Popping crystals

02 June 2014. NUS scientists have unraveled the mystery of why certain molecules form crystals that pop when light shines on them.

A team led by Professor Jagadese J. VITTAL from the Department of Chemistry in NUS in collaboration with an international team of scientists, have successfully unraveled the photochemical reaction responsible for the bursting effect in crystals formed by certain types of molecules when they are exposed to ultraviolet (UV) light. The UV light causes bond formation between pairs of molecules in the crystal (see diagram). This causes a rotation of the molecular pair and a strain energy to accumulate in the crystal. The stored strain energy is then released suddenly, which causes the crystal to pop. The same effect could also be used to propel microscopic crystals over distances several hundred times of their own size when exposed to UV light.

This effect is related to the bursting of popcorn kernels at high temperatures. It is termed the “photosalient effect” and is one pathway for the conversion of light into mechanical motion. The discovery paves the way to exploit bond-forming photochemical reactions in solids for the photosalient effect.

The team hopes to eventually develop new materials that could convert solar energy effectively into mechanical energy. This will enable the development of actuators and mechanical devices that bend, change shape, or store mechanical energy from sunlight.

Their findings were published as the cover story in the English version of German scientific journal Angewandte Chemie International Edition.


A schematic diagram showing the popping nature of the crystals under UV light, a property that is very similar to the popping of corns on a hot plate. (Image credit: National University of Singapore)