New materials stretch the imagination
Jun 16, 2000
Rubber bands, human muscle and most other materials get thinner when they are stretched. These materials are said to have a positive Poisson's ratio. However, some materials have negative Poisson's ratios and become fatter when they are stretched. Now Ray Baughman of the Honeywell Technology Center in New Jersey, and co-workers in Brazil, Sweden and the US, have predicted that negative Poisson's ratios can be found in both the high-density matter inside neutron stars and in very low-density crystals of trapped ions (R H Baughman et al. 2000 Science 288 2018). The team has confirmed its predictions for the ion crystals in experiments.
The Poisson's ratio of a material is defined as nij = -ej/ ei, where ej is the lateral strain in the j direction that is caused when a strain ei is applied in the longitudinal i direction. Negative Poisson's ratios have previously been observed in foam- and polymer-based structures, in the non-cubic phases of arsenic and other crystals, and in certain cubic metals. While these materials have the counterintuitive property of expanding laterally when stretched, none of them are "incompressible" when stretched. A rubber band, on the other hand, is incompressible because the volume does not change when the band is stretched. Baughman and co-workers predict that incompressible materials can also have negative Poisson's rations - another highly counterintuitive property.
The team investigate the effect of a wide range of processes - including temperature changes, charge screening, crystal strains and ion mixing - on the Poisson's ratios of a variety of different systems. These systems range from star crystals, which can have densities as high as 1011 grams per cubic centimetre, to crystals of ions and colloids with densities of just 10-15 grams per cc. The work could have implications in both astrophysics and the development of new optical devices.