A self-propelled thermophoretic microgear.

An asymmetric microgear will spontaneously and unidirectionally rotate if it is heated in a cool surrounding solvent. The resulting temperature gradient along the edges of the gear teeth translates in a directed thermophoretic force, which will exert a net torque on the gear. By means of computer simulations, the validity of this scenario is proved. The rotational direction and speed are dependent on gear–solvent interactions, and can be analytically related to system parameters like the thermal diffusion factor, the solvent viscosity, or the temperature difference. This microgear provides a simple way to extract net work from non-isothermal solutions, and can become a valuable tool in microfluids.