Fragile matter
Encyclopedia
In materials science
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...

, fragile matter is a granular material
Granular material
A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact . The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations...

 that is jammed solid. Everyday examples include bean
Bean
Bean is a common name for large plant seeds of several genera of the family Fabaceae used for human food or animal feed....

s getting stuck in a hopper in a whole food shop, or milk powder getting jammed in an upside-down bottle. The term was coined by physicist Michael Cates, who asserts that such circumstances warrant a new class of materials. The jamming thus described can be unjammed by mechanical means, such as tapping or shaking the container, or poking it with a stick.

Cates proposed that such jammed systems differ from ordinary solids in that if the direction of the applied stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

 changes, the jam will break up. Sometimes the change of direction required is very small.

Perhaps the simplest example is a pile of sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, which is solid in the sense that the pile sustains its shape despite the force of gravity. Slight tilting or vibration is enough to enable the grains to shift, collapsing the pile.

Not all jammed systems are fragile, i.e. foam
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...

. Shaving
Shave
Shave may refer to:*to shave refers to the act of shaving*"Shave," a song by Enon from their 2003 album Hocus Pocus*Shave , a periodical magazine...

 foam is jammed because the bubbles are tightly packed together under the isotropic stress imposed by atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

. If it were a fragile solid, it would respond plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

ally to shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...

, however small. But because bubbles deform, foam actually responds elastically
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....

provided that the stress is below a threshold value. Fragile matter is also not to be confused with cases in which the particles have adhered to one another ("caking").
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