Light-induced crawling of crystals on a glass surface

Motion is an essential process for many living organisms and for artificial robots and machines. To date, creating self-propelled motion in nano-to-macroscopic-sized objects has been a challenging issue for scientists. Herein, we report the directional and continuous motion of crystals on a glass surface when irradiated simultaneously with two different wavelengths, using simple azobenzenes as a photoresponsive organic compound. The direction of the motion can be controlled by the position of the light sources, and the crystals can even climb vertical surfaces. The motion is driven by crystallization and melting at the front and rear edges of the crystal, respectively, via photochemical conversion between the crystal and liquid phases induced by the trans-cis isomerization of azobenzenes. This finding could lead to remote-controlled micrometre-sized vehicles and valves on solid substrates.