Crystal expansion makes light work of moving microbeads

By exploiting local thermal expansion and mesophase changes, scientists from Japan are able to move microbeads dispersed in a liquid crystal using UV light – despite neither material being light-responsive.

Most materials that respond to heat, electric convection, light or pressure need complicated syntheses or set-ups. But Yoshiko Takenaka and T Yamamoto from the National Institute of Advanced Industrial Science and Technology found that they can move simple silicon dioxide microbeads in a 4-cyano-4’-pentylbiphenyl liquid crystal (5CB) using UV light and no added light-responsive materials.

The UV light heats 5CB by around 1°C, putting it at the cusp of a phase change; it is only at this intermediate state that the microbeads move. Just before 5CB’s molecules rearrange themselves from a nematic (thread-like order in one direction) to an isotropic (the same in all directions) order, the liquid crystal’s thermal expansion coefficient increases to a point where it moves the microbeads with it.The distance the beads travel is proportional to the intensity of the UV light and the beads return to their original position when the light is turned off.

This system is simple and straightforward to use and could find applications in heat sensors or microfluidic pumps.