Auxetics
Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Such materials are expected to have mechanical properties such as high energy absorption and fracture 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 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.[1] Auxetic materials can be single molecules or a particular structure of macroscopic matter.
Scientists have known about auxetic materials for over 100 years,[2] 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" [3] by R.S. Lakes from the University of Iowa. The use of the word auxetic to refer to this property probably began in 1991.[4]
Typically, auxetic materials have low density, which is what allows the hinge-like areas of the auxetic microstructures to flex.[5]
Examples of auxetic materials include:
- Certain rocks and minerals[2]
- Living bone tissue (although this is only suspected)[2]
- Specific variants of polytetrafluorethylene polymers such as Gore-Tex[6]
- 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. Ref Baum et al 1984, Tappi journal, Öhrn, O. E. (1965): Thickness variations of paper on stretching, Svensk Papperstidn. 68(5), 141.
See also
References
- ^ Quinion, Michael (1996-11-09), Auxetic, retrieved 2009-01-02.
- ^ a b c Burke, Maria (1997-06-07), "A stretch of the imagination", New Scientist, 154 (2085): 36
- ^ Lakes, R.S. (1987-02-27), "Foam structures with a negative Poisson's ratio", Science, 235 (4792): 1038, doi:10.1126/science.235.4792.1038, PMID 17782252.
- ^ Anissimov, Michael, What are Auxetic Materials, retrieved 2009-01-12.
- ^ A stretch of the imagination - 07 June 1997 - New Scientist Space
- ^ Auxetic materials, retrieved 2009-01-02.