Laser-Induced Photothermal Pulling of Dyed Droplets on a Superhydrophobic Surface.

Understanding the interaction between laser beams and liquid droplets has significant implications for applications in microfluidics and optical manipulation. Laser beams have previously been reported to act as tractor beams to pull microscopic particles toward the light source or serve as a heating source when directed from above to induce lateral droplet motion via photothermal effects. However, it remains unknown whether a laser beam can move a droplet toward its source. In this study, we show that a laser beam can pull a dyed droplet on a superhydrophobic surface toward the light source through a sequence of photothermal effects. By directing a green laser beam near the bottom front of a dyed droplet, we observe that the droplet moves toward the light source in two distinct stages. Initially, the dyed droplet advances due to contact angle hysteresis and coalescence with condensation satellite droplets. Subsequently, the droplet motion is stimulated by iterative bubble bursting, coalescence, and relaxation, a combination of effects not reported earlier. We experimentally investigate this motion phenomenon and analyze the influence of laser power and focal point position on droplet motion, offering new insights into laser-induced droplet manipulation.