Lightly reactive

most materials expand when heated and contract when cooled. But what about a material which contracts when in light and expands in the dark? Scientists in Germany and uk have developed precisely such a material.

If the material can be made into other compounds, it could prove immensely useful in micromachines and even provide robots with muscles. Materials changing shape and size under external stimuli are well known. Piezoelectric materials, the type used in quartz watches, change their shape and size under the influence of electrical voltages. But the change is minimal. Ordinary materials expand by a few percent when heated. Certain organic chemicals, known as nematic elastomers on the other hand can deform by upto 400 per cent when heated. To make these elastomers, scientists use a liquid of polymers. The polymers have other molecules attached to them. By adding other molecules to link the chains, the liquid is converted into a solid. Heating and cooling the liquid, whose composition has been suitably modified, can produce neamtic elastomers. Scientists at University of Frieburg in Germany made such an elastomer, also adding photosensitive molecules. These are molecules that absorb a photon of light and use this energy to reposition the chemical bonds between the atoms. This causes a kink in the polymer chain.

The scientists found that the material contracted under the action of light by about 25 per cent. When the light was withdrawn, it regained its original shape in less than an hour. Scientists at the University of Cambridge have explained this effect. With the increasing use of micromachines in industry, the trend is to shift from making parts that move to materials that can change shape and size under certain conditions. If the elastomer could be made into other compounds and the time taken by it to regain its shape can be reduced, then it could find many applications in industry (Physical Review Letters , Vol 87, No 015501).