Upper Limit of Light-Levitated Droplet Motion.

The light-enabled droplet levitation shows promising potential in applications in biotechnology, clinical medicine, and nanomaterials. In particular, light-levitated droplets have good followability with a moving laser beam, resulting in flexibility in manipulating their motion. However, it is still unclear whether there exists an upper limit to the light-levitated droplet motion with a moving laser beam. Therefore, the motion of light-levitated droplets above the free interface is studied to determine the upper limit of motions of the droplets with a moving laser beam. We demonstrate that an inefficient interface temperature response because of a very high moving speed of the laser beam and the resultant small upward vertical component of vapor flow are responsible for the existence of an upper-limit velocity. Above the upper limit, the light-levitated droplets are unable to stably move with the laser beam and finally disappear. By contrast, the droplets can stably move with the laser beam in a wide range at or below this upper limit. In addition, an almost linear relationship between the upper-limit velocity of the light-levitated droplets and the input laser power is presented. The findings of the present study are informative for the implementation of this light levitation technology.