Forward and backward laser-guided motion of an oil droplet.

Directed motion of an oil droplet floating in an aqueous solution is generated by using a laser beam. Interestingly, the direction of the droplet motion can be switched between forward and backward by changing the optical path of the laser through the droplet. This motion is caused above a certain critical power of the laser, and above this value the velocity increases almost linearly with the power. The mechanism of this directed motion is explained as follows: the oil droplet is locally heated by a narrow laser beam, this local heating induces a specific mode of convection inside the droplet, and this generated convective motion produces translational directed motion of the droplet.