In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
that have a negative Poisson's ratio
Poisson's ratio , named after Siméon Poisson, is the ratio, when a sample object is stretched, of the contraction or transverse strain , to the extension or axial strain ....
. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Auxetic materials can be single molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s or a particular structure of macroscopic matter.
Such materials are expected to have mechanical properties such as high energy absorption
A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
resistance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.
Auxetics can be illustrated with an inelastic string wound around an elastic cord. When the ends of the structure are pulled apart, the inelastic string straightens while the elastic cord stretches and winds around it, increasing the structure's effective volume.
The term auxetic
derives from the Greek
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
word αὐξητικός (auxetikos) which means "that which tends to increase" and has its root in the word αὔξησις, or auxesis
, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter
The University of Exeter is a public university in South West England. It belongs to the 1994 Group, an association of 19 of the United Kingdom's smaller research-intensive universities....
Scientists have known about auxetic materials for over 100 years, but have only recently given them special attention. The earliest published example of a synthetic auxetic material was in Science
in 1987, entitled "Foam structures with a Negative Poisson's Ratio" by R.S. Lakes from the University of Iowa
The University of Iowa is a public state-supported research university located in Iowa City, Iowa, United States. It is the oldest public university in the state. The university is organized into eleven colleges granting undergraduate, graduate, and professional degrees...
. The use of the word auxetic
to refer to this property probably began in 1991.
Typically, auxetic materials have low density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, which is what allows the hinge-like areas of the auxetic microstructures to flex.
Examples of auxetic materials include:
- Certain rocks and minerals
- Living bone tissue (although this is only suspected)
- Specific variants of polytetrafluorethylene polymers such as Gore-Tex
Gore-Tex is a waterproof/breathable fabric, and a registered trademark of W. L. Gore and Associates. It was co-invented by Wilbert L. Gore, Rowena Taylor, and Gore's son, Robert W. Gore. Robert Gore was granted on April 27, 1976, for a porous form of polytetrafluoroethylene with a...
- Paper, all types. If a paper is stretched in an in-plane direction it will expand in its thickness direction due to its network structure.
- Acoustic metamaterials
Acoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound in the form of sonic, infrasonic, or ultrasonic waves, as these might occur in gases, liquids, and solids. The hereditary line into acoustic metamaterials follows from theory and research...
Zetix is a fabric invented by Auxetics Technologies, Ltd., a UK company.Zetix is an auxetic material that is so strong it absorbs and disperses the energy from explosions without breaking...
a type of auxetic material commercially manufactured.