquid are added, as this lowers the friction of the particles and can lead to mutual attraction due to cohesion . This explains the stability of sand castles , for example .

Granular matter has only been an active research area for a few decades, so that many phenomena are known from everyday life or physical experiments, but are not yet clearly explained theoretically.

Alternatively, the mechanical behavior of granular matter can also be described using the theory of porous media . Here, the grains are not considered individually, but are included in the description averaged over their volume fraction .

field of use

Castle of dripped sand

Knowledge of granular matter can be of great importance for manufacturing processes and warehousing in industry.

Another important application is the physics of geophysical mass movements , such as debris flows and avalanches . Also rockfalls and sediment in fall watering can as - are modeled particles - huge. These models form the basis for hazard zone labeling and disaster control .

In transport, the term loose material and the underlying mechanical and physical processes are important for the safety and stability of cargo ships and aircraft . The explosive effect (see also dust explosion ) of granular matter, due to its large surface, oxidation effect and static electricity , was only recognized as a risk factor after serious accidents.

literature

  • P. Grassmann: Physical basics of chemical engineering technology. Verlag HR Sauerländer, Aarau / Frankfurt am Main 1961, in particular Chapter 5: Surface-rich bodies and finely divided substances.
  • J. Schwedes: Flow behavior of bulk solids in bunkers. Verlag Chemie, Weinheim (Bergstrasse) 1968.
  • M. Stieß: Mechanical Process Engineering - Particle Technology 1st 3rd edition. Springer-Verlag, Berlin / Heidelberg 2009, ISBN 978-3-540-32551-2 .
  • R. Bunge: Mechanical processing. Wiley-VCH Verlag, Weinheim (Bergstrasse) 2012, ISBN 978-3-527-33209-0

Web links