In materials science, fragile matter is a

granular material A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when grains collide). The constituents that compose gra ...

that is jammed solid. Everyday examples include beans 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 Michael Elmhirst Cates (born 5 May 1961) is a British physicist. He is the 19th Lucasian Professor of Mathematics at the University of Cambridge and has held this position since 1 July 2015. He was previously Professor of Natural Philosophy at ...

, 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 may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

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 is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

, 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 Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the ...

Shaving Shaving is the removal of hair, by using a razor or any other kind of bladed implement, to slice it down—to the level of the skin or otherwise. Shaving is most commonly practiced by men to remove their facial hair and by women to remove th ...

foam is jammed because the bubbles are tightly packed together under the isotropic stress imposed by

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

. If it were a fragile solid, it would respond

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

ally to

shear stress Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ...

, however small. But because bubbles deform, foam actually responds elastically 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").

References

* {{cite book , last = Cates , first = Michael , authorlink = Michael Cates , title = Soft and Fragile Matter , publisher = CRC Press , year = 2000 , isbn = 0-7503-0724-2 Granularity of materials